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On the genetics of the Rett syndrome
On the Genetics of the Rett Syndrome Ulrike Hillig, MD Ever since its recognition as a separate disease, the Rett syndrome has been reported to afflict girls only. For this reason, heredity has consistently remained at the forefront of discussions regarding putative causes. On account of the gravity of the disorder and the want for adequate therapy, the clarification of its genesis is of great practical importance in consideration of the possibility of descendants. J Wahlstrom reports a breakpoint at Xp22 in two sisters with the disease and in their healthy mother. This finding is questionable as being typical of the syndrome in light of recent chromosomal findings. In addition to Wahlstrom's two-step-mutation hypothesis, an earlier suggestion of an X-chromosomal dominant new mutation is under discussion again. Further genetic findings are expected from a yet-to-be-completed interpretation of all relevant data hitherto observed in approximately 160 probands and their families. Hillig U. On the genetics of the Rett syndrome. Brain Dev 1985;7:368-71
The Rett syndrome has been recognized as an entity for two decades in Austria (the native country of A Rett, who first described the syndrome), western Europe, particularly in Scandinavia, and, for several years now, also in Japan. Although it can be distinguished from other childhood mental retardations and although it has been researched carefully, its genesis has eluded explanation. It has not yet been possible to assess definitely its etiologyneither in the clinical form nor as it occurs in families. On the one hand there was no specific pathological substrate present, and on the other hand, the underlying defect could not be identified. The statements at the symposium in 1984 in Vienna by Y Nomura concerning the pathophysiology, by P Riederer and W Birkmayer about the biochemistry and by J Wilson regarding the neuropathology of the syndrome raise the possibility that this disease is caused by a neurotransmitter defect. Definite pathological From the Institut fUr Humangenetik und Genetische Poliklinik der Philipps-Universitiit, Marburg.
Key words: Rett syndrome, X-chromosome, fragile site, X-chromosomal dominant mutation, genetic counseling. Correspondence address: Dr. Ulrike Hillig, Institut fUr Humangenetik und Genetische Poliklinik der Philipps-Universitiit Marburg, Bahnhofstrasse 7A, D-3550 Marburg, FRG.
anatomical alterations with depigmentation in the substantia nigra and the nucleus coeruleus were found . Therefore our previous assumption, based on the clinical presentation and course of the disease (viz, the syndrome as a functional defect in children with a primarily normal stature) would stand confirmed. The disease starts with functional defects of the central nervous system manifesting themselves as pathological alterations in speech and motoric behavior, intellectual and psychical performance, and as the disease progresses there are also physical symptoms such as microcephalia, occasionally retarded growth, and so on. The fact that the roughly 160 cases definitely diagnosed up to the present time were all girls, led A Rett at first to think the disease must be genetically determined. Therefore, the question of a genetic cause or at least a hereditary participation in the syndrome has remained compelling ever since its diagnosis became feasible, particularly because there has been no possibility of effective treatment for the involved children. We have remained unable to offer any promising therapy to counteract or alleviate the Rett syndrome. We can only treat the patients' symptoms to a limited extent and help them and their parents bear this cross. At present, the syndrome must be described as intractable to treatment. The afflicted girls are 100% certain to require nursing care. Nevertheless, puberty progresses
(in general) in a normal way, but the patients do not propagate because of their physical and mental weakness and their lack of libido. It also seems doubtful that they would be able to carry a child full term should they in fact become pregnant. The oldest patients suffering from the Rett syndrome are now within the range of 25-27 yrs of age. In view of the serious course of the disease, the parents and close relatives should be informed about the risk of recurrence in descendants (i.e., genetic counseling). Exact genetic advice, however, is not possible as long as there is any question as to a hereditary factor in the Rett syndrome. At first it was generally assumed that this disease could only appear sporadically. Because of Hagberg's observation (1981 in Sweden) of two afflicted siblings who have the same healthy mother, but are of different male parentage, this opinion has been revised, all the more so since a similar observation (in this case the girls had both parents in common) was made by F Hanefeld in West Germany. Chromosome analyses of the patients were of course made with Giemsa and other staining methods. The respective findings were, however, all unrevealing. At the symposium in April 1983 in Vienna it was suggested that prometaphase stainings of patients chromosomes be investigated. Thereby it could be ascertained once and for all whether or not a chromosomal aberration exists submicroscopically which had therefore been imperceptible before, in which case we would then be dealing with a phenomenon comparable to Prader-Willi syndrome which affects the short arm of chromosome No 15. For that reason, the investigations by J Wahlstrom were also necessary in order to track down fragile sites in children with Rett syndrome. In association with this, one must mention the Martin-Bell or fragile-X syndrome, which is characterized by a specific pattern of mental, psychomotoric and speech retardation with relatively discrete physical attributes, and which is limited only to the male sex. The previous result of about 3% fra Xp22 in the afflicted half-sisters with Rett syndrome in the family published by Hagberg are not yet convincing. On the other hand, the reported finding of 10% of homogeneous sites of the fracture (fra Xp22) on an X-chromosome obtained from a fibroblast tissue culture from the
