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HEREDITY IN HYPERTENSION
Saturday HEREDITY IN HYPERTENSION Sir ROBERT PLATT Bt, M.D. Sheff., M.Sc. Manc., LL.D., F.R.C.P. PROFESSOR OF
MEDICINE, UNIVERSITY OF MANCHESTER
THAT heredity is important in the aetiology of hypertension is known to all physicians who are interested in the families of their patients. In 1923 Weitz recorded data from his cases, and in 1947 I drew a distinction between essential hypertension and secondary hypertension, the parental history of hypertensive disease being much commoner in the former than in the latter. Other authors are quoted in these papers. Since then the concept of essential hypertension as a hereditary disease has been of diagnostic value in distinguishing it from other forms of
hypertension. Nevertheless some doubt remains on the relative contributions of heredity and environment, and on the of the hereditary mechanism; and this has prompted the present study. nature
Severe
Hypertension
April I963
extraordinary resemblance to each other (fig. 1). They lived about 100 miles apart. Arthur died in January, 1952. Geoffrey developed the same symptoms and died in December, 1953. The nine blood-groups studied were identical. The brothers B are 47 and also live about 100 miles apart. They have always been known as identical. Richard has, gout mildly, but came because of hypertension. His blood-pressure was 230/130 mm. Hg. His twin John was not interviewed, but when he visited his brother his blood-pressure was 205/130 mm. Hg and his serum uric acid 8 mg. John has now developed gout. All fifteen blood-groups tested are identical. There was no evidence of renal disease to suggest that gout was the basis of the hypertension, but it remains an unlikely an
possibility.
-
The sisters Mrs. C and Mrs. D were 37 when investigated and are perhaps the most interesting of all. Mrs. C TABLE I-SEVERE HYPERTENSION IN DIZYGOTIC TWINS
in Twins
Previous studies of hypertension in twins were made before screening tests for the diagnosis of secondary hypertension were in general use, and were mostly on patients with only moderately raised blood-pressure such as might well be at the high end of the normal range. Nevertheless they have usually shown concordance in identical twins (Kahler and Weber 1940). In our own experience of severe hypertension we have come across three pairs of identical twins and four pairs of dizygotic twins. The evidence they present is unique and important. The brothers A were 48 when Arthur was admitted to hospital for treatment of malignant essential hypertension. His blood-pressure on admission was 260/150 mm. Hg. He was visited by his twin brother Geoffrey, who was
symptom-free but whose blood-pressure we found to be 210/130 mm. Hg. This was strong concordance consider-
ing that
the renal damage of the malignant phase would have raised Arthur’s pressure beyond its former level. The twins had always been considered identical and still bore
Fig. I-The brothers 7287
27
A.
*
Twin sister of
propositus.
through four hypertensive (but successful) pregnancies. Four months after the last pregnancy her bloodpressure was 200/130 mm. Hg. On the same day her identical twin sister’s pressure, taken by another observer, was 210/130 mm. Hg. This sister had never been pregnant. All fifteen blood-groups tested were identical. In no case were the twins sent for consultation because both were known to have hypertension, and in no case were the initial blood-pressures of both twins taken by the In all three the co-twin was symptomsame observer. free when first observed. The four pairs of dizygotic twins are all known to differ in important physical characters, and have never been regarded as identical. Their data are shown in table 1. Three of the four pairs are sibs (not co-twins) of the patient who came for treatment. Family X clearly shows discordance between the twins. Families Y and Z follow the rule that sibs of patients with secondary hypertension usually have normal blood-pressure. This evidence from twins, though numerically small, is alone sufficient to establish the hereditary nature of essential hypertension beyond reasonable doubt. went
The Sibs of Patients with Severe Hypertension During the past two-and-a-half years an attempt has been made to record the casual blood-pressure of the sibs
900 of every
patient attending the hypertension clinic at the Royal Infirmary, Manchester, and of every patient coming for the first time to me, or referred to me by a colleague. The study now comprises 178 propositi and 350 sibs. Patients of any age were accepted, but those of African or Asiatic race were excluded. The only other criterion of entry was that the initial diastolic blood-pressure of the propositi must be 125 mm. Hg or more. The study was thus confined to patients with quite severe hypertension. This was done because there is no reliable dividing line in terms of blood-pressure between " normal " and abnormal ", and the milder cases of hypertension must "
therefore include cases in which there would be a difference of opinion. All the propositi went through the usual screening tests to distinguish between essential and secondary hypertension, but aortograms were done only for younger patients, or where there was some likelihood of unilateral renal disease. Renal biopsy was reserved for rare cases in which the clinical situation seemed to warrant it. The propositi were divided into three groups-essential hypertension, secondary hypertension, and " doubtful ". I do not claim infallibility in the separation of these groups, and can only say that, after all reasonable care has been taken, cases remain in which essential and secondary hypertension cannot with certainty be distinguished. This applies, for instance, to a middle-aged woman whose investigation is negative but whose past history includes
pyelonephritis. Control studies of the normal population were not made, partly because they have been done extensively by other authors, and partly because it was hoped that secondary cases would form a useful contrast or comparison with the cases of essential hypertension. Because the secondary cases are much less common and have a very different agedistribution their value as a comparison with essential hypertension has not been as great as was hoped. Table n gives the ages, sex, and mean blood-pressure of the 178 propositi. We are studying patients whose mean diastolic pressure is 140-150 mm. Hg. They are in the range in which investigation and treatment for high blood-pressure is needed. The sexes are almost equally represented. Fig. 2, which is of great importance, contrasts the age-distribution of patients diagnosed as having essential hypertension with the age-distribution of those diagnosed as having secondary hypertension. It shows that in essential hypertension the age at which blood-pressure reaches serious TABLE
II-178
PROPOSITI WITH SEVERE HYPERTENSION
24 of the doubtful and
hypertension.
secondary
cases were
in the
malignant phase
of
Fig. 2-Age-distribution of propositi with
severe
hypertension.
