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Genetic mediation of cholesterol metabolism in the baboon (Papio cynocephalus)
Atherosclerosis, 41 (1982) 403-414 Scientific @ Elsevier/North-Holland
403 Publishers,
Ltd.
GENETIC MEDIATION OF CHOLESTEROL METABOLISM IN THE BABOON (PAPZO C YNOCEPHA L US)
BRYAN
L. FLOW
and GLEN
E. MOTT
Department of Pathology, The and the Southwest Foundation (U.S.A.) (Received (Revised, (Accepted
University of Texas Health Science Center for Research and Education, San Antonio,
at San Antonio TX 78284
29 May, 1981) received 20 July, 1981) 20 July, 1981)
Summary Genetic effects on serum cholesterol concentration and measures of cholesterol metabolism were investigated in 83 normocholesterolemic juvenile baboons (Pupio cynocephalus), the progeny of 6 sires. Sire effects (P < 0.05) were observed for serum cholesterol concentration, cholesterol turnover rate, cholesterol production rate, and several cholesterol pool parameters derived from a two-pool model. The estimates of heritability (h’) for the half-time of the first (t’,A) and second exponentials (t:B) derived from serum [ 14C]cholesterol decay curves and the cholesterol turnover rate were 0.67, 0.73 and 0.71, respectively. These heritabilities and those for serum cholesterol concentration (h* = 0.44) and cholesterol production rate (h* = 0.56) indicate these characters are moderately to highly heritable. Serum cholesterol concentration was correlated genetically with cholesterol turnover rate (rg = -0.56), production rate (rg = -0.41), t$A (l”g= 0.53), and t+B (rg = 0.39). Correlations among observed values, or phenotypic correlations, were low. Path analyses revealed that the low phenotypic correlation (rp = 0.02) between serum cholesterol concentration and cholesterol turnover rate was due to genetic and environmental contributions of similar magnitude but opposite in sign. The low phenotypic correlations of serum cholesterol with cholesterol production rate, t$A, and t$B (r, = -0.24, 0.23, and 0.19, respectively) were due primarily to the genetic contribution since environmental contributions were near zero. This research was suppoked by grants HG15914 and HL-19362 from the National Heart. Lung and Blood Institute. Address for correspondence and reprints: Glen E. Mott, Ph.D., Department of Pathology, The University of Texas Health Science Center, San Antonio, TX 78284, U.S.A.
OOZl-9150/82/0000+I000/$02.75@1982
Elsevier/North-Holland
Scientific
Publishers,
Ltd.
404
Application of similar genetic and path analytic techniques to human populations should enhance our understanding of the low phenotypic relationships previously observed between serum cholesterol and parameters of cholesterol metabolism.
Key words:
Baboon - Cholesterol terol metabolism
turnover
- Genetics
-. Heredity
- Regulation
of choles-
Introduction Genetic mediation of serum cholesterol concentration has been demonstrated in man [l-3] and in a number of animal species [4-101 including nonhuman primates [ 11,121. A few well defined genetic defects in cholesterol metabolism have been identified [13,14] but the mechanisms of regulation of serum cholesterol concentration in most individuals are not well understood. A significant sire effect on serum cholesterol concentration [ 151 and development of lines of progeny with high and low serum cholesterol concentrations [16] indicate that serum cholesterol concentration in baboons (Papio sp.) also is mediated genetically. This report describes previously unidentified genetic and environmental relationships between serum cholesterol and parameters of cholesterol metabolism in baboons. Materials and Methods Selection of adult breeders Adult female baboons were randomly were of East African origin and most of selected for their reproductive potential. associated characteristics were not criteria
assigned to 7 sires [ 151. All breeders them were feral. Sires and dams were Serum cholesterol concentration and for consideration.
Diets We assigned each infant at birth to one of three infant formulas or to breast feeding [15]. The effects of infant diet, sex, and sire on infant serum cholesterol concentration have been described previously [ 151. At 14 weeks of age, infants were weaned and each was maintained on one of four adult diets. These diets were formulated with Special Monkey Chow 25 (No. 5045-6), Ralston-Purina Co., St. Louis, MO, to which saturated (P/S = 0.26) or unsaturated (P/S = 2.1) plant oils were added with either 1.0 mg or 0.01 mg cholesterol/kcal. The effects of infant and adult diets and sex on progeny serum cholesterol concentration and variables of cholesterol metabolism during the juvenile period will be presented in a separate report. Serum cholesterol concentrations of the adult breeders were measured 3 times at monthly intervals while they were fed a chow-based diet which contained 20% saturated fat and 1 mg cholesterol/kcal.
405
Serum
lipid analyses
Serum was saponified and extracted with petroleum ether [17], and cholesterol measured by the color development method of Searcy and Bergquist [18]. The procedure met the criteria of the Center for Disease Control Lipid Standardization Program. The sire or dam serum cholesterol value was the mean of the 3 analyses made while the animals were fed the diet described for the adult breeders. The midparent value was the mean of the sire and dam values. Serum cholesterol concentration of the progeny was the mean of measurements at 39, 42, and 45 months of age. The value at each age was the mean of blind duplicate determinations. The coefficient of variation of the duplicate measurements was approximately 5%. Analyses
of cholesterol
metabolism
At 3.5 years of age, cholesterol absorption was measured by the method of Borgstrom [19] and cholesterol turnover rate by the method (equations 4, 5, and 7) described by Grundy and Ahrens [20]. A number of parameters of cholesterol metabolism were estimated from the specific radioactivity decay curves for serum [ 14C]cholesterol. The radioactive dose was prepared by adding 35 PCi of [4-14C]cholestero1 (Sigma Chemical Co., St. Louis, MO) dissolved in 100 1-11 of acetone to 3 ml of sterile autologous serum and mixing slowly with a stirring bar for 20 h. The amount of radioactivity injected intravenously was determined as the difference between the radioactivity added to the syringe and the residual radioactivity in the syringe after injection. The serum cholesterol specific radioactivity was measured 25 times over a 4-month period. Cholesterol production rate, half-time of the first exponential (t%A), half-time of the second exponential (t+B) and cholesterol mass of pool A were estimated by fitting 2 exponential functions to the serum [ 14C]cholesterol-specific radioactivity curve [21,22]. A minimum estimate of the cholesterol mass of pool B also was calculated with the assumption that there was no cholesterol synthesis in pool B [23]. The specific radioactivity data were also fitted to a 3-pool model [ 241 by a similar procedure, but in many animals the 3-pool model did not provide a better fit of the data than the 2-pool model. Therefore, only the results of the 2-pool fit are presented. Effects
of growth
on cholesterol
metabolism
The mean body weight of animals in this study was 10.5 kg. Among sire progeny groups there were no differences for body weight or body weight change during the metabolism studies. The animals were growing at an average rate of 812 mg/kg body weight/day with a range among sire progeny groups of 582-948 mg/kg body weight/day or less than 0.1% of the body weight. An average cholesterol content of baboon tissue of 1.03 mg/g from a previous experiment (unpublished observations) was used to estimate that only 0.84 mg cholesterol/kg body weight/day would be incorporated into new tissue with a maximum difference among sire progeny groups of 0.38 mg cholesterol/kg body weight/day. This represents a small percentage of the average cholesterol turnover rates of 28.5-36.7 mg/kg body weight/day observed in this study. Thus we conclude that the sire effects and genetic parameters are not due to the effects of differences in growth rate on cholesterol metabolism.