two girls' healthy mother seems to be more significant. Nearly all people have folic-acidsensitive fragile sites on their chromosomes under the conditions of a folate-deficiency cell culture. This has been shown in recent examinations [1, 2]. One can infer from these examinations that fra Xp22 is one of the most frequent fragile sites. Further chromosomal analyses on cell cultures of patients with Rett syndrome and their relatives should, however, be performed to determine whether one can find a specific, reproducible fracture on an X or on another chromosome for this syndrome. The hypothesis of J Wahlstrom of a two-step mutation as the cause for the Rett syndrome is also very interesting and worthy of examination: the idea of first an autosomal recessive (gametic) mutation which in rare cases is assumed to be accompanied by a second, somatic X-chromosomal mutation. In the case of male embryos, the combination of both mutated genes takes on the properties of a lethal factor and probably leads to the prenatal death of the fetus, because boys with Rett syndrome could not previously be detected. With the indication of possible DNA-recombination of the mutated genes, Wahlstrom compares the very rare familial incidence of the syndrome with the occurrence of a retinoblastoma or a neuroblastoma. Both of these diseases appear independently of the carrier's sex and are combined with a proven structural chromosomal aberration as well as with tumor development during childhood. Comparable chromosomal aberrations among children with Rett syndrome have perhaps not yet been sufficiently researched. A malignoma, however, is not one of the pathologies associated with this syndrome. Attempts to explain the fact that the occurrence of the Rett syndrome is strictly limited to the female sex were first made at the symposium in 1983 by A Rett and his colleague, W Killian, and of course also by B Hagberg and others, in the form of the hypothesis of a possible X-chromosomal dominant inheritance. Because not even one mother of the patients has shown minimal symptoms of this syndrome (in contrast to incontinentia pigmenti Bloch-Sulzberger), we have been proceeding from X-chromosomal dominant new mutations among the patients. Because girls afflicted with the syndrome have not propagated up to now,
Hillig: Genetics of the Rett syndrome 369
there has not been any opportunity to observe whether or not they can pass this X-chromosomal dominant gene to their descendants. In my opinion, therefore, the Rett syndrome presents a lethal factor in both sexes, but girls survive longer than boys do, who probably die in utero. However, till now it has been impossible to show whether abortions of male fetuses are caused by an X-chromosomal dominant gene which would lead to the syndrome. W Rosenkranz objects to the fact that a serious defect at the level of neurotransmitters can cause an early abortion. The intrauterine death of the fetuses at a later point in time is conceivable because the failure of vital general nerve functions at the beginning of their activity could lead to the death of the involved male fetuses. Effects on the sex ratio among the siblings of patients are not expected in the case of such an X-chromosomal dominant mutation. If this mutation happens during the maternal gametogenesis as a single-gene mutation on an Xchromosome, it would affect male and female zygotes with equal frequency. The male fetuses would be aborted, the reason for which would not however be obvious and so would remain unregistered, whereas the female fetuses begin to demonstrate the symptoms of the syndrome not later than in their fourth year of life. If this mutation on the X-chromosome happens only in the paternal gametogenesis, it would be expected to involve daughters only in the first generation. Affected sons could be expected in the second generation if the involved girls procreated. The risk of an X-chromosomal new mutation which can occur in the paternal as well as in the maternal gametogenesis is doubled in the case of daughters (XX). Supposing the mother of a patient is a conductor, we anticipate an accumulation of male abortions in the case of a normal sequence of further pregnancies. The statement of Rett and Killian that the paternal age of 32.5 yrs at the time of birth of the Austrian patients is rather high perhaps allows the conclusion that the X-chromosomal dominant mutation which seems to be responsible for the Rett syndrome does occur more frequently in the paternal but also sometimes in the maternal gametogenesis. Therefore, in the genetic counseling of these families, it must always be determined whether the mother is a conductor.