levels is often 45 or over, and about two-thirds of the propositi are in the range 45-59. This of course is what any physician would expect. There is an interesting group, apparently indistinguishable from essential hypertension, in which these high levels of blood-pressure develop at an earlier age. The numbers are small but do not seem to increase over the age-groups from 30 to 45. This suggests that in the individual with essential hypertension the rise of blood-pressure with age is not linear. Indeed it is not linear in the general population (Hamilton, Pickering, Fraser Roberts, and Sowry 1954, Miall and Oldham 1955). The graph also shows that if we are to study blood-pressure in relatives, some of whom will develop essential hypertension of severe degree, we may not find them showing unequivocal evidence of high blood-pressure until they have entered the dangerous years-some of them not until their middle fifties. The study of relatives must therefore be a study of sibs, and not of parents and children, for the children will be too young and the affected parents probably dead. The agedistribution of secondary hypertension speaks for itself and shows that the causes can operate at any age. Table ill shows the mean pressures of the propositi with sibs divided into groups according to age and sex from which it will be seen that the mean blood-pressure varies very little between the groups, as we would expect because the series has been selected by height of blood-pressure. Those patients who have reached the malignant stage of hypertension are not separately considered. Naturally they are found in the higher range of blood-pressure (see table 11). They show a similar age-distribution. The blood-pressures of sibs were taken by a standard method to the nearest 5 mm., the subject lying on a couch. The method was explained by letter to the many doctors who willingly collaborated in the ascertainment of the blood-pressure of sibs who lived too far away to come to the Manchester Royal Infirmary. The lowest of a series of readings was used. It would have been better to use the type of machine which records the pressure unseen by the observer, but such instruments could not have been supplied to every doctor who took a reading. Although this series is almost certainly a larger collection of sibs of severe hypertensives than has previously been assembled, the numbers are still small when divided among the various age and sex categories, and also between sibs of patients with essential, and with secondary, hypertension. Moreover, in spite of every possible care a good many sibs are still missing. Of the sibs of propositi with essential hypertension, 238 have been studied, 59 are dead (at least 17i
901
of these deaths being probably from hypertension), and 40 are untraced. (Sibs dying in early life are not included.) The graphs in fig. 3 set out the frequency distribution of systolic pressure among the sibs of propositi with essential ten-year age-groups. Note that the ageMen and women are included together the increase to numbers, but graphs for males and females separately have been compiled and do not show any characteristics not illustrated in fig. 3.* Most patients with essential hypertension are in the age-group 45-60, and naturally most of their sibs are in the same range. Numbers of sibs in the younger decades are therefore smaller, and the graphs may be less reliable. In each case graphs of the frequency distribution of systolic bloodpressure in the general population are shown, scaled down to correspond with the numbers of the sibs. These are taken from the study of Hamilton et al. (1954), and not
hypertension in groups overlap.
TABLE III-MEAN PRESSURES BY AGE-GROUPS OF 84 PROPOSITI WITH SIBS (ESSENTIAL HYPERTENSION ONLY)
Interpretation
of
Frequency Distributions
It is clear from these curves that frequency distributions be adequately expressed by a simple statement of The abnormal mean pressures in different age-groups. of must be taken into at the levels high pressure bulge account: it is found in every study of blood-pressure in populations (e.g., Boe et al. 1957), but much more definitely in studies of the sibs of hypertensives (see, for instance, Platt 1959 and 1961). It does not disappear when blood-pressure is presented on a logarithmic scale. The first temptation is to intepret the cases in the middle range as the product of multifactorial inheritance, and to consider those at the high end of the curve as a separate group. But this interpretation is unsatisfactory because the individuals in the middle range are the sibs of those at the high end. Any genetical interpretation must therefore account for both the high and the middle ranges. The possibility arises of a multifactorial inheritance together with some superimposed environmental factor accounting for the severe hypertensives. While this cannot be ruled out, the evidence from identical twins very strongly points to a genetic rather than an environmental The action of a simple cause for severe hypertension. dominant gene will not account for these distributions. The genetic mechanism which we propose to account for the distribution curves is that of a gene which in the homozygous form causes severe hypertension and in the heterozygous form causes moderate elevation of bloodpressure. If we take the frequency of severe hypertension in females aged 45-60 in the general population as 6%, cannot
from the study of Miall and Oldham (1958), as the latter was done in a more inbred Welsh community which might be less comparable with the present study than the figures from St. Mary’s Hospital. In any case the differences are not great. The most obvious feature of these graphs is that none of them shows a Gaussian distribution. As we follow the successive age-groups they seem to show two important features, both of which can be seen to a much smaller extent in the curves for the distribution of blood-pressure in the general population. The first is the development of a bulge in the middle ranges of blood-pressure which, as we reach the 40-year-olds, is most noticeable at a systolic pressure of 160 mm. Hg. When we reach the " hypertensive age we find that more sibs are now in this middle range than at either end of the curve. A second feature is the development of an increasingly noticeable bulge at the high end of the curve, quite outside any normal Gaussian distribution. The effect of these two features is to give a kind of irregular trimodal distribution which is best seen in the graphs for men and women separately at ages 45-59 (fig. 4). These trends seem to be so clear that they are not likely to be due to artefacts caused by the admitted variability of blood-pressure "
measurements.
The
curves
also
seem to
show
a
right, there being fewer at the lower expect from the general population.
general shift end than
one
the would
to
’ Mean blood-pressures for
men and women in the normal population diverge a little as middle age is approached, the mean systolic pressure in men from 45 to 59 being about 5 mm. lower than that in women (Hamilton et al. 1954); but this itself may be partly an artefact, for there are fewer men than women in their series, and the lower mean pressures are partly due to a deficiency of men with high pressures. The prognosis of hypertension in men is known to be worse than in women.
Fig. 3-Frequency distribution of systolic blood-pressure of sibs of with essential hypertension, compared with that of the general population. Arranged in ten-year age-groups (males and females together). Sibs - - -. General population -.
propositi
902
would be four times as common in the general population The frequency-distribution curves for as in the sibs. systolic pressure in the general population can also be closely imitated by proportions 0-58:0-36:0-06, distributed around means of 130 mm. Hg, 160 mm. Hg, and over 200 mm. Hg. It is conceded that within fairly wide ranges of gene frequency, curves similar to those of the middleaged sibs in fig. 4 could be constructed which would fit the theory of an incompletely dominant gene with three phenotypes. No point is made of the exact proportions. The real point is that no such curve could be constructed on the basis either of a purely dominant gene or of multifactorial inheritance, and the new theory could neatly explain the fact that though some sibs inherit a very high pressure similar to that of the propositus, the majority inherit only a moderate rise of pressure. Diastolic Pressure Fig. 4-Frequency distribution of systolic blood-pressure of sibs aged 45-59, compared with that of the general population.