406
Gene tic analyses Heritabilities (h’) and genetic (rg), phenotypic (rP), and environmental (r,) correlations were estimated by procedures described for a paternal half-sib design [25]. The analyses of variance and the analyses of variance of cross products were performed by least squares procedures [26,27]. The linear model included the effects of infant diet, dietary cholesterol, dietary fat, sex, sire, and the two-factor interactions of infant diet by dietary cholesterol and dietary cholesterol by dietary fat. We included in the model only those twofactor interactions which were significant for one or more of the dependent variables. All effects were considered fixed except sires which were assumed to be random. Data for 83 progeny of 6 sires were log-transformed to better meet the assumptions of the model. The 7th sire was excluded from the genetic analyses since he produced only 3 progeny. Heritability of serum cholesterol concentration also was estimated by the regression of progeny values on the mid-parent value [28]. Since serum lipid concentrations in juvenile animals were compared with those in adult animals, heritability was estimated by standardized [16,29] partial regression coefficients for the regression of offspring values on mid-parent values. The model included the mid-parent value and the effects of infant diet, adult dietary cholesterol, adult dietary fat, sex, and all the two-factor interactions. Path analysis The phenotypic correlation, or the correlation of observed values, represents the net effect of genetic and environmental factors. The genetic (hxrghy) and environmental (e&e,) contributions to phenotypic correlation (rP) were expressed as: rP = hxrghy + exreey where h, and h, represent the correlations between additive genetic effects and phenotype for characters x and y; rg represents the genetic correlation between x and y; e, and e, represent the correlations between environmental (including non-additive genetic) effects and phenotype, and r, represents the correlation among environmental (including non-additive genetic) effects [ 281. Assumptions of this statistical model include the absence of interactions and correlations among the genotypes and environments. The correlation (h) between additive genetic effects and phenotype was determined by taking the square root of the paternal half-sib estimate of heritability (h’) of the character. Similarly, we estimated the correlation (e) between environmental effects and phenotype of the character by d/1- h2. Results Genetic mediation of cholesterol metabolism Differences (P < 0.05) were observed among sire progeny groups for serum cholesterol concentration, cholesterol turnover rate, cholesterol production rate, t;A, t;B, and mass of pool A (Table 1). There were no significant differences (P > 0.10) among sire progeny groups for the percent of dietary cholesterol absorbed or for the minimum estimate of the cholesterol mass of pool B.
squares
means.
16
A776
groups
8
A956
b Least
17
A947
a Sire progeny
19
A943
15
8
a
A982
A772
gr0UP
of
Values
141
in parenthesis
in decreasing
(100-122)
111
(104-138)
120
(110-132)
120
(110-132)
121
(110-145)
126
(127-156)b
order are 95%
28.5
cholesterol
b
intervals.
serum
confidence
of their
(27.4-33.4)
30.2
(31.642.1)
36.4
(31.1-37.7)
34.2
(33.540.1)
36.7
(29.3-39.1)
33.8
(25.7-31.6)
concentrations.
(32.0-35.7)
33.8
(34.940.8)
37.8
(35.4-39.3)
37.3
(33.8-37.2)
35.5
(32.0-37.4)
34.6
(31.1-34.8)
32.9
(mg/kg/daY)
Production
b
rate
3.51
(2.89-3.52)
3.19
(2.63-3.50)
3.04
(2.93-3.54)
3.22
(2.45-2.93)
2.68
(2.65-3.52)
3.06
(3.16-3.89)
b
23.9
(22.4-24.6)
23.5
(20.0-22.9)
21.4
(20.8-22.8)
21.8
(20.7-22.5)
21.6
(22.2-25.4)
23.8
(22.7-25.1)
(days)
t+B
Turnover (mg/kg/daY)
SelUm
cholesterol
rate
exponential,
GROUP
tf A (days)
PROGENY
exponential,
SIRE
(meld0
BY
concentration
METABOLISM Half-time
CHOLESTEROL
of second
OF
first
MEASURES Half-time
AND
of
CONCENTRATION
are ranked
Number
progeny
Sire
CHOLESTEROL
SERUM
progeny
1
TABLE
b
Mass
of A
(308-359)
332
(295-367)
329
(322-373)
347
(279-320)
299
(265-330)
296
(320-375)
347
(mg/kg)
pool
b
408
Sire progeny group means for the percent of dietary cholesterol absorbed ranged from 42 to 45% and those for cholesterol mass of pool B ranged from 386 to 427 mg/kg. Heritability of serum cholesterol determined from the standardized regression of offspring value on mid-parent mean was 0.43 and the paternal half-sib estimate was 0.44 (SE = 0.43). These heritabilities and that for cholesterol turnover rate (h’ = 0.71, SE = 0.53), production rate (h2 = 0.56, SE = 0.47), t;A (h2 = 0.67, SE = 0.52), t;B (h2 = 0.73, SE = 0.54) and cholesterol mass of pool A (h2 = 0.55, SE = 0.48) suggest the heritabilities of these characters are in the moderate to high range. Heritability of cholesterol absorption or mass of pool B could not be estimated because the differences observed among sire progeny groups were small relative to within group variability (results not shown). Correlations among measures of cholesterol metabolism Negative genetic correlations were observed between cholesterol turnover or production rate with t:A and t;B (Table 2). The phenotypic correlations among these variables were also negative but lower in value than the genetic relationships. Reasons for genetic correlations exceeding the theoretical range of minus one to plus one have been discussed by Hill and Thompson [30]. Strong genetic correlations were observed for cholesterol mass of pool A with turnover rate and t$A (Table 2). However, the genetic correlations for cholesterol mass of pool A with production rate and t$B were low. Cholesterol turnover rate and cholesterol production rate were closely correlated genetically (rg = 0.98). The phenotypic correlation (rP = 0.44) between these metabolic variables was due chiefly to the genetic contribution (h,r,h, = 0.62), while the environmental component (e,r,e, = -0.18) was small (Fig. 1).
TABLE
2
GENETIC
AND
METABOLISM Variable
correlated
Half-time
a Genetic
BABOoNS
CORRELATIONS
a
AMONG
MEASURES
OF
CHOLESTEROL
(N = 83)
Cholesterol
Cholesterol
Half-time
Half-time
Cholesterol
turnover
production
of first
of second
mass
of
rate
rate
exponential
exponential
pool
A
(t$A)
(t$B)
of second
Cholesterol of pool
PHENOTYPIC IN
mass
A correlations
(*SE)
are above
the diagonal.
Phenotypic
correlations
are below
the diagonal.
409
Phenotype (TO)
rp=o.44
> Phenotype (PI3 Environment
Genetic
A (PR)
062
Contribution
Environmental Phenotypic
Contribution
-0.1 8 044
CorrelOtiOn
Fig. 1. Path analysis representing genetic [h(TO)rgh(pR)l and environmental [e(TO)ree(PR)l contributions to the phenotypic correlation (rp) between cholesterol turnover (TO) and cholesterol production rate (PR). Genetic correlation, environmental correlation. correlation of genotype (defined as the additive genetic effects) and phenotype, and correlation of environment and phenotype are represented by rg, re, h. and e, respectively. The environmental component in this model is a residual term which includes environmental effects and also nonadditive genetic effects.