370 Brain & Development, Vol 7, No 3, 1985
In my opinion, it is not possible at present to comment definitively on the examinations hitherto reported by the Austrian (Killian) and Swedish (Wahlstrom) teams concerning the sex-ratio in siblings. In an X-chromosomal dominant new mutation, the affected girls generally show a less serious form of the disease and, particularly with regard to Rett syndrome, they survive longer than boys do. If, corresponding to the Lyon hypothesis, the one Xchromosome which carries the Rett gene is predominantly inactivated, the onset of the disease in girls could be retarded and the progress could be less serious. On the other hand, the beginning of the disease can also be very early, quite soon after birth in the case of predominant inactivation of the normal allele. This has been observed by Y Nomura. Supposing an X-chromosomal dominant new mutation does cause the Rett syndrome, it cannot explain the fact that a healthy woman bore two daughters with the syndrome, although the fathers were two healthy men (cf B Hagberg). Either the mother is indeed a conductor or the extraordinary case of two subsequent new mutations at the same gene locus in the gametogenesis of each pregnancy has taken place. For a long time it has been conspicuous that families with Rett patients are very small. A possible explanation for this fact was put forward very early by Rett himself: as this syndrome is generally characterized by a serious course, the parents' psyche and their procreative behavior are impaired so much that they choose not to have further children. An extranuclear mutation must also be taken into consideration. Extra-chromosomal heredity has, however, up to now, been without significance in the genetics of human diseases. The cytoplasm of the human zygote is of oocyte origin. That is, it comes from the mother. A mitochondrial mutation, steered by a further X-chromosomal single-gene mutation, could have graver effects on a male zygote and could cause its embryonal death, whereas female zygotes survive longer because they still have a normal allele on their second Xchromosome. But before we deal with further possible speculations, all genetically relevant data of the families of the roughly 160 patients with Rett syndrome should be registered and considered. In the context of the central reg-
istration which now has been agreed upon, the following family data should also be ascertained : 1. Birth data (length, weight, head circumference, Apgar score) of the patients and their siblings. 2. Questionable symptoms prior to manifestation of the syndrome. 3. Birth sequence and pregnancy data of all siblings of the patients in order to get indications for possible early abortions . 4. Contraceptive means. 5. Parents' contact with possibly mutagenic agents before conception; age, physical condition, therapies. 6. Tracing of teratogenic influences on the mother during pregnancy. 7. Data of the grandparents as to 5 and 6. 8. Tracing of consanguinity by collecting data
pertaining to the parents, grandparents and, if possible, further ancestors (places of birth, of origin, maiden names ... ). At present genetic counseling for Rett syndrome parents is still difficult. The empirical recurrence risk for further daughters can be said to be minimal. Therefore the termination of a further pregnancy with a female fetus cannot be justified.