and if it were due to an incompletely dominant gene of this type, we would expect the three phenotypes (normal blood-pressure, moderate hypertension, and severe hypertension) to occur in sibs of severe hypertensives in the approximate ratio of 14%, 47%, and 38% respectively. (The basis of this calculation follows in the next section.) Using these proportions, satisfactory hypothetical curves can be drawn to represent very closely the distribution of sibs in fig. 4, except that the number of sibs at the high end of this curve is rather less than we would expect from the calculation; but we know, in the present series, that at least 8 of the female sibs of this age have died of hypertensive disease, so that a deficiency can easily be accounted for. A similar curve can be constructed for men, there being fewer both in the general population and among sibs at the high end of the curve. As men and women present themselves for treatment in approximately equal numbers (table 11), this discrepancy is presumably due to the worse prognosis in males with severe hypertension. Genetic Calculations
If
hypertension were very rare and due to homozygosity for a gene of incomplete dominance, the frequency of the three phenotypes-normal bloodpressure (normal homozygote), moderate hypertension (heterozygote), and severe hypertension (abnormal homozygote)-among sibs of severe hypertensives should be 1 :2 : 1; but severe hypertension is not rare, and the expected ratio in the sibs resulting from all possible matings, assuming equal fertility, will be: severe
Frequency distributions for diastolic pressure similar those of fig. 3 have been constructed. They are by themselves more difficult to interpret because the range of
to
pressure is smaller. In the younger ages the shift towards a bulge in the middle range is clear, as well as the emergence of a group at the high end. In middle age the main curve is widened and shifted to the right as compared with the normal population. It could consist of two overlapping groups with modes at 80 mm. Hg and 100 mm. Hg, and a third group in the higher range. For the sake of brevity only fig. 5 is shown which corresponds to fig. 4 for systolic pressures. In short, although the trends in the curves for diastolic pressure are less clear than in those for systolic pressure, there is nothing which makes them incompatible with the theory put forward, and they do not fit with the proposition of either a dominant gene or of multifactorial inheritance. Interested readers may have the data on application. The single-gene theory in its new form is put up only tentatively, and it will probably have to be modified in the light of future experience. The two most needed aids to further knowledge are a long-term study-twenty years at least-of the sibs and children of hypertensives, and some better tests for essential hypertension than blood-pressure measurement: for essential hypertension
"
where p and q are the gene frequencies for the normal " and " hypertensive " genes respectively. Examination of systolic blood-pressure for females in the general population suggests that about 6% are in the range of severe hypertension at the age of 45-60, taking a pressure of about 190 mm. Hg as the dividing line, which seems reasonable from the graphs. Taking q2 as 0-0576, that being the nearest convenient square to 6%, we get q=0-24 and the proportions in sibs as 0-14:0-47:0-38. In the general population, the proportions should be 0-58:0-36:0-06, assuming the normal distribution p2 : 2pq: q2. Thus severe hypertension should be about six times as common in the sibs of severe hypertensives as in the general population, while normal blood-pressure
Fig. 5-Frequency distribution of diastolic blood-pressure of sibs aged 45-59, compared with that of the general population.
903
population studies of arterial pressure have added to confusion by being quoted as if they had direct relevance to the problem of hypertension. In the whole of the series recorded in their 1958 paper there are only 4 persons under gene involved. 60 with systolic pressure of 200 mm. Hg or over, and only Environment and Heredity one with a pressure as high as the mean of the hundred and One of the difficulties in any study of blood-pressure is seventy-eight propositi examined in this paper. that over short periods of time it is greatly influenced by Before adopting the new single-gene theory we must environment, and it is affected by other factors, such as it to some other tests. First, would not such a gene obesity. Distribution curves are based on casual pressures, put in a fairly common and serious disease like essential hyperand their interpretation requires the assumption that numbers will cancel out the temporary environmental tension have been already eliminated from the population effects, or that the effects will be similar in the various by natural selection ? On the contrary, it is almost certain that a disease which so accurately begins to kill not in old populations. Hamilton et al. (1954) thought that heredity played age but in the post-reproductive years must be hereditary. selection has (nearly) eliminated it from younger only a small part in essential hypertension, and quoted a Natural but would be slow if not powerless to affect it people regression of 0-2 for the resemblance of sibs to propositi. much further. Moreover, there may be some heterozygote (A dominant gene would give 0-5.) This we must examine as yet undetermined: it might be that the advantage further as the conclusion is contrary to clinical experielimination of ageing relatives has survival value to the ence : their opinions are based on scores adjusted for age and sex, worked out from mean pressures of all available younger generation, or that the fertility of the heteroresistance to infective disease is enhanced. first-degree relatives (i.e., sibs, parents, and children). zygote or his the Secondly, theory infers that both parents of a The data I have presented are sufficient to cast doubt with severe hypertension must have carried the patient on any method which depends on means, or on an therefore be or have been in the interand should assumption that in essential hypertension the blood- gene, or of mediate blood-pressure. In this series high range pressure of people of 50 can be predicted from that of to take the blood-pressure of people of 30. But there are many other reasons why, even we did not usuallyas try we had not then seen its possible if the validity of the age and sex scores could be allowed, a surviving parents, The histories show that many died parental significance. regression of 0-5 would not have been realised, even if the of known or disease. probable hypertensive Many disease had an entirely hereditary basis. in the into their but intermediate survived seventies, In the first place doubts have been raised as to whether their propositi were a homogeneous group representative range of blood-pressure this is to be expected. In the of essential hypertension (Platt 1959). In the second series of Hamilton et al. the surviving parents whose bloodplace the large numbers of missing relatives would be pressures were taken show very few exceptions to the rule. there is an exception, the diagnosis of the prolikely to reduce the mean pressures. For instance, from Where is 110 hypertensive propositi only 63 parents were examined positus either doubtful because of age, or the propositus -i.e., less than a third. The original observations which himself is in the intermediate range of pressure and so called attention to heredity in hypertension (and which might be heterozygous. these authors have ignored in their calculations) would Diagnostic Value of the Blood-pressures of Sibs Using diastolic pressure as a better measure of clinical suggest that many of these parents had died of hypertensive disease. Thirdly, an inherited disease does not severity than systolic pressure, the records of our cases the In with same of degree severity. hyper- show that in the age-group 45-65 about 40% of the sibs of always appear for the sib who comes treatment-i.e., the severe essential hypertensives will have diastolic pressures tension, to have the highest pressure of the of 110 mm. Hg or more, a level found in only about 5% of propositus-is likely in the younger age-groups the validity of the general population of this age. The likelihood of one familyand scores adjusted for age and sex is even more or more of three sibs showing such a diastolic pressure is using questionable, partly because of the different frequency about 14% of the general population and 78% if the distributions (see fig. 3) and partly for reasons given by propositus has severe essential hypertension. ConHamilton et al. (1954, p. 44). Thus we see that for parents, versely, if a patient has three normal sibs the chances are sibs, and children, regressions worked out by the methods about 4 to 1 against essential hypertension as his diagnosis. of Hamilton et al. are all likely to have been underTABLE IV-MEAN PRESSURES OF SIBS OF THE PROPOSITI CLASSIFIED AS estimates. For these and other reasons the full significance SECONDARY AND DOUBTFUL HYPERTENSION of heredity in hypertension has been missed in these otherwise important studies. if it is simply an excess of one of the numerous normal determinants of blood-pressure. There may of course be several genes and not a single must
have
a
cause,
even
"
Further Examination of the
"
Single-gene Theory
The evidence here put forward, including that from twins, clearly shows that essential hypertension is a hereditary disease. The single-gene theory in its new form could explain not only the facts but also the frequency of moderate degrees of hypertension in the middleaged population, and thus resolve the difficulty of Miall and Oldham (1963) in deciding between the frequencies of hypertension given by various authors. Miall and Oldham’s In our series the sibs in the higher ranges of pressure
were
usually
less affected than the propositus. Only one living sib is in the malignant phase, but several dead sibs probably had malignant
hypertension.