Correlation of serum cholesterol with measures of cholesterol metabolism Negative genetic correlations vyere observed for serum cholesterol concentration with cholesterol production rate and turnover rate (Table 3). There was a strong positive environmental correlation between serum cholesterol concentration and cholesterol turnover rate (r, = 0.81), but we observed a lower environmental correlation (r, = -0.09) for serum cholesterol with production rate. The low phenotypic correlation (rp = 0.02) between serum cholesterol
TABLE
3
GENETIC, PHENOTYPIC, AND ENVIRONMENTAL RELATIONSHIPS OF SERUM CHOLESTEROL CONCENTRATIONS WITH MEASURES OF CHOLESTEROL METABOLISM IN BABOONS AT YEARS OF AGE Variable
correlated
Correlation with serum cholesterol concentration (mg/dl) Genetic (rg)
Cholesterol turnover rate (mg/kg/day)
-0.56
Cholesterol production rate (mg/kg/day)
4.41
a
Environmental (a
0.81 -0.09
b
Components of the phenotypic correlation Phenotypic
@p)
0.02 -0.24
Genetic
Environmental
(h,rghy) (e,r,ey)
-0.31 -0.20
0.33 -0.04
Half-time of first exponential (t:A)
0.53
-0.13
0.23
0.29
-0.06
Half-time of second exponential (tl B)
0.39
-Q.O8
0.19
0.22
a.03
Mass of pool (mglkg)
0.11
0.57
0.34
0.05
A
a Standard errors of the genetic correlations range from 0.54 to 0.69. b Environmental correlation represents the net effect of environmental
and nonadditive
0.29
genetic
effects.
3
410
= -0.56
Phenotype / (TO)
=0.81 Genetic
-
Contribution
Environmental
U-0)
Phenotypic
Contribution COrrelOtiOn
-0.31 to.33 0.02
Fig. 2. Path analysis representing genetic [h(sc)rgh(TO)l and environmental [a(SC) contributions to the phenotypic correlation (rp) between serum cholesterol concentration (SC) and cholesterol turnover rate (TO). Genetic correlation, environmental correlation, correlation of genotype (defined as the additive genetic effects) and phenotype. and correlations of environment and phenotype are repreThe environmental component in this model is a residual term sented by rg, re, h, and e, respectively. which includes environmental effects and also nonadditive genetic effects.
concentration and turnover rate was due to genetic and environmental contributions of similar magnitude but opposite in sign (Table 3, Fig. 2). Positive genetic correlations were observed for serum cholesterol concentration with t+A and t+B, but the genetic correlation of serum cholesterol with the cholesterol mass of pool A was very low (Table 3). The low phenotypic correlations for serum cholesterol concentration with t$A and t;B were primarily the result of the genetic contributions since the environmental components were near zero. In contrast, the phenotypic correlation of serum cholesterol concentration with cholesterol mass of pool A was determined chiefly by the environmental correlation. Discussion Genetic mediation of serum terol metabolism in baboons
cholesterol
concentration
and measures
of choles-
The sire effects in the present study clearly demonstrate genetic mediation of serum cholesterol concentration and cholesterol metabolism in baboons. The large standard errors of the genetic parameters were expected since these estimates were determined with data from a small number of progeny (N = 83) and sire families (N = 6). However, the number of individuals in this study is large compared to that for other animal or human studies of cholesterol metabolism. The paternal half-sib estimate for the heritability of serum cholesterol concentration is consistent with our previous estimate [16] of realized heritability (h* = 0.41) and with that obtained by the regression of offspring on mid-parent (h* = 0.43). Similar values have been obtained for the heritability of serum cholesterol in other species [4,7,10,31] and in man [32]. In squirrel monkeys [ll], the heritability of serum cholesterol concentration when corrected as described by Patton et al. [7] was 0.46, an estimate similar to our results in baboons.
411
Heritabilities have not been previously reported for measures of cholesterol metabolism. Our results suggest that turnover rate, production rate, t$A, t:B, and cholesterol mass of pool A are moderately to highly heritable. These estimates of heritability and sire effects indicate that genetic factors explain a large part of the variability for serum cholesterol concentration and these measures of cholesterol metabolism in these baboons. Genetic and phenotypic correlations among measures of cholesterol metabolism Cholesterol production rate [ 211 and cholesterol turnover rate [ 201 measure fecal excretion of bile salts and cholesterol and its bacterial metabolites. Production rate also includes steroid losses by skin sloughing [33], steroid hormone production, or degradation of the steroid nucleus by intestinal bacteria. Evidence of degradation of the steroid nucleus by bacteria has not been observed in the baboon (Mott, unpublished observation). Thus, the slightly higher values (Table 1) generally observed for cholesterol production rate compared to cholesterol turnover rate probably are due to relatively small losses from skin sloughing or steroid hormone production. These additional sources of variation which contribute to production rate may explain the somewhat lower heritability of cholesterol production rate (h2 = 0.56) compared to that for cholesterol turnover rate (h2 = 0.71). The phenotypic correlation (rp = 0.44) between cholesterol turnover rate and production rate (Fig. 1) was determined principally by the strong genetic correlation (rg = 0.98) which confirms that turnover rate and production rate measure similar phenomena. However, the negative environmental correlation (r, = -0.49) suggests that there are other sources of variation which affect turnover rate and production rate in opposite ways. In humans, low phenotypic correlations have been observed between serum cholesterol concentration and cholesterol turnover rate, production rate, t$A, and t;B [23,34-361. The phenotypic correlations of serum cholesterol concentration with measures of cholesterol metabolism in our baboons also were low, but the genetic correlations and genetic contributions (hxrphy) to the phenotypic correlations (Table 3) indicate these characters are not genetically independent. In the special case of familial hypercholesterolemia in humans, a single genetic abnormality dominates the physiologic control of serum cholesterol. Thus, the relationships of cholesterol metabolism to serum cholesterol concentration can be clearly identified. For example, Sodhi et al. [35] reported significantly greater cholesterol turnover rate by normal individuals than by a group of subjects selected for hypercholesterolemia. Similarly, Bhattacharyya [37] and Carter [38] reported that the t’,A was significantly longer in familial hypercholesterolemic patients than in normal subjects. These physiologic relationships in humans with familial hypercholesterolemia are in agreement with the genetic relationships we observed in our normocholesterolemic baboons. The low phenotypic correlations between serum cholesterol and cholesterol turnover in normocholesterolemic individuals may result from different primary regulatory mechanisms among individuals or among families. We observed an inverse relationship between serum cholesterol concentration and
412
cholesterol production rate for 5 of the 6 sire progeny groups (Table 1). There also appears to be a similar relationship for serum cholesterol with turnover rate. However, the progeny of one sire (A776) had low serum cholesterol concentration and also low turnover and production rates, a finding that suggests a different regulatory process in that sire group compared to the other sire groups. Environmental
bases of correlation