References 1. Barbi G, Steinbach P, Vogel W. Nonrandom distribution of methotrexate-induced aberrations on human chromosomes. Detection of further folic acid sensitive fragile sites. Hum Genet 1984;68: 290-4. 2. Hecht F, Kaiser Hecht B. Autosomal fragile sites not a current indication for prenatal diagnosis. Hum Genet 1984;67:352-3.
Hillig: Genetics of the Ret? syndrome
371
The Rett syndrome has been recognized as an entity for two decades in Austria (the native country of A Rett, who first described the syndrome), western Europe, particularly in Scandinavia, and, for several years now, also in Japan. Although it can be distinguished from other childhood mental retardations and although it has been researched carefully, its genesis has eluded explanation. It has not yet been possible to assess definitely its etiologyneither in the clinical form nor as it occurs in families. On the one hand there was no specific pathological substrate present, and on the other hand, the underlying defect could not be identified. The statements at the symposium in 1984 in Vienna by Y Nomura concerning the pathophysiology, by P Riederer and W Birkmayer about the biochemistry and by J Wilson regarding the neuropathology of the syndrome raise the possibility that this disease is caused by a neurotransmitter defect. Definite pathological From the Institut fUr Humangenetik und Genetische Poliklinik der Philipps-Universitiit, Marburg.
Key words: Rett syndrome, X-chromosome, fragile site, X-chromosomal dominant mutation, genetic counseling. Correspondence address: Dr. Ulrike Hillig, Institut fUr Humangenetik und Genetische Poliklinik der Philipps-Universitiit Marburg, Bahnhofstrasse 7A, D-3550 Marburg, FRG.
anatomical alterations with depigmentation in the substantia nigra and the nucleus coeruleus were found . Therefore our previous assumption, based on the clinical presentation and course of the disease (viz, the syndrome as a functional defect in children with a primarily normal stature) would stand confirmed. The disease starts with functional defects of the central nervous system manifesting themselves as pathological alterations in speech and motoric behavior, intellectual and psychical performance, and as the disease progresses there are also physical symptoms such as microcephalia, occasionally retarded growth, and so on. The fact that the roughly 160 cases definitely diagnosed up to the present time were all girls, led A Rett at first to think the disease must be genetically determined. Therefore, the question of a genetic cause or at least a hereditary participation in the syndrome has remained compelling ever since its diagnosis became feasible, particularly because there has been no possibility of effective treatment for the involved children. We have remained unable to offer any promising therapy to counteract or alleviate the Rett syndrome. We can only treat the patients' symptoms to a limited extent and help them and their parents bear this cross. At present, the syndrome must be described as intractable to treatment. The afflicted girls are 100% certain to require nursing care. Nevertheless, puberty progresses
(in general) in a normal way, but the patients do not propagate because of their physical and mental weakness and their lack of libido. It also seems doubtful that they would be able to carry a child full term should they in fact become pregnant. The oldest patients suffering from the Rett syndrome are now within the range of 25-27 yrs of age. In view of the serious course of the disease, the parents and close relatives should be informed about the risk of recurrence in descendants (i.e., genetic counseling). Exact genetic advice, however, is not possible as long as there is any question as to a hereditary factor in the Rett syndrome. At first it was generally assumed that this disease could only appear sporadically. Because of Hagberg's observation (1981 in Sweden) of two afflicted siblings who have the same healthy mother, but are of different male parentage, this opinion has been revised, all the more so since a similar observation (in this case the girls had both parents in common) was made by F Hanefeld in West Germany. Chromosome analyses of the patients were of course made with Giemsa and other staining methods. The respective findings were, however, all unrevealing. At the symposium in April 1983 in Vienna it was suggested that prometaphase stainings of patients chromosomes be investigated. Thereby it could be ascertained once and for all whether or not a chromosomal aberration exists submicroscopically which had therefore been imperceptible before, in which case we would then be dealing with a phenomenon comparable to Prader-Willi syndrome which affects the short arm of chromosome No 15. For that reason, the investigations by J Wahlstrom were also necessary in order to track down fragile sites in children with Rett syndrome. In association with this, one must mention the Martin-Bell or fragile-X syndrome, which is characterized by a specific pattern of mental, psychomotoric and speech retardation with relatively discrete physical attributes, and which is limited only to the male sex. The previous result of about 3% fra Xp22 in the afflicted half-sisters with Rett syndrome in the family published by Hagberg are not yet convincing. On the other hand, the reported finding of 10% of homogeneous sites of the fracture (fra Xp22) on an X-chromosome obtained from a fibroblast tissue culture from the