In the sibs of secondary hypertensives the highest pressure is 175 mm. Hg and 110 mm. Hg diastolic (in the age-group 55-59). 5 sibs of doubtful " cases have systolic pressures over 200 mm. Hg and 7 have diastolic pressures of 110 mm. Hg or over.
systolic "
904
Secondary and
"
Doubtful
"
METHICILLIN RESISTANCE IN STAPHYLOCOCCI
Hypertension
In these categories are a mixed group of cases, some clearly renal or endocrine, some almost certainly essential but giving a past history of renal calculus or urinary infection which would seem to make it unwise to place them in the " essential " group in a paper which sets out to distinguish between that group and others. Thus of other authors tend to cases come pyelonephritic may into the " doubtful " group in this series. Certainly the heredity of the established secondary cases is quite different from that of essential hypertensives, and indistinguishable from the normal population. As numbers are small only mean pressures of sibs are given (table iv). Cruz-Coke (1961) showed that many pyelonephritic cases seemed to have’a hereditary background, and this is confirmed by the latest paper of Hamilton et al. (1963). The study of this series of hypertensive cases confirms the view of Gibson and Platt (1959) that pregnancy toxaemia is rarely if ever a cause of permanent severe hypertension. The recognition of renal-artery stenosis is fairly new. The numerous failures of surgery to influence the bloodpressure in such cases suggest that many are superimposed on essential hypertension. Examination of sibs may turn out to have some relevance to the selection of cases for surgery.
M. PATRICIA JEVONS M.D. Lond., Dip.Bact. A. W. COE M. T. PARKER Cantab., Dip.Bact.
M.D.
OF THE CROSS-INFECTION REFERENCE LABORATORY, CENTRAL PUBLIC HEALTH LABORATORY, COLINDALE, LONDON,
IN the last quarter of 1960 a search was made for methicillin-resistant strains of Staphylococcus aureus among cultures sent to the Staphylococcus Reference Laboratory for phage-typing (Jevons 1961). Of the 5440 tested, 3 (all from one hospital) were resistant. We now report the results of examining over 27,000 strains in the two years ended last September. Testing had to be suspended for several short periods; but we examined 27,479 (78%) of the 35,300 cultures received. They included 18,691 from hospital laboratories in Great Britain, 646 from the Food Hygiene Laboratory, Colindale, 314 from British veterinary laboratories, and 470 from abroad. Care was taken to exclude any strains made resistant to methicillin in the laboratory or sent to us because they were methicillin resistant.
Summary
Methods
Evidence from the study of twins and of 350 sibs of 178 patients with severe hypertension shows that essential hypertension is a specific inherited disorder of middle age. Secondary hypertension, on the other hand, is not, in general, hereditary: neither is it particularly prevalent in any age-group.
The hereditary mechanism of essential hypertension does not seem explicable by multifactorial inheritance or by the action of a single dominant gene. The facts at present known could be explained by the action of a single gene, with incomplete dominance and a frequency of about 0-24, which in the homozygous form gives rise to severe hypertension and in the heterozygous form to moderate elevation of blood-pressure. Better understanding of the mechanisms involved in essential and in secondary hypertension would help to test the validity of this theory. More important than the theory is the demonstration of the paramount importance of genetic factors in essential hypertension. Reasons are given why other authors have come to other conclusions. Thanks
are
due to Marion Winterbottom who did all the work of
assembling and tracing the 350 sibs; to the patients and their relatives; to a very large number of doctors in Britain, Australia, Canada, the United States, and Belgium who willingly cooperated and took blood-pressures from sibs who could not come to the Manchester Royal Infirmary; to Dr. A. J. Bateman and Dr. A. M. Adelstein for advice; to my colleagues who have referred cases; and to the department of medical illustration for producing the graphs. REFERENCES
Bøe, J., Humerfelt, S., Wederwang, F. (1957) Acta med. scand. 157, 1. Cruz-Coke, R. (1961) Acta genet. Basel, 11, 58. Gibson, G. B., Platt, R. (1959) Brit. med. J. ii, 159. Hamilton, M., Pickering, G. W., Fraser Roberts, J. A., Sowry, G. (1954) Clin. Sci. 13, 11, 37, 273. (1963) ibid. 24, 91. Kahler, O. H., Weber, R. (1940) Z. klin. Med. 137, 380. Miall, W. E., Oldham, P. D. (1955) Clin. Sci. 14, 459. (1958) ibid. 17, 409. (1963) Brit. med. J. i, 75. Platt, R. (1947) Quart. J. Med. 16, 111. (1959) Lancet, ii, 55. (1961) Ann. intern. Med. 55, 1. Weitz, W. (1923) Z. klin. Med. 96, 151. —
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N.W.9
S. C.
Screening Test Filterpaper discs, 7 mm.
in diameter, were each impregnated with 10 -g. methicillin and immediately freeze-dried. Samples of each batch were tested for inhibition of the Oxford staphylococcus (N.C.T.C. 6571); a batch was accepted if it gave a zone 22-28 mm. in diameter. Discs were stored at 4°C over calcium chloride, and were used within a month of preparation. Staphylococci sent for phage-typing were subcultured in tubes of nutrient broth, and incubated for four hours at 37°C. These broth cultures were used to flood nutrient-agar plates for the routine phage-typing test (see Blair and Williams 1961), and a methicillin disc was placed on an unoccupied area of each plate. The plates were incubated at 30°C overnight. Any growth within 3 mm. of the edge of the disc was taken as presumptive evidence of resistance. Most resistant cultures grew continuously up to the disc, but a few, which contained a relatively small proportion of resistant individuals (Knox and Smith 1961) showed only discrete colonies near the disc. The most common cause of confusion, however, was impurity of the culture submitted to us. For this reason, and also to exclude the possibility that a faulty disc had been used, all apparently resistant cultures were inoculated on to a blood-agar plate; the disc sensitivity test was repeated from a single wellseparated colony on the plate, and a tube coagulase test was
performed. Tube Dilution Method All cultures apparently resistant in two successive disc tests were examined by a tube dilution method, with a moderately large inoculum. 0-02 ml. of a six-hour broth culture was inoculated into 1 ml. amounts of nutrient broth containing doubling dilutions of methicillin from 100 jjLg. to 1-56 (lg. per ml. A parallel series of dilutions was inoculated from a sixhour culture of the Oxford staphylococcus. The tubes were incubated for eighteen hours at 37°C. The lowest concentration of antibiotic in which there was no visible growth was taken as the minimum inhibitory concentration (M.l.c.). Organisms with a M.l.c. of 12-5 -g. per ml. or more were regarded as resistant. Additional Tests Representative cultures of 16 of the 25 independently isolated methicillin-resistant strains were also tested for resistance to other antibiotics withOxoid’ Multodiscs containing 10 .-. each of
streptomycin, tetracycline, chloramphenicol, erythromycin, and 1-5 units of penicillin. They were
and also
MEDICINE, UNIVERSITY OF MANCHESTER
THAT heredity is important in the aetiology of hypertension is known to all physicians who are interested in the families of their patients. In 1923 Weitz recorded data from his cases, and in 1947 I drew a distinction between essential hypertension and secondary hypertension, the parental history of hypertensive disease being much commoner in the former than in the latter. Other authors are quoted in these papers. Since then the concept of essential hypertension as a hereditary disease has been of diagnostic value in distinguishing it from other forms of
hypertension. Nevertheless some doubt remains on the relative contributions of heredity and environment, and on the of the hereditary mechanism; and this has prompted the present study. nature
Severe
Hypertension
April I963
extraordinary resemblance to each other (fig. 1). They lived about 100 miles apart. Arthur died in January, 1952. Geoffrey developed the same symptoms and died in December, 1953. The nine blood-groups studied were identical. The brothers B are 47 and also live about 100 miles apart. They have always been known as identical. Richard has, gout mildly, but came because of hypertension. His blood-pressure was 230/130 mm. Hg. His twin John was not interviewed, but when he visited his brother his blood-pressure was 205/130 mm. Hg and his serum uric acid 8 mg. John has now developed gout. All fifteen blood-groups tested are identical. There was no evidence of renal disease to suggest that gout was the basis of the hypertension, but it remains an unlikely an
possibility.