The environmental contribution to the phenotypic correlation (e,r,e,) includes not only environmental effects but also the non-additive genetic components (dominance and epistasis) [28]. In this instance the environmental effects do not include diet because the environmental correlations were determined after adjusting for the dietary effects. The environmental term may include effects of interactions (epistasis) with other physiological factors. One physiologic effect that influences the relationship between serum cholesterol concentration and cholesterol turnover rate in humans is hypertriglyceridemia. In a review of cholesterol metabolism Sodhi et al. [35] concluded that the cholesterol turnover and production rates of normocholesterolemic subjects were greater than those of hypercholesterolemic subjects, but cholesterol turnover and production rates of combined hypercholesterolemic-hypertriglyceridemic subjects were greater than those of both normo- and hypercholesterolemic subjects. However, this apparent difference in cholesterol turnover may be due to methodologic discrepancies which occur in hypertriglyceridemic individuals [ 391. Individual variation in cholesterol absorption may obscure the genetic relationships of serum cholesterol concentration with measures of cholesterol metabolism. Turnover rate, production rate, and serum cholesterol concentrations of baboons fed a high cholesterol diet are greater than those of animals fed a low cholesterol diet (Mott, unpublished observations). The data in the present report are adjusted for diet effects, but individual differences in cholesterol absorption may cause serum cholesterol and cholesterol turnover or production rates to increase or decrease together in the same manner as for effects of dietary cholesterol. However, an effect of cholesterol absorption on production rate may be masked by cholesterol losses due to skin sloughing or steroid hormone production which contribute only to production rate. This rationale, although speculative, may explain the difference in magnitude (Table 3) between the environmental correlation of serum cholesterol with turnover rate (r, = 0.81) and that with production rate (r, = -0.09). In this group of 83 baboons, the large individual variability and low repeatibility of cholesterol absorption [40] and serum triglyceride [41] concentrations prevented detection of differences of these variables among sire groups. Thus, additional measures are required to assess accurately the effects of individual variability in cholesterol absorption or serum triglyceride concentration on the relationships between serum cholesterol concentration and measures of cholesterol metabolism. Relevance
to human cholesterol
metabolism
Although the precise genetic relationships among various metabolic param-
413
eters and serum cholesterol concentration will vary among species, there are several reasons to believe that these results in baboons provide insight into genetic control of cholesterol metabolism in humans with moderate serum cholesterol concentrations. Cholesterol metabolism and response to dietary cholesterol in baboons are similar to those in man [42]. The heritability of serum cholesterol is similar in the two species and low phenotypic relationships of metabolic variables with serum cholesterol concentration have been observed in both species. Therefore, the use of similar genetic and path analytic techniques in human populations would likely identify similar relationships between serum cholesterol and cholesterol metabolism. Acknowledgements This work was performed with the technical assistance of Cynthia Mersinger Farley, Evelyn Jackson, Don Smith, Merle Taylor, Lisa Simmons, Charles Jimenez, and Steve Ingram. Dr. Douglas A. Eggen, Louisiana State University Medical Center, New Orleans, provided the computer program used to fit data to a 2-pool model of cholesterol metabolism. Drs. T.C. Cartwright, H.C. McGill, Jr., and H.S. Wigodsky offered helpful suggestions during preparation of the manuscript. References 1 Elston, R.C., Namboodiri. K.K., Glueck, C.J., Fallat, R.. Tsang, R. and Leuba, V.. Study of the genetic transmission of hypercholesterolemia and hypertriglyceridemia in a 195 member kindred, Ann. Hum. Genet., 39 (1975) 67. 2 Goldstein, J.L.. Genetic aspects of hyperlipidemia in coronary heart disease. Hosp. Pratt.. 8 (1973) 53. 3 Goldstein, J.L., Hazzard, W.R., Schrott, H.G., Bierman. E.L. and Motulsky. A.G.. Hyperlipidemia in coronary heart disease, Part 2 (Genetic analysis of lipid levels in 176 families and delineation of a new inherited disorder. combined hyperlipidemia). J. Clin. Invest.. 52 (1973) 1544. levels, 4 Dunnington. E.A., White, J.M. and Vinson, W.E., Genetic parameters of serum cholesterol activity and growth in mice, Genetics. 85 (1977) 659. 5 Estep, G.D.. Fanguy, R.C. and Ferguson. T.M., The effect of age and heredity upon serum cholesterol levels in chickens, Poultry Sci., 48 (1969) 1908. 6 Hollands. K.G., Grunder, A.A. and Williams, C.J., Response to five generations of selection for blood cholesterol levels in white leghorns, Poultry Sci., 59 (1980) 1316. 7 Patton, N.M., Brown. R.V. and Middleton. C.C., Familial cholesterolemia in pigeons, Atherosclerosis, 19 (1974) 307. 8 Rothschild, M.F. and Chapman, A.B., Factors influencing serum cholesterol levels in swine. J. Hered.. 67 (1976) 47. 9 Stufflebean, C.E. and Lasley, J.F., Hereditary basis of serum cholesterol level in beef cattle, J. Hered.. 60 (1969) 15. 10 Wilcox, F.H., Cherms. F.L., VanVleck, L.D., Harvey, W.R. and Shaffner, C.S., Estimates of genetic parameters of serum cholesterol level. Poultry Sci., 42 (1963) 37. 11 Clarkson, T.B., Lofland, H.B., Bullock, B.C. and Goodman, H.O., Genetic control of plasma cholesterol - Studies on squirrel monkeys, Arch. Path., 92 (1971) 37. 12 Morris, M.D., Rudel. L.L.. Walls, R.C. and Clarkson. T.B.. The effects of dietary cholesterol on plasma cholesterol and lipoproteins in progeny of rhesus monkeys with naturally occurring hypercholesterolemia. Circulation, Suppl. II. 52 (1975) II-1 5. 13 Goldstein, J.L. and Brown, MS., Hyperlipidemia in coronary heart disease - A biochemical genetic approach, J. Lab. Clin. Med., 85 (1975) 15. 14 Stanbury, J.B.. Wyngaarden. J.B. and Fredrickson, D.S. (Eds.), The Metabolic Basis of Inherited Disease, McGraw-Hill, New York, NY, 1978, P. 544. 15 Mott. G.E., McMahan, C.A. and McGill, H.C., Diet and sire effects on serum cholesterol and cholesterol absorption in infant baboons (Papio cynocephalus). Circ. Res.. 43 (1978) 364.
414 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
33
34 35 36 37 38
39
40 41
42
Flow. B.L., Cartwright. T.C.. Kuehl, T.J., Mott. G.E., Kraemer. D.C.. Kruski, A.W., Williams, J.D. and McGill, H.C.. Genetic effects on serum cholesterol concentrations in baboons, J. Hered.. 72 (1981) 97. Abell, L.L., Levy, B.B., Brodie, B.B. and Kendall, F.E.. A simplified method for the estimation of total cholesterol in serum and demonstration of its specificity, J. Biol. Chem., 195 (1952) 357. Searcy, R.L. and Bergquist. L.M.. A new color reaction for the quantitation of serum cholesterol, Clin. Chim. Acta. 5 (1960) 192. Borgstrom. B., Quantification of cholesterol absorption in man by fecal analysis after the feeding of a single isotope-labeled meal, J. Lipid Res.. 10 (1969) 331. Grundy, S.M. and Ahrens, E.H., Measurements of cholesterol turnover, synthesis, and absorption in man. carried out by isotope kinetic and sterol balance methods. J. Lipid Res., 10 (1969) 91. Goodman, D.S. and Noble, R.P., Turnover of plasma cholesterol in man. J. Clin. Invest.. 47 (1968) 231. Gurpide, E.. Mann. J. and Sandberg, E.. Determination of kinetic parameters in a two-pool system by administration of one or more tracers. Biochemistry, 3 (1964) 1250. Nest& P.J.. Whyte. H.M. and Goodman, D.S.. Distribution and turnover of cholesterol in humans, J. Clin. Invest.. 