two girls' healthy mother seems to be more significant. Nearly all people have folic-acidsensitive fragile sites on their chromosomes under the conditions of a folate-deficiency cell culture. This has been shown in recent examinations [1, 2]. One can infer from these examinations that fra Xp22 is one of the most frequent fragile sites. Further chromosomal analyses on cell cultures of patients with Rett syndrome and their relatives should, however, be performed to determine whether one can find a specific, reproducible fracture on an X or on another chromosome for this syndrome. The hypothesis of J Wahlstrom of a two-step mutation as the cause for the Rett syndrome is also very interesting and worthy of examination: the idea of first an autosomal recessive (gametic) mutation which in rare cases is assumed to be accompanied by a second, somatic X-chromosomal mutation. In the case of male embryos, the combination of both mutated genes takes on the properties of a lethal factor and probably leads to the prenatal death of the fetus, because boys with Rett syndrome could not previously be detected. With the indication of possible DNA-recombination of the mutated genes, Wahlstrom compares the very rare familial incidence of the syndrome with the occurrence of a retinoblastoma or a neuroblastoma. Both of these diseases appear independently of the carrier's sex and are combined with a proven structural chromosomal aberration as well as with tumor development during childhood. Comparable chromosomal aberrations among children with Rett syndrome have perhaps not yet been sufficiently researched. A malignoma, however, is not one of the pathologies associated with this syndrome. Attempts to explain the fact that the occurrence of the Rett syndrome is strictly limited to the female sex were first made at the symposium in 1983 by A Rett and his colleague, W Killian, and of course also by B Hagberg and others, in the form of the hypothesis of a possible X-chromosomal dominant inheritance. Because not even one mother of the patients has shown minimal symptoms of this syndrome (in contrast to incontinentia pigmenti Bloch-Sulzberger), we have been proceeding from X-chromosomal dominant new mutations among the patients. Because girls afflicted with the syndrome have not propagated up to now,
Hillig: Genetics of the Rett syndrome 369
there has not been any opportunity to observe whether or not they can pass this X-chromosomal dominant gene to their descendants. In my opinion, therefore, the Rett syndrome presents a lethal factor in both sexes, but girls survive longer than boys do, who probably die in utero. However, till now it has been impossible to show whether abortions of male fetuses are caused by an X-chromosomal dominant gene which would lead to the syndrome. W Rosenkranz objects to the fact that a serious defect at the level of neurotransmitters can cause an early abortion. The intrauterine death of the fetuses at a later point in time is conceivable because the failure of vital general nerve functions at the beginning of their activity could lead to the death of the involved male fetuses. Effects on the sex ratio among the siblings of patients are not expected in the case of such an X-chromosomal dominant mutation. If this mutation happens during the maternal gametogenesis as a single-gene mutation on an Xchromosome, it would affect male and female zygotes with equal frequency. The male fetuses would be aborted, the reason for which would not however be obvious and so would remain unregistered, whereas the female fetuses begin to demonstrate the symptoms of the syndrome not later than in their fourth year of life. If this mutation on the X-chromosome happens only in the paternal gametogenesis, it would be expected to involve daughters only in the first generation. Affected sons could be expected in the second generation if the involved girls procreated. The risk of an X-chromosomal new mutation which can occur in the paternal as well as in the maternal gametogenesis is doubled in the case of daughters (XX). Supposing the mother of a patient is a conductor, we anticipate an accumulation of male abortions in the case of a normal sequence of further pregnancies. The statement of Rett and Killian that the paternal age of 32.5 yrs at the time of birth of the Austrian patients is rather high perhaps allows the conclusion that the X-chromosomal dominant mutation which seems to be responsible for the Rett syndrome does occur more frequently in the paternal but also sometimes in the maternal gametogenesis. Therefore, in the genetic counseling of these families, it must always be determined whether the mother is a conductor.