-
The sisters Mrs. C and Mrs. D were 37 when investigated and are perhaps the most interesting of all. Mrs. C TABLE I-SEVERE HYPERTENSION IN DIZYGOTIC TWINS
in Twins
Previous studies of hypertension in twins were made before screening tests for the diagnosis of secondary hypertension were in general use, and were mostly on patients with only moderately raised blood-pressure such as might well be at the high end of the normal range. Nevertheless they have usually shown concordance in identical twins (Kahler and Weber 1940). In our own experience of severe hypertension we have come across three pairs of identical twins and four pairs of dizygotic twins. The evidence they present is unique and important. The brothers A were 48 when Arthur was admitted to hospital for treatment of malignant essential hypertension. His blood-pressure on admission was 260/150 mm. Hg. He was visited by his twin brother Geoffrey, who was
symptom-free but whose blood-pressure we found to be 210/130 mm. Hg. This was strong concordance consider-
ing that
the renal damage of the malignant phase would have raised Arthur’s pressure beyond its former level. The twins had always been considered identical and still bore
Fig. I-The brothers 7287
27
A.
*
Twin sister of
propositus.
through four hypertensive (but successful) pregnancies. Four months after the last pregnancy her bloodpressure was 200/130 mm. Hg. On the same day her identical twin sister’s pressure, taken by another observer, was 210/130 mm. Hg. This sister had never been pregnant. All fifteen blood-groups tested were identical. In no case were the twins sent for consultation because both were known to have hypertension, and in no case were the initial blood-pressures of both twins taken by the In all three the co-twin was symptomsame observer. free when first observed. The four pairs of dizygotic twins are all known to differ in important physical characters, and have never been regarded as identical. Their data are shown in table 1. Three of the four pairs are sibs (not co-twins) of the patient who came for treatment. Family X clearly shows discordance between the twins. Families Y and Z follow the rule that sibs of patients with secondary hypertension usually have normal blood-pressure. This evidence from twins, though numerically small, is alone sufficient to establish the hereditary nature of essential hypertension beyond reasonable doubt. went
The Sibs of Patients with Severe Hypertension During the past two-and-a-half years an attempt has been made to record the casual blood-pressure of the sibs
900 of every
patient attending the hypertension clinic at the Royal Infirmary, Manchester, and of every patient coming for the first time to me, or referred to me by a colleague. The study now comprises 178 propositi and 350 sibs. Patients of any age were accepted, but those of African or Asiatic race were excluded. The only other criterion of entry was that the initial diastolic blood-pressure of the propositi must be 125 mm. Hg or more. The study was thus confined to patients with quite severe hypertension. This was done because there is no reliable dividing line in terms of blood-pressure between " normal " and abnormal ", and the milder cases of hypertension must "
therefore include cases in which there would be a difference of opinion. All the propositi went through the usual screening tests to distinguish between essential and secondary hypertension, but aortograms were done only for younger patients, or where there was some likelihood of unilateral renal disease. Renal biopsy was reserved for rare cases in which the clinical situation seemed to warrant it. The propositi were divided into three groups-essential hypertension, secondary hypertension, and " doubtful ". I do not claim infallibility in the separation of these groups, and can only say that, after all reasonable care has been taken, cases remain in which essential and secondary hypertension cannot with certainty be distinguished. This applies, for instance, to a middle-aged woman whose investigation is negative but whose past history includes
pyelonephritis. Control studies of the normal population were not made, partly because they have been done extensively by other authors, and partly because it was hoped that secondary cases would form a useful contrast or comparison with the cases of essential hypertension. Because the secondary cases are much less common and have a very different agedistribution their value as a comparison with essential hypertension has not been as great as was hoped. Table n gives the ages, sex, and mean blood-pressure of the 178 propositi. We are studying patients whose mean diastolic pressure is 140-150 mm. Hg. They are in the range in which investigation and treatment for high blood-pressure is needed. The sexes are almost equally represented. Fig. 2, which is of great importance, contrasts the age-distribution of patients diagnosed as having essential hypertension with the age-distribution of those diagnosed as having secondary hypertension. It shows that in essential hypertension the age at which blood-pressure reaches serious TABLE
II-178
PROPOSITI WITH SEVERE HYPERTENSION
24 of the doubtful and
hypertension.
secondary
cases were
in the
malignant phase
of
Fig. 2-Age-distribution of propositi with
severe
hypertension.