48 (1969) 982. Goodman, D.S., Noble, R.P. and Dell, R.B.. Three-pool model of the long-term turnover of plasma cholesterol in man, J. Lipid Res., 14 (1973) 178. Becker, W.A., Manual of Procedures in Quantitative Genetics, Washington State University Press, Pullman, WA, 1967. Harvey, W.R.. Least squares analysis of data with unequal subclass numbers, U.S. A&c. Res. Serv. Publ. ARS H-4.1975. Harvey, W.R., User’s guide for LSML 76 mixed model least squares and maximum likelihood computer program, Ohio State University, Columbus, OH, 1977. F&zoner. D.S., Introduction to Quantitative Genetics, Ronald Press Co., New York, NY, 1960. Nie, N.H.. Hull, C.H., Jenkins, J.G., Steinbrenner, K. and Bent. D.H.. Statistical Package for the Social Sciences. 2nd edition, McGraw-Hill, New York, NY. 1970. Hill, W.G. and Thompson, R.. Probabilities of non-positive definite between-group or genetic covariante matrices, Biometrics. 34 (1978) 429. Roberts, D.C.K., West, C.E., Redgrave, T.G. and Smith, J.B., Plasma cholesterol concentration in normal and cholesterol-fed rabbits, Atherosclerosis. 19 (1974) 369. Sergeev, AS., Luchkina. E.M., Lunga. I.N., Mazovetskii, A.G., Koshechkin. V.A. and Chepurnenko, N.V.. Genetic analysis of the structure of relations between some physiological traits, Part 2 (Analysis of characteristics of glucose tolerance and plasma cholesterol levels), Genetika. 16 (1980) 908. Bhattacharyya. A.K.. Connor, W.E. and Spector, A.A., Excretion of sterols from the skin of normal and hypercholesterolemic humans - Implications for sterol balance studies. J. Clin. Invest., 51 (1972) 2060. Goodman, D.S., Smith. F.R., Seplowitz. A.H.. Ramakrishnan, R. and Dell, R.B., Prediction of the parameters of whole body cholesterol metabolism in humans, J. Lipid Res., 21 (1980) 699. Sodhi, H.S., Kudchodkar, B.J. and Mason, D.T., Cholesterol metabolism in clinical hyperlipidemias. Adv. Lipid Res., 17 (1980) 107. Ho, K.J., Biss, K. and Taylor, C.B.. Serum cholesterol levels in US males, Arch. Path., 97 (1974) 306. Bhattacharyya. A.K., Connor, W.E. and Spector, A.A.. Abnormalities of cholesterol turnover in hypercholesterolemic (Type II) patients, J. Lab. Clin. Med., 88 (1976) 202. Carter, G.A.. Connor, W.E.. Bhattacharyya. A.K. and Lin, D.S.. The cholesterol turnover, synthesis. and absorption in two sisters with familial hypercholesterolemia (type IIa), J. Lipid Res., 20 (1979) 66. Davidson, N.O., Samuel, P., Lieberman. S.. Shane, S.P., Grouse. J.R. and Ahrens, E.H., Measurement of bile acid production in hyperlipidemic man - Does phenotype or methodology make the difference?, J. Lipid Res., 22 (1981) 620. Mott. G.E., Jackson, E.M. and Morris, M.D., Cholesterol absorption in baboons, J. Lipid Res., 21 (1980) 635. Flow, B.L., Genetic Mediation of Body Weight, Serum Cholesterol Concentration, Cholesterol Absorption, and Lipoprotein Cholesterol Concentration in the Baboon (Z’apio c~nocephalus). Ph.D. dissertation. Texas A & M University, May. 1980. Eggen, D.A., Cholesterol metabolism in rhesus monkey, squirrel monkey, and baboon. J. Lipid Res., 15 (1974) 139.
403 Publishers,
Ltd.
GENETIC MEDIATION OF CHOLESTEROL METABOLISM IN THE BABOON (PAPZO C YNOCEPHA L US)
BRYAN
L. FLOW
and GLEN
E. MOTT
Department of Pathology, The and the Southwest Foundation (U.S.A.) (Received (Revised, (Accepted
University of Texas Health Science Center for Research and Education, San Antonio,
at San Antonio TX 78284
29 May, 1981) received 20 July, 1981) 20 July, 1981)
Summary Genetic effects on serum cholesterol concentration and measures of cholesterol metabolism were investigated in 83 normocholesterolemic juvenile baboons (Pupio cynocephalus), the progeny of 6 sires. Sire effects (P < 0.05) were observed for serum cholesterol concentration, cholesterol turnover rate, cholesterol production rate, and several cholesterol pool parameters derived from a two-pool model. The estimates of heritability (h’) for the half-time of the first (t’,A) and second exponentials (t:B) derived from serum [ 14C]cholesterol decay curves and the cholesterol turnover rate were 0.67, 0.73 and 0.71, respectively. These heritabilities and those for serum cholesterol concentration (h* = 0.44) and cholesterol production rate (h* = 0.56) indicate these characters are moderately to highly heritable. Serum cholesterol concentration was correlated genetically with cholesterol turnover rate (rg = -0.56), production rate (rg = -0.41), t$A (l”g= 0.53), and t+B (rg = 0.39). Correlations among observed values, or phenotypic correlations, were low. Path analyses revealed that the low phenotypic correlation (rp = 0.02) between serum cholesterol concentration and cholesterol turnover rate was due to genetic and environmental contributions of similar magnitude but opposite in sign. The low phenotypic correlations of serum cholesterol with cholesterol production rate, t$A, and t$B (r, = -0.24, 0.23, and 0.19, respectively) were due primarily to the genetic contribution since environmental contributions were near zero. This research was suppoked by grants HG15914 and HL-19362 from the National Heart. Lung and Blood Institute. Address for correspondence and reprints: Glen E. Mott, Ph.D., Department of Pathology, The University of Texas Health Science Center, San Antonio, TX 78284, U.S.A.
OOZl-9150/82/0000+I000/$02.75@1982
Elsevier/North-Holland
Scientific
Publishers,
Ltd.
404
Application of similar genetic and path analytic techniques to human populations should enhance our understanding of the low phenotypic relationships previously observed between serum cholesterol and parameters of cholesterol metabolism.
Key words:
Baboon - Cholesterol terol metabolism
turnover
- Genetics
-. Heredity
- Regulation
of choles-
Introduction Genetic mediation of serum cholesterol concentration has been demonstrated in man [l-3] and in a number of animal species [4-101 including nonhuman primates [ 11,121. A few well defined genetic defects in cholesterol metabolism have been identified [13,14] but the mechanisms of regulation of serum cholesterol concentration in most individuals are not well understood. A significant sire effect on serum cholesterol concentration [ 151 and development of lines of progeny with high and low serum cholesterol concentrations [16] indicate that serum cholesterol concentration in baboons (Papio sp.) also is mediated genetically. This report describes previously unidentified genetic and environmental relationships between serum cholesterol and parameters of cholesterol metabolism in baboons. Materials and Methods Selection of adult breeders Adult female baboons were randomly were of East African origin and most of selected for their reproductive potential. associated characteristics were not criteria
assigned to 7 sires [ 151. All breeders them were feral. Sires and dams were Serum cholesterol concentration and for consideration.
Diets We assigned each infant at birth to one of three infant formulas or to breast feeding [15]. The effects of infant diet, sex, and sire on infant serum cholesterol concentration have been described previously [ 151. At 14 weeks of age, infants were weaned and each was maintained on one of four adult diets. These diets were formulated with Special Monkey Chow 25 (No. 5045-6), Ralston-Purina Co., St. Louis, MO, to which saturated (P/S = 0.26) or unsaturated (P/S = 2.1) plant oils were added with either 1.0 mg or 0.01 mg cholesterol/kcal. The effects of infant and adult diets and sex on progeny serum cholesterol concentration and variables of cholesterol metabolism during the juvenile period will be presented in a separate report. Serum cholesterol concentrations of the adult breeders were measured 3 times at monthly intervals while they were fed a chow-based diet which contained 20% saturated fat and 1 mg cholesterol/kcal.