370 Brain & Development, Vol 7, No 3, 1985
In my opinion, it is not possible at present to comment definitively on the examinations hitherto reported by the Austrian (Killian) and Swedish (Wahlstrom) teams concerning the sex-ratio in siblings. In an X-chromosomal dominant new mutation, the affected girls generally show a less serious form of the disease and, particularly with regard to Rett syndrome, they survive longer than boys do. If, corresponding to the Lyon hypothesis, the one Xchromosome which carries the Rett gene is predominantly inactivated, the onset of the disease in girls could be retarded and the progress could be less serious. On the other hand, the beginning of the disease can also be very early, quite soon after birth in the case of predominant inactivation of the normal allele. This has been observed by Y Nomura. Supposing an X-chromosomal dominant new mutation does cause the Rett syndrome, it cannot explain the fact that a healthy woman bore two daughters with the syndrome, although the fathers were two healthy men (cf B Hagberg). Either the mother is indeed a conductor or the extraordinary case of two subsequent new mutations at the same gene locus in the gametogenesis of each pregnancy has taken place. For a long time it has been conspicuous that families with Rett patients are very small. A possible explanation for this fact was put forward very early by Rett himself: as this syndrome is generally characterized by a serious course, the parents' psyche and their procreative behavior are impaired so much that they choose not to have further children. An extranuclear mutation must also be taken into consideration. Extra-chromosomal heredity has, however, up to now, been without significance in the genetics of human diseases. The cytoplasm of the human zygote is of oocyte origin. That is, it comes from the mother. A mitochondrial mutation, steered by a further X-chromosomal single-gene mutation, could have graver effects on a male zygote and could cause its embryonal death, whereas female zygotes survive longer because they still have a normal allele on their second Xchromosome. But before we deal with further possible speculations, all genetically relevant data of the families of the roughly 160 patients with Rett syndrome should be registered and considered. In the context of the central reg-
istration which now has been agreed upon, the following family data should also be ascertained : 1. Birth data (length, weight, head circumference, Apgar score) of the patients and their siblings. 2. Questionable symptoms prior to manifestation of the syndrome. 3. Birth sequence and pregnancy data of all siblings of the patients in order to get indications for possible early abortions . 4. Contraceptive means. 5. Parents' contact with possibly mutagenic agents before conception; age, physical condition, therapies. 6. Tracing of teratogenic influences on the mother during pregnancy. 7. Data of the grandparents as to 5 and 6. 8. Tracing of consanguinity by collecting data
pertaining to the parents, grandparents and, if possible, further ancestors (places of birth, of origin, maiden names ... ). At present genetic counseling for Rett syndrome parents is still difficult. The empirical recurrence risk for further daughters can be said to be minimal. Therefore the termination of a further pregnancy with a female fetus cannot be justified.
References 1. Barbi G, Steinbach P, Vogel W. Nonrandom distribution of methotrexate-induced aberrations on human chromosomes. Detection of further folic acid sensitive fragile sites. Hum Genet 1984;68: 290-4. 2. Hecht F, Kaiser Hecht B. Autosomal fragile sites not a current indication for prenatal diagnosis. Hum Genet 1984;67:352-3.
Hillig: Genetics of the Ret? syndrome
371