levels is often 45 or over, and about two-thirds of the propositi are in the range 45-59. This of course is what any physician would expect. There is an interesting group, apparently indistinguishable from essential hypertension, in which these high levels of blood-pressure develop at an earlier age. The numbers are small but do not seem to increase over the age-groups from 30 to 45. This suggests that in the individual with essential hypertension the rise of blood-pressure with age is not linear. Indeed it is not linear in the general population (Hamilton, Pickering, Fraser Roberts, and Sowry 1954, Miall and Oldham 1955). The graph also shows that if we are to study blood-pressure in relatives, some of whom will develop essential hypertension of severe degree, we may not find them showing unequivocal evidence of high blood-pressure until they have entered the dangerous years-some of them not until their middle fifties. The study of relatives must therefore be a study of sibs, and not of parents and children, for the children will be too young and the affected parents probably dead. The agedistribution of secondary hypertension speaks for itself and shows that the causes can operate at any age. Table ill shows the mean pressures of the propositi with sibs divided into groups according to age and sex from which it will be seen that the mean blood-pressure varies very little between the groups, as we would expect because the series has been selected by height of blood-pressure. Those patients who have reached the malignant stage of hypertension are not separately considered. Naturally they are found in the higher range of blood-pressure (see table 11). They show a similar age-distribution. The blood-pressures of sibs were taken by a standard method to the nearest 5 mm., the subject lying on a couch. The method was explained by letter to the many doctors who willingly collaborated in the ascertainment of the blood-pressure of sibs who lived too far away to come to the Manchester Royal Infirmary. The lowest of a series of readings was used. It would have been better to use the type of machine which records the pressure unseen by the observer, but such instruments could not have been supplied to every doctor who took a reading. Although this series is almost certainly a larger collection of sibs of severe hypertensives than has previously been assembled, the numbers are still small when divided among the various age and sex categories, and also between sibs of patients with essential, and with secondary, hypertension. Moreover, in spite of every possible care a good many sibs are still missing. Of the sibs of propositi with essential hypertension, 238 have been studied, 59 are dead (at least 17i
901
of these deaths being probably from hypertension), and 40 are untraced. (Sibs dying in early life are not included.) The graphs in fig. 3 set out the frequency distribution of systolic pressure among the sibs of propositi with essential ten-year age-groups. Note that the ageMen and women are included together the increase to numbers, but graphs for males and females separately have been compiled and do not show any characteristics not illustrated in fig. 3.* Most patients with essential hypertension are in the age-group 45-60, and naturally most of their sibs are in the same range. Numbers of sibs in the younger decades are therefore smaller, and the graphs may be less reliable. In each case graphs of the frequency distribution of systolic bloodpressure in the general population are shown, scaled down to correspond with the numbers of the sibs. These are taken from the study of Hamilton et al. (1954), and not
hypertension in groups overlap.
TABLE III-MEAN PRESSURES BY AGE-GROUPS OF 84 PROPOSITI WITH SIBS (ESSENTIAL HYPERTENSION ONLY)
Interpretation
of
Frequency Distributions
It is clear from these curves that frequency distributions be adequately expressed by a simple statement of The abnormal mean pressures in different age-groups. of must be taken into at the levels high pressure bulge account: it is found in every study of blood-pressure in populations (e.g., Boe et al. 1957), but much more definitely in studies of the sibs of hypertensives (see, for instance, Platt 1959 and 1961). It does not disappear when blood-pressure is presented on a logarithmic scale. The first temptation is to intepret the cases in the middle range as the product of multifactorial inheritance, and to consider those at the high end of the curve as a separate group. But this interpretation is unsatisfactory because the individuals in the middle range are the sibs of those at the high end. Any genetical interpretation must therefore account for both the high and the middle ranges. The possibility arises of a multifactorial inheritance together with some superimposed environmental factor accounting for the severe hypertensives. While this cannot be ruled out, the evidence from identical twins very strongly points to a genetic rather than an environmental The action of a simple cause for severe hypertension. dominant gene will not account for these distributions. The genetic mechanism which we propose to account for the distribution curves is that of a gene which in the homozygous form causes severe hypertension and in the heterozygous form causes moderate elevation of bloodpressure. If we take the frequency of severe hypertension in females aged 45-60 in the general population as 6%, cannot
from the study of Miall and Oldham (1958), as the latter was done in a more inbred Welsh community which might be less comparable with the present study than the figures from St. Mary’s Hospital. In any case the differences are not great. The most obvious feature of these graphs is that none of them shows a Gaussian distribution. As we follow the successive age-groups they seem to show two important features, both of which can be seen to a much smaller extent in the curves for the distribution of blood-pressure in the general population. The first is the development of a bulge in the middle ranges of blood-pressure which, as we reach the 40-year-olds, is most noticeable at a systolic pressure of 160 mm. Hg. When we reach the " hypertensive age we find that more sibs are now in this middle range than at either end of the curve. A second feature is the development of an increasingly noticeable bulge at the high end of the curve, quite outside any normal Gaussian distribution. The effect of these two features is to give a kind of irregular trimodal distribution which is best seen in the graphs for men and women separately at ages 45-59 (fig. 4). These trends seem to be so clear that they are not likely to be due to artefacts caused by the admitted variability of blood-pressure "
measurements.
The
curves
also
seem to
show
a
right, there being fewer at the lower expect from the general population.
general shift end than
one
the would
to
’ Mean blood-pressures for
men and women in the normal population diverge a little as middle age is approached, the mean systolic pressure in men from 45 to 59 being about 5 mm. lower than that in women (Hamilton et al. 1954); but this itself may be partly an artefact, for there are fewer men than women in their series, and the lower mean pressures are partly due to a deficiency of men with high pressures. The prognosis of hypertension in men is known to be worse than in women.
Fig. 3-Frequency distribution of systolic blood-pressure of sibs of with essential hypertension, compared with that of the general population. Arranged in ten-year age-groups (males and females together). Sibs - - -. General population -.
propositi
902
would be four times as common in the general population The frequency-distribution curves for as in the sibs. systolic pressure in the general population can also be closely imitated by proportions 0-58:0-36:0-06, distributed around means of 130 mm. Hg, 160 mm. Hg, and over 200 mm. Hg. It is conceded that within fairly wide ranges of gene frequency, curves similar to those of the middleaged sibs in fig. 4 could be constructed which would fit the theory of an incompletely dominant gene with three phenotypes. No point is made of the exact proportions. The real point is that no such curve could be constructed on the basis either of a purely dominant gene or of multifactorial inheritance, and the new theory could neatly explain the fact that though some sibs inherit a very high pressure similar to that of the propositus, the majority inherit only a moderate rise of pressure. Diastolic Pressure Fig. 4-Frequency distribution of systolic blood-pressure of sibs aged 45-59, compared with that of the general population.