405
Serum
lipid analyses
Serum was saponified and extracted with petroleum ether [17], and cholesterol measured by the color development method of Searcy and Bergquist [18]. The procedure met the criteria of the Center for Disease Control Lipid Standardization Program. The sire or dam serum cholesterol value was the mean of the 3 analyses made while the animals were fed the diet described for the adult breeders. The midparent value was the mean of the sire and dam values. Serum cholesterol concentration of the progeny was the mean of measurements at 39, 42, and 45 months of age. The value at each age was the mean of blind duplicate determinations. The coefficient of variation of the duplicate measurements was approximately 5%. Analyses
of cholesterol
metabolism
At 3.5 years of age, cholesterol absorption was measured by the method of Borgstrom [19] and cholesterol turnover rate by the method (equations 4, 5, and 7) described by Grundy and Ahrens [20]. A number of parameters of cholesterol metabolism were estimated from the specific radioactivity decay curves for serum [ 14C]cholesterol. The radioactive dose was prepared by adding 35 PCi of [4-14C]cholestero1 (Sigma Chemical Co., St. Louis, MO) dissolved in 100 1-11 of acetone to 3 ml of sterile autologous serum and mixing slowly with a stirring bar for 20 h. The amount of radioactivity injected intravenously was determined as the difference between the radioactivity added to the syringe and the residual radioactivity in the syringe after injection. The serum cholesterol specific radioactivity was measured 25 times over a 4-month period. Cholesterol production rate, half-time of the first exponential (t%A), half-time of the second exponential (t+B) and cholesterol mass of pool A were estimated by fitting 2 exponential functions to the serum [ 14C]cholesterol-specific radioactivity curve [21,22]. A minimum estimate of the cholesterol mass of pool B also was calculated with the assumption that there was no cholesterol synthesis in pool B [23]. The specific radioactivity data were also fitted to a 3-pool model [ 241 by a similar procedure, but in many animals the 3-pool model did not provide a better fit of the data than the 2-pool model. Therefore, only the results of the 2-pool fit are presented. Effects
of growth
on cholesterol
metabolism
The mean body weight of animals in this study was 10.5 kg. Among sire progeny groups there were no differences for body weight or body weight change during the metabolism studies. The animals were growing at an average rate of 812 mg/kg body weight/day with a range among sire progeny groups of 582-948 mg/kg body weight/day or less than 0.1% of the body weight. An average cholesterol content of baboon tissue of 1.03 mg/g from a previous experiment (unpublished observations) was used to estimate that only 0.84 mg cholesterol/kg body weight/day would be incorporated into new tissue with a maximum difference among sire progeny groups of 0.38 mg cholesterol/kg body weight/day. This represents a small percentage of the average cholesterol turnover rates of 28.5-36.7 mg/kg body weight/day observed in this study. Thus we conclude that the sire effects and genetic parameters are not due to the effects of differences in growth rate on cholesterol metabolism.
406
Gene tic analyses Heritabilities (h’) and genetic (rg), phenotypic (rP), and environmental (r,) correlations were estimated by procedures described for a paternal half-sib design [25]. The analyses of variance and the analyses of variance of cross products were performed by least squares procedures [26,27]. The linear model included the effects of infant diet, dietary cholesterol, dietary fat, sex, sire, and the two-factor interactions of infant diet by dietary cholesterol and dietary cholesterol by dietary fat. We included in the model only those twofactor interactions which were significant for one or more of the dependent variables. All effects were considered fixed except sires which were assumed to be random. Data for 83 progeny of 6 sires were log-transformed to better meet the assumptions of the model. The 7th sire was excluded from the genetic analyses since he produced only 3 progeny. Heritability of serum cholesterol concentration also was estimated by the regression of progeny values on the mid-parent value [28]. Since serum lipid concentrations in juvenile animals were compared with those in adult animals, heritability was estimated by standardized [16,29] partial regression coefficients for the regression of offspring values on mid-parent values. The model included the mid-parent value and the effects of infant diet, adult dietary cholesterol, adult dietary fat, sex, and all the two-factor interactions. Path analysis The phenotypic correlation, or the correlation of observed values, represents the net effect of genetic and environmental factors. The genetic (hxrghy) and environmental (e&e,) contributions to phenotypic correlation (rP) were expressed as: rP = hxrghy + exreey where h, and h, represent the correlations between additive genetic effects and phenotype for characters x and y; rg represents the genetic correlation between x and y; e, and e, represent the correlations between environmental (including non-additive genetic) effects and phenotype, and r, represents the correlation among environmental (including non-additive genetic) effects [ 281. Assumptions of this statistical model include the absence of interactions and correlations among the genotypes and environments. The correlation (h) between additive genetic effects and phenotype was determined by taking the square root of the paternal half-sib estimate of heritability (h’) of the character. Similarly, we estimated the correlation (e) between environmental effects and phenotype of the character by d/1- h2. Results Genetic mediation of cholesterol metabolism Differences (P < 0.05) were observed among sire progeny groups for serum cholesterol concentration, cholesterol turnover rate, cholesterol production rate, t;A, t;B, and mass of pool A (Table 1). There were no significant differences (P > 0.10) among sire progeny groups for the percent of dietary cholesterol absorbed or for the minimum estimate of the cholesterol mass of pool B.
squares
means.
16
A776
groups
8
A956
b Least
17
A947
a Sire progeny
19
A943
15
8
a
A982
A772
gr0UP
of
Values
141
in parenthesis
in decreasing
(100-122)
111
(104-138)
120
(110-132)
120
(110-132)
121
(110-145)
126
(127-156)b
order are 95%
28.5
cholesterol
b
intervals.
serum
confidence
of their
(27.4-33.4)
30.2
(31.642.1)
36.4
(31.1-37.7)
34.2
(33.540.1)
36.7
(29.3-39.1)
33.8
(25.7-31.6)
concentrations.
(32.0-35.7)
33.8
(34.940.8)
37.8
(35.4-39.3)
37.3
(33.8-37.2)
35.5
(32.0-37.4)
34.6
(31.1-34.8)
32.9
(mg/kg/daY)
Production
b
rate
3.51
(2.89-3.52)
3.19
(2.63-3.50)
3.04
(2.93-3.54)
3.22
(2.45-2.93)
2.68
(2.65-3.52)
3.06
(3.16-3.89)
b
23.9
(22.4-24.6)
23.5
(20.0-22.9)
21.4
(20.8-22.8)
21.8
(20.7-22.5)
21.6
(22.2-25.4)
23.8
(22.7-25.1)
(days)
t+B
Turnover (mg/kg/daY)
SelUm
cholesterol
rate
exponential,
GROUP
tf A (days)
PROGENY
exponential,
SIRE
(meld0
BY
concentration
METABOLISM Half-time
CHOLESTEROL
of second
OF
first
MEASURES Half-time
AND
of
CONCENTRATION
are ranked
Number
progeny
Sire
CHOLESTEROL
SERUM
progeny
1
TABLE
b
Mass
of A
(308-359)
332
(295-367)
329
(322-373)
347
(279-320)
299
(265-330)
296
(320-375)
347
(mg/kg)
pool
b
408
Sire progeny group means for the percent of dietary cholesterol absorbed ranged from 42 to 45% and those for cholesterol mass of pool B ranged from 386 to 427 mg/kg. Heritability of serum cholesterol determined from the standardized regression of offspring value on mid-parent mean was 0.43 and the paternal half-sib estimate was 0.44 (SE = 0.43). These heritabilities and that for cholesterol turnover rate (h’ = 0.71, SE = 0.53), production rate (h2 = 0.56, SE = 0.47), t;A (h2 = 0.67, SE = 0.52), t;B (h2 = 0.73, SE = 0.54) and cholesterol mass of pool A (h2 = 0.55, SE = 0.48) suggest the heritabilities of these characters are in the moderate to high range. Heritability of cholesterol absorption or mass of pool B could not be estimated because the differences observed among sire progeny groups were small relative to within group variability (results not shown). Correlations among measures of cholesterol metabolism Negative genetic correlations were observed between cholesterol turnover or production rate with t:A and t;B (Table 2). The phenotypic correlations among these variables were also negative but lower in value than the genetic relationships. Reasons for genetic correlations exceeding the theoretical range of minus one to plus one have been discussed by Hill and Thompson [30]. Strong genetic correlations were observed for cholesterol mass of pool A with turnover rate and t$A (Table 2). However, the genetic correlations for cholesterol mass of pool A with production rate and t$B were low. Cholesterol turnover rate and cholesterol production rate were closely correlated genetically (rg = 0.98). The phenotypic correlation (rP = 0.44) between these metabolic variables was due chiefly to the genetic contribution (h,r,h, = 0.62), while the environmental component (e,r,e, = -0.18) was small (Fig. 1).