and if it were due to an incompletely dominant gene of this type, we would expect the three phenotypes (normal blood-pressure, moderate hypertension, and severe hypertension) to occur in sibs of severe hypertensives in the approximate ratio of 14%, 47%, and 38% respectively. (The basis of this calculation follows in the next section.) Using these proportions, satisfactory hypothetical curves can be drawn to represent very closely the distribution of sibs in fig. 4, except that the number of sibs at the high end of this curve is rather less than we would expect from the calculation; but we know, in the present series, that at least 8 of the female sibs of this age have died of hypertensive disease, so that a deficiency can easily be accounted for. A similar curve can be constructed for men, there being fewer both in the general population and among sibs at the high end of the curve. As men and women present themselves for treatment in approximately equal numbers (table 11), this discrepancy is presumably due to the worse prognosis in males with severe hypertension. Genetic Calculations
If
hypertension were very rare and due to homozygosity for a gene of incomplete dominance, the frequency of the three phenotypes-normal bloodpressure (normal homozygote), moderate hypertension (heterozygote), and severe hypertension (abnormal homozygote)-among sibs of severe hypertensives should be 1 :2 : 1; but severe hypertension is not rare, and the expected ratio in the sibs resulting from all possible matings, assuming equal fertility, will be: severe
Frequency distributions for diastolic pressure similar those of fig. 3 have been constructed. They are by themselves more difficult to interpret because the range of
to
pressure is smaller. In the younger ages the shift towards a bulge in the middle range is clear, as well as the emergence of a group at the high end. In middle age the main curve is widened and shifted to the right as compared with the normal population. It could consist of two overlapping groups with modes at 80 mm. Hg and 100 mm. Hg, and a third group in the higher range. For the sake of brevity only fig. 5 is shown which corresponds to fig. 4 for systolic pressures. In short, although the trends in the curves for diastolic pressure are less clear than in those for systolic pressure, there is nothing which makes them incompatible with the theory put forward, and they do not fit with the proposition of either a dominant gene or of multifactorial inheritance. Interested readers may have the data on application. The single-gene theory in its new form is put up only tentatively, and it will probably have to be modified in the light of future experience. The two most needed aids to further knowledge are a long-term study-twenty years at least-of the sibs and children of hypertensives, and some better tests for essential hypertension than blood-pressure measurement: for essential hypertension
"
where p and q are the gene frequencies for the normal " and " hypertensive " genes respectively. Examination of systolic blood-pressure for females in the general population suggests that about 6% are in the range of severe hypertension at the age of 45-60, taking a pressure of about 190 mm. Hg as the dividing line, which seems reasonable from the graphs. Taking q2 as 0-0576, that being the nearest convenient square to 6%, we get q=0-24 and the proportions in sibs as 0-14:0-47:0-38. In the general population, the proportions should be 0-58:0-36:0-06, assuming the normal distribution p2 : 2pq: q2. Thus severe hypertension should be about six times as common in the sibs of severe hypertensives as in the general population, while normal blood-pressure
Fig. 5-Frequency distribution of diastolic blood-pressure of sibs aged 45-59, compared with that of the general population.
903
population studies of arterial pressure have added to confusion by being quoted as if they had direct relevance to the problem of hypertension. In the whole of the series recorded in their 1958 paper there are only 4 persons under gene involved. 60 with systolic pressure of 200 mm. Hg or over, and only Environment and Heredity one with a pressure as high as the mean of the hundred and One of the difficulties in any study of blood-pressure is seventy-eight propositi examined in this paper. that over short periods of time it is greatly influenced by Before adopting the new single-gene theory we must environment, and it is affected by other factors, such as it to some other tests. First, would not such a gene obesity. Distribution curves are based on casual pressures, put in a fairly common and serious disease like essential hyperand their interpretation requires the assumption that numbers will cancel out the temporary environmental tension have been already eliminated from the population effects, or that the effects will be similar in the various by natural selection ? On the contrary, it is almost certain that a disease which so accurately begins to kill not in old populations. Hamilton et al. (1954) thought that heredity played age but in the post-reproductive years must be hereditary. selection has (nearly) eliminated it from younger only a small part in essential hypertension, and quoted a Natural but would be slow if not powerless to affect it people regression of 0-2 for the resemblance of sibs to propositi. much further. Moreover, there may be some heterozygote (A dominant gene would give 0-5.) This we must examine as yet undetermined: it might be that the advantage further as the conclusion is contrary to clinical experielimination of ageing relatives has survival value to the ence : their opinions are based on scores adjusted for age and sex, worked out from mean pressures of all available younger generation, or that the fertility of the heteroresistance to infective disease is enhanced. first-degree relatives (i.e., sibs, parents, and children). zygote or his the Secondly, theory infers that both parents of a The data I have presented are sufficient to cast doubt with severe hypertension must have carried the patient on any method which depends on means, or on an therefore be or have been in the interand should assumption that in essential hypertension the blood- gene, or of mediate blood-pressure. In this series high range pressure of people of 50 can be predicted from that of to take the blood-pressure of people of 30. But there are many other reasons why, even we did not usuallyas try we had not then seen its possible if the validity of the age and sex scores could be allowed, a surviving parents, The histories show that many died parental significance. regression of 0-5 would not have been realised, even if the of known or disease. probable hypertensive Many disease had an entirely hereditary basis. in the into their but intermediate survived seventies, In the first place doubts have been raised as to whether their propositi were a homogeneous group representative range of blood-pressure this is to be expected. In the of essential hypertension (Platt 1959). In the second series of Hamilton et al. the surviving parents whose bloodplace the large numbers of missing relatives would be pressures were taken show very few exceptions to the rule. there is an exception, the diagnosis of the prolikely to reduce the mean pressures. For instance, from Where is 110 hypertensive propositi only 63 parents were examined positus either doubtful because of age, or the propositus -i.e., less than a third. The original observations which himself is in the intermediate range of pressure and so called attention to heredity in hypertension (and which might be heterozygous. these authors have ignored in their calculations) would Diagnostic Value of the Blood-pressures of Sibs Using diastolic pressure as a better measure of clinical suggest that many of these parents had died of hypertensive disease. Thirdly, an inherited disease does not severity than systolic pressure, the records of our cases the In with same of degree severity. hyper- show that in the age-group 45-65 about 40% of the sibs of always appear for the sib who comes treatment-i.e., the severe essential hypertensives will have diastolic pressures tension, to have the highest pressure of the of 110 mm. Hg or more, a level found in only about 5% of propositus-is likely in the younger age-groups the validity of the general population of this age. The likelihood of one familyand scores adjusted for age and sex is even more or more of three sibs showing such a diastolic pressure is using questionable, partly because of the different frequency about 14% of the general population and 78% if the distributions (see fig. 3) and partly for reasons given by propositus has severe essential hypertension. ConHamilton et al. (1954, p. 44). Thus we see that for parents, versely, if a patient has three normal sibs the chances are sibs, and children, regressions worked out by the methods about 4 to 1 against essential hypertension as his diagnosis. of Hamilton et al. are all likely to have been underTABLE IV-MEAN PRESSURES OF SIBS OF THE PROPOSITI CLASSIFIED AS estimates. For these and other reasons the full significance SECONDARY AND DOUBTFUL HYPERTENSION of heredity in hypertension has been missed in these otherwise important studies. if it is simply an excess of one of the numerous normal determinants of blood-pressure. There may of course be several genes and not a single must
have
a
cause,
even
"
Further Examination of the
"
Single-gene Theory
The evidence here put forward, including that from twins, clearly shows that essential hypertension is a hereditary disease. The single-gene theory in its new form could explain not only the facts but also the frequency of moderate degrees of hypertension in the middleaged population, and thus resolve the difficulty of Miall and Oldham (1963) in deciding between the frequencies of hypertension given by various authors. Miall and Oldham’s In our series the sibs in the higher ranges of pressure
were
usually
less affected than the propositus. Only one living sib is in the malignant phase, but several dead sibs probably had malignant
hypertension.
In the sibs of secondary hypertensives the highest pressure is 175 mm. Hg and 110 mm. Hg diastolic (in the age-group 55-59). 5 sibs of doubtful " cases have systolic pressures over 200 mm. Hg and 7 have diastolic pressures of 110 mm. Hg or over.
systolic "
904
Secondary and
"
Doubtful
"
METHICILLIN RESISTANCE IN STAPHYLOCOCCI
Hypertension
In these categories are a mixed group of cases, some clearly renal or endocrine, some almost certainly essential but giving a past history of renal calculus or urinary infection which would seem to make it unwise to place them in the " essential " group in a paper which sets out to distinguish between that group and others. Thus of other authors tend to cases come pyelonephritic may into the " doubtful " group in this series. Certainly the heredity of the established secondary cases is quite different from that of essential hypertensives, and indistinguishable from the normal population. As numbers are small only mean pressures of sibs are given (table iv). Cruz-Coke (1961) showed that many pyelonephritic cases seemed to have’a hereditary background, and this is confirmed by the latest paper of Hamilton et al. (1963). The study of this series of hypertensive cases confirms the view of Gibson and Platt (1959) that pregnancy toxaemia is rarely if ever a cause of permanent severe hypertension. The recognition of renal-artery stenosis is fairly new. The numerous failures of surgery to influence the bloodpressure in such cases suggest that many are superimposed on essential hypertension. Examination of sibs may turn out to have some relevance to the selection of cases for surgery.
M. PATRICIA JEVONS M.D. Lond., Dip.Bact. A. W. COE M. T. PARKER Cantab., Dip.Bact.
M.D.
OF THE CROSS-INFECTION REFERENCE LABORATORY, CENTRAL PUBLIC HEALTH LABORATORY, COLINDALE, LONDON,
IN the last quarter of 1960 a search was made for methicillin-resistant strains of Staphylococcus aureus among cultures sent to the Staphylococcus Reference Laboratory for phage-typing (Jevons 1961). Of the 5440 tested, 3 (all from one hospital) were resistant. We now report the results of examining over 27,000 strains in the two years ended last September. Testing had to be suspended for several short periods; but we examined 27,479 (78%) of the 35,300 cultures received. They included 18,691 from hospital laboratories in Great Britain, 646 from the Food Hygiene Laboratory, Colindale, 314 from British veterinary laboratories, and 470 from abroad. Care was taken to exclude any strains made resistant to methicillin in the laboratory or sent to us because they were methicillin resistant.
Summary
Methods
Evidence from the study of twins and of 350 sibs of 178 patients with severe hypertension shows that essential hypertension is a specific inherited disorder of middle age. Secondary hypertension, on the other hand, is not, in general, hereditary: neither is it particularly prevalent in any age-group.
The hereditary mechanism of essential hypertension does not seem explicable by multifactorial inheritance or by the action of a single dominant gene. The facts at present known could be explained by the action of a single gene, with incomplete dominance and a frequency of about 0-24, which in the homozygous form gives rise to severe hypertension and in the heterozygous form to moderate elevation of blood-pressure. Better understanding of the mechanisms involved in essential and in secondary hypertension would help to test the validity of this theory. More important than the theory is the demonstration of the paramount importance of genetic factors in essential hypertension. Reasons are given why other authors have come to other conclusions. Thanks
are
due to Marion Winterbottom who did all the work of
assembling and tracing the 350 sibs; to the patients and their relatives; to a very large number of doctors in Britain, Australia, Canada, the United States, and Belgium who willingly cooperated and took blood-pressures from sibs who could not come to the Manchester Royal Infirmary; to Dr. A. J. Bateman and Dr. A. M. Adelstein for advice; to my colleagues who have referred cases; and to the department of medical illustration for producing the graphs. REFERENCES
Bøe, J., Humerfelt, S., Wederwang, F. (1957) Acta med. scand. 157, 1. Cruz-Coke, R. (1961) Acta genet. Basel, 11, 58. Gibson, G. B., Platt, R. (1959) Brit. med. J. ii, 159. Hamilton, M., Pickering, G. W., Fraser Roberts, J. A., Sowry, G. (1954) Clin. Sci. 13, 11, 37, 273. (1963) ibid. 24, 91. Kahler, O. H., Weber, R. (1940) Z. klin. Med. 137, 380. Miall, W. E., Oldham, P. D. (1955) Clin. Sci. 14, 459. (1958) ibid. 17, 409. (1963) Brit. med. J. i, 75. Platt, R. (1947) Quart. J. Med. 16, 111. (1959) Lancet, ii, 55. (1961) Ann. intern. Med. 55, 1. Weitz, W. (1923) Z. klin. Med. 96, 151. —
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N.W.9
S. C.
Screening Test Filterpaper discs, 7 mm.
in diameter, were each impregnated with 10 -g. methicillin and immediately freeze-dried. Samples of each batch were tested for inhibition of the Oxford staphylococcus (N.C.T.C. 6571); a batch was accepted if it gave a zone 22-28 mm. in diameter. Discs were stored at 4°C over calcium chloride, and were used within a month of preparation. Staphylococci sent for phage-typing were subcultured in tubes of nutrient broth, and incubated for four hours at 37°C. These broth cultures were used to flood nutrient-agar plates for the routine phage-typing test (see Blair and Williams 1961), and a methicillin disc was placed on an unoccupied area of each plate. The plates were incubated at 30°C overnight. Any growth within 3 mm. of the edge of the disc was taken as presumptive evidence of resistance. Most resistant cultures grew continuously up to the disc, but a few, which contained a relatively small proportion of resistant individuals (Knox and Smith 1961) showed only discrete colonies near the disc. The most common cause of confusion, however, was impurity of the culture submitted to us. For this reason, and also to exclude the possibility that a faulty disc had been used, all apparently resistant cultures were inoculated on to a blood-agar plate; the disc sensitivity test was repeated from a single wellseparated colony on the plate, and a tube coagulase test was
performed. Tube Dilution Method All cultures apparently resistant in two successive disc tests were examined by a tube dilution method, with a moderately large inoculum. 0-02 ml. of a six-hour broth culture was inoculated into 1 ml. amounts of nutrient broth containing doubling dilutions of methicillin from 100 jjLg. to 1-56 (lg. per ml. A parallel series of dilutions was inoculated from a sixhour culture of the Oxford staphylococcus. The tubes were incubated for eighteen hours at 37°C. The lowest concentration of antibiotic in which there was no visible growth was taken as the minimum inhibitory concentration (M.l.c.). Organisms with a M.l.c. of 12-5 -g. per ml. or more were regarded as resistant. Additional Tests Representative cultures of 16 of the 25 independently isolated methicillin-resistant strains were also tested for resistance to other antibiotics withOxoid’ Multodiscs containing 10 .-. each of
streptomycin, tetracycline, chloramphenicol, erythromycin, and 1-5 units of penicillin. They were
and also