TABLE
2
GENETIC
AND
METABOLISM Variable
correlated
Half-time
a Genetic
BABOoNS
CORRELATIONS
a
AMONG
MEASURES
OF
CHOLESTEROL
(N = 83)
Cholesterol
Cholesterol
Half-time
Half-time
Cholesterol
turnover
production
of first
of second
mass
of
rate
rate
exponential
exponential
pool
A
(t$A)
(t$B)
of second
Cholesterol of pool
PHENOTYPIC IN
mass
A correlations
(*SE)
are above
the diagonal.
Phenotypic
correlations
are below
the diagonal.
409
Phenotype (TO)
rp=o.44
> Phenotype (PI3 Environment
Genetic
A (PR)
062
Contribution
Environmental Phenotypic
Contribution
-0.1 8 044
CorrelOtiOn
Fig. 1. Path analysis representing genetic [h(TO)rgh(pR)l and environmental [e(TO)ree(PR)l contributions to the phenotypic correlation (rp) between cholesterol turnover (TO) and cholesterol production rate (PR). Genetic correlation, environmental correlation. correlation of genotype (defined as the additive genetic effects) and phenotype, and correlation of environment and phenotype are represented by rg, re, h. and e, respectively. The environmental component in this model is a residual term which includes environmental effects and also nonadditive genetic effects.
Correlation of serum cholesterol with measures of cholesterol metabolism Negative genetic correlations vyere observed for serum cholesterol concentration with cholesterol production rate and turnover rate (Table 3). There was a strong positive environmental correlation between serum cholesterol concentration and cholesterol turnover rate (r, = 0.81), but we observed a lower environmental correlation (r, = -0.09) for serum cholesterol with production rate. The low phenotypic correlation (rp = 0.02) between serum cholesterol
TABLE
3
GENETIC, PHENOTYPIC, AND ENVIRONMENTAL RELATIONSHIPS OF SERUM CHOLESTEROL CONCENTRATIONS WITH MEASURES OF CHOLESTEROL METABOLISM IN BABOONS AT YEARS OF AGE Variable
correlated
Correlation with serum cholesterol concentration (mg/dl) Genetic (rg)
Cholesterol turnover rate (mg/kg/day)
-0.56
Cholesterol production rate (mg/kg/day)
4.41
a
Environmental (a
0.81 -0.09
b
Components of the phenotypic correlation Phenotypic
@p)
0.02 -0.24
Genetic
Environmental
(h,rghy) (e,r,ey)
-0.31 -0.20
0.33 -0.04
Half-time of first exponential (t:A)
0.53
-0.13
0.23
0.29
-0.06
Half-time of second exponential (tl B)
0.39
-Q.O8
0.19
0.22
a.03
Mass of pool (mglkg)
0.11
0.57
0.34
0.05
A
a Standard errors of the genetic correlations range from 0.54 to 0.69. b Environmental correlation represents the net effect of environmental
and nonadditive
0.29
genetic
effects.
3
410
= -0.56
Phenotype / (TO)
=0.81 Genetic
-
Contribution
Environmental
U-0)
Phenotypic
Contribution COrrelOtiOn
-0.31 to.33 0.02
Fig. 2. Path analysis representing genetic [h(sc)rgh(TO)l and environmental [a(SC) contributions to the phenotypic correlation (rp) between serum cholesterol concentration (SC) and cholesterol turnover rate (TO). Genetic correlation, environmental correlation, correlation of genotype (defined as the additive genetic effects) and phenotype. and correlations of environment and phenotype are repreThe environmental component in this model is a residual term sented by rg, re, h, and e, respectively. which includes environmental effects and also nonadditive genetic effects.
concentration and turnover rate was due to genetic and environmental contributions of similar magnitude but opposite in sign (Table 3, Fig. 2). Positive genetic correlations were observed for serum cholesterol concentration with t+A and t+B, but the genetic correlation of serum cholesterol with the cholesterol mass of pool A was very low (Table 3). The low phenotypic correlations for serum cholesterol concentration with t$A and t;B were primarily the result of the genetic contributions since the environmental components were near zero. In contrast, the phenotypic correlation of serum cholesterol concentration with cholesterol mass of pool A was determined chiefly by the environmental correlation. Discussion Genetic mediation of serum terol metabolism in baboons
cholesterol
concentration
and measures
of choles-
The sire effects in the present study clearly demonstrate genetic mediation of serum cholesterol concentration and cholesterol metabolism in baboons. The large standard errors of the genetic parameters were expected since these estimates were determined with data from a small number of progeny (N = 83) and sire families (N = 6). However, the number of individuals in this study is large compared to that for other animal or human studies of cholesterol metabolism. The paternal half-sib estimate for the heritability of serum cholesterol concentration is consistent with our previous estimate [16] of realized heritability (h* = 0.41) and with that obtained by the regression of offspring on mid-parent (h* = 0.43). Similar values have been obtained for the heritability of serum cholesterol in other species [4,7,10,31] and in man [32]. In squirrel monkeys [ll], the heritability of serum cholesterol concentration when corrected as described by Patton et al. [7] was 0.46, an estimate similar to our results in baboons.
411
Heritabilities have not been previously reported for measures of cholesterol metabolism. Our results suggest that turnover rate, production rate, t$A, t:B, and cholesterol mass of pool A are moderately to highly heritable. These estimates of heritability and sire effects indicate that genetic factors explain a large part of the variability for serum cholesterol concentration and these measures of cholesterol metabolism in these baboons. Genetic and phenotypic correlations among measures of cholesterol metabolism Cholesterol production rate [ 211 and cholesterol turnover rate [ 201 measure fecal excretion of bile salts and cholesterol and its bacterial metabolites. Production rate also includes steroid losses by skin sloughing [33], steroid hormone production, or degradation of the steroid nucleus by intestinal bacteria. Evidence of degradation of the steroid nucleus by bacteria has not been observed in the baboon (Mott, unpublished observation). Thus, the slightly higher values (Table 1) generally observed for cholesterol production rate compared to cholesterol turnover rate probably are due to relatively small losses from skin sloughing or steroid hormone production. These additional sources of variation which contribute to production rate may explain the somewhat lower heritability of cholesterol production rate (h2 = 0.56) compared to that for cholesterol turnover rate (h2 = 0.71). The phenotypic correlation (rp = 0.44) between cholesterol turnover rate and production rate (Fig. 1) was determined principally by the strong genetic correlation (rg = 0.98) which confirms that turnover rate and production rate measure similar phenomena. However, the negative environmental correlation (r, = -0.49) suggests that there are other sources of variation which affect turnover rate and production rate in opposite ways. In humans, low phenotypic correlations have been observed between serum cholesterol concentration and cholesterol turnover rate, production rate, t$A, and t;B [23,34-361. The phenotypic correlations of serum cholesterol concentration with measures of cholesterol metabolism in our baboons also were low, but the genetic correlations and genetic contributions (hxrphy) to the phenotypic correlations (Table 3) indicate these characters are not genetically independent. In the special case of familial hypercholesterolemia in humans, a single genetic abnormality dominates the physiologic control of serum cholesterol. Thus, the relationships of cholesterol metabolism to serum cholesterol concentration can be clearly identified. For example, Sodhi et al. [35] reported significantly greater cholesterol turnover rate by normal individuals than by a group of subjects selected for hypercholesterolemia. Similarly, Bhattacharyya [37] and Carter [38] reported that the t’,A was significantly longer in familial hypercholesterolemic patients than in normal subjects. These physiologic relationships in humans with familial hypercholesterolemia are in agreement with the genetic relationships we observed in our normocholesterolemic baboons. The low phenotypic correlations between serum cholesterol and cholesterol turnover in normocholesterolemic individuals may result from different primary regulatory mechanisms among individuals or among families. We observed an inverse relationship between serum cholesterol concentration and
412
cholesterol production rate for 5 of the 6 sire progeny groups (Table 1). There also appears to be a similar relationship for serum cholesterol with turnover rate. However, the progeny of one sire (A776) had low serum cholesterol concentration and also low turnover and production rates, a finding that suggests a different regulatory process in that sire group compared to the other sire groups. Environmental
bases of correlation
The environmental contribution to the phenotypic correlation (e,r,e,) includes not only environmental effects but also the non-additive genetic components (dominance and epistasis) [28]. In this instance the environmental effects do not include diet because the environmental correlations were determined after adjusting for the dietary effects. The environmental term may include effects of interactions (epistasis) with other physiological factors. One physiologic effect that influences the relationship between serum cholesterol concentration and cholesterol turnover rate in humans is hypertriglyceridemia. In a review of cholesterol metabolism Sodhi et al. [35] concluded that the cholesterol turnover and production rates of normocholesterolemic subjects were greater than those of hypercholesterolemic subjects, but cholesterol turnover and production rates of combined hypercholesterolemic-hypertriglyceridemic subjects were greater than those of both normo- and hypercholesterolemic subjects. However, this apparent difference in cholesterol turnover may be due to methodologic discrepancies which occur in hypertriglyceridemic individuals [ 391. Individual variation in cholesterol absorption may obscure the genetic relationships of serum cholesterol concentration with measures of cholesterol metabolism. Turnover rate, production rate, and serum cholesterol concentrations of baboons fed a high cholesterol diet are greater than those of animals fed a low cholesterol diet (Mott, unpublished observations). The data in the present report are adjusted for diet effects, but individual differences in cholesterol absorption may cause serum cholesterol and cholesterol turnover or production rates to increase or decrease together in the same manner as for effects of dietary cholesterol. However, an effect of cholesterol absorption on production rate may be masked by cholesterol losses due to skin sloughing or steroid hormone production which contribute only to production rate. This rationale, although speculative, may explain the difference in magnitude (Table 3) between the environmental correlation of serum cholesterol with turnover rate (r, = 0.81) and that with production rate (r, = -0.09). In this group of 83 baboons, the large individual variability and low repeatibility of cholesterol absorption [40] and serum triglyceride [41] concentrations prevented detection of differences of these variables among sire groups. Thus, additional measures are required to assess accurately the effects of individual variability in cholesterol absorption or serum triglyceride concentration on the relationships between serum cholesterol concentration and measures of cholesterol metabolism. Relevance
to human cholesterol
metabolism
Although the precise genetic relationships among various metabolic param-
413
eters and serum cholesterol concentration will vary among species, there are several reasons to believe that these results in baboons provide insight into genetic control of cholesterol metabolism in humans with moderate serum cholesterol concentrations. Cholesterol metabolism and response to dietary cholesterol in baboons are similar to those in man [42]. The heritability of serum cholesterol is similar in the two species and low phenotypic relationships of metabolic variables with serum cholesterol concentration have been observed in both species. Therefore, the use of similar genetic and path analytic techniques in human populations would likely identify similar relationships between serum cholesterol and cholesterol metabolism. Acknowledgements This work was performed with the technical assistance of Cynthia Mersinger Farley, Evelyn Jackson, Don Smith, Merle Taylor, Lisa Simmons, Charles Jimenez, and Steve Ingram. Dr. Douglas A. Eggen, Louisiana State University Medical Center, New Orleans, provided the computer program used to fit data to a 2-pool model of cholesterol metabolism. Drs. T.C. Cartwright, H.C. McGill, Jr., and H.S. Wigodsky offered helpful suggestions during preparation of the manuscript. References 1 Elston, R.C., Namboodiri. K.K., Glueck, C.J., Fallat, R.. Tsang, R. and Leuba, V.. Study of the genetic transmission of hypercholesterolemia and hypertriglyceridemia in a 195 member kindred, Ann. Hum. Genet., 39 (1975) 67. 2 Goldstein, J.L.. Genetic aspects of hyperlipidemia in coronary heart disease. Hosp. Pratt.. 8 (1973) 53. 3 Goldstein, J.L., Hazzard, W.R., Schrott, H.G., Bierman. E.L. and Motulsky. A.G.. Hyperlipidemia in coronary heart disease, Part 2 (Genetic analysis of lipid levels in 176 families and delineation of a new inherited disorder. combined hyperlipidemia). J. Clin. Invest.. 52 (1973) 1544. levels, 4 Dunnington. E.A., White, J.M. and Vinson, W.E., Genetic parameters of serum cholesterol activity and growth in mice, Genetics. 85 (1977) 659. 5 Estep, G.D.. Fanguy, R.C. and Ferguson. T.M., The effect of age and heredity upon serum cholesterol levels in chickens, Poultry Sci., 48 (1969) 1908. 6 Hollands. K.G., Grunder, A.A. and Williams, C.J., Response to five generations of selection for blood cholesterol levels in white leghorns, Poultry Sci., 59 (1980) 1316. 7 Patton, N.M., Brown. R.V. and Middleton. C.C., Familial cholesterolemia in pigeons, Atherosclerosis, 19 (1974) 307. 8 Rothschild, M.F. and Chapman, A.B., Factors influencing serum cholesterol levels in swine. J. Hered.. 67 (1976) 47. 9 Stufflebean, C.E. and Lasley, J.F., Hereditary basis of serum cholesterol level in beef cattle, J. Hered.. 60 (1969) 15. 10 Wilcox, F.H., Cherms. F.L., VanVleck, L.D., Harvey, W.R. and Shaffner, C.S., Estimates of genetic parameters of serum cholesterol level. Poultry Sci., 42 (1963) 37. 11 Clarkson, T.B., Lofland, H.B., Bullock, B.C. and Goodman, H.O., Genetic control of plasma cholesterol - Studies on squirrel monkeys, Arch. Path., 92 (1971) 37. 12 Morris, M.D., Rudel. L.L.. Walls, R.C. and Clarkson. T.B.. The effects of dietary cholesterol on plasma cholesterol and lipoproteins in progeny of rhesus monkeys with naturally occurring hypercholesterolemia. Circulation, Suppl. II. 52 (1975) II-1 5. 13 Goldstein, J.L. and Brown, MS., Hyperlipidemia in coronary heart disease - A biochemical genetic approach, J. Lab. Clin. Med., 85 (1975) 15. 14 Stanbury, J.B.. Wyngaarden. J.B. and Fredrickson, D.S. (Eds.), The Metabolic Basis of Inherited Disease, McGraw-Hill, New York, NY, 1978, P. 544. 15 Mott. G.E., McMahan, C.A. and McGill, H.C., Diet and sire effects on serum cholesterol and cholesterol absorption in infant baboons (Papio cynocephalus). Circ. Res.. 43 (1978) 364.
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