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The clinical consequences of X-chromosome inactivation: Duchenne muscular dystrophy in one of monozygotic twins
Journal of the Neurological Sciences, 1987, 79:337-344 Elsevier
337
JNS 02837
The clinical consequences of X-chromosome inactivation" Duchenne muscular dystrophy in one of monozygotic twins Sergio D.J. Pena l, George Karpati 2, Stirling Carpenter 2 and F. Clarke Fraser 3 1Department of Biochemistry, Universidade Federal de Minas Gerais, Belo Horizonte (Brazil); 2Department of Neurology and Neurosurgery, McGill University, and Montreal Neurological Institute, Montreal and 3Departments of Pediatrics, Biology and Center for Human Genetics, McGill University, Montreal (Canada) (Received 5 January, 1987) (Revision, received 10 February, 1987) (Accepted 10 February, 1987)
SUMMARY
We have ascertained retrospectively a female patient, one of identical twins, who was diagnosed at age 23 years as having Duchenne muscular dystrophy (DMD). A muscle biopsy at that time showed a pattern in which large areas of destroyed muscle fibers replaced with adipose tissue were interspersed with normal-appearing muscle fascicles. The visualization of Barr bodies in the muscle biopsy, plus the patient's normal menstrual history served to rule out Turner's syndrome. The clinical expression of DMD in only one of monozygotic twins is strongly suggestive of uneven lyonization, with an excess of paternally derived X-chromosomes being inactivated in the patient. This view is supported by the appearance of the muscle biopsy. Twinning may conceivably predispose to uneven lyonization by reducing the size of the muscle cell anlage at the time of X-chromosome inactivation. Alternatively, lyonization may occur before the splitting of the embryonic mass, and by chance, the two embryonic centers could end up with a significantly different proportion of active maternal and paternal X-chromosomes.
Key words: Duchenne dystrophy; Monozygous twins; X-chromosome inactivation
Supported by the Medical Research Council of Canada, The Muscular Dystrophy Association of Canada and the Killam Fund of the Montreal Neurological Institute. Correspondence and reprint requests to: Dr. George Karpati, 3801 University Street, Montreal (Quebec), Canada, H3A 2B4. 0022-510X/87/$03.50 © 1987 Elsevier Science Publishers B.V. (Biomedical Division)
338 INTRODUCTION
It is well established that during early embryonic development one of the two X-chromosomes in all female somatic cells is inactivated, a phenomenon known as lyonization (Gartler and Riggs 1983). The inactivation is random and clonal; either the paternal or the maternal X-chromosome can be inactivated in a given cell, but once one or the other has been inactivated, such differentiation will be maintained in all the progeny of that cell. The inactivated X-chromosome becomes heterochromatic and can be detected microscopically in the interphase nucleus as the sex chromatin or Barr body. Duchenne muscular dystrophy is an X-linked recessive disease and as such, characteristically affects boys. There are, however, numerous reports of affected girls with bona fide disease. Homozygosityfor the Duchenne gene is a theoretical possibility, but unlikely on statistical grounds. Most affected females can be shown to have a sex chromosome abnormality such as a 45,X karyotype or a structurally abnormal X-chromosome. Recently, there have been several reports of affected girls with X-autosomal translocations with breakpoints on Xp21. The Duchenne locus has consequently been assigned to that region of the short arm of the X-chromosome (Canki et al. 1979; Verellen et al. 1977). This assignment has been confirmed by linkage of the disease to 11 different sites of DNA polymorphism, all of which map to the same region of the X-chromosome (Davies 1985). Approximately 8 ~o of the female heterozygotes for DMD show some clinical manifestations, which range from mild hypertrophy of the calf muscles to proximal muscle weakness and wasting (Emery 1981). It has been proposed that excess random inactivation of the X-chromosome carrying the normal allele might explain such clinical manifestations in DMD carder females (Hazama et al. 1979; Moser and Emery 1974). This hypothesis was supported by the reports by Gomez et al. (1977) of severe clinical manifestations in one of monozygous female twins carders for DMD. In 1982, we reported in abstract a second such patient (Pena et al. 1982). Two further patients have subsequently been reported (Bum et al. 1986; Chutkow et al. 1986). In the present paper we provide a full report of our patient and discuss possible mechanisms for the association of twinning and uneven X-chromosome inactivation. PATIENTS AND RESULTS
The case was brought to our attention again during a genetic counselling session in 1981 and was studied retrospectively from medical records. The proposita was hospitalized in the Montreal Neurological Hospital in 1954, at the age of 23 years. She first showed calf hypertrophy and proximal muscle weakness at the age of 6 years, progressing steadily and leading to loss of ambulation in the mid-teens. She had menarche at age 14 and then regular menstrual periods. At age 23 she presented with severe muscle weakness and wasting. Paraffin sections of a muscle biopsy from the gastrocnemius showed muscle fiber necrosis with phagocytosis and numerous hypercontracted fibers as well as atrophic and hypertrophic fibers in many randomly scattered fascicles. Large areas of muscle were replaced by fat cells. However,
339
A
B Fig. 1. A: 'Mosaic' appearance of the muscle biopsy where several well-preserved fascicles are interspersed with large areas in which destroyed muscle fibers were replaced by adipocytes. Hematoxylin-eosin, × 100. B: Higher power photomicrograph illustrates the practically normal appearance of a large muscle fascicle. Hematoxylin-eosin, × 350.
340 there were also several normal appearing muscle fascicles which gave a peculiar "mosaic" appearance to the specimen (Fig. 1). The myonuclei exhibited Barr bodies (Fig. 2). The family history disclosed several cases of Duchenne muscular dystrophy in the family, including one maternal uncle and two of the patient's brothers (Fig. 3). The patient had a healthy twin sister, to whom she was completely similar during infancy and early childhood. Blood group studies and dermatoglyphic analysis supported the
Fig. 2. In muscle cell nuclei Barr bodies are seen adjacent to the nuclear membrane (arrows). Hematoxylin-eosin, x 600.
11
I!Z •
•
DuchenneMuscularDystrophy(DMD)
(~) Carrierfor DMD
Fig. 3. Pedigree. The proposita (1II-8) is indicated by an arrow.
341 TABLE 1 BLOOD GROUP AND DERMATOGLYPHICS FINDINGS ON THE PROPOSITA (J), HER TWIN SISTER (T) AND THEIR PARENTS
Blood groups
Dermatoglyphics Total ridge count (TRC)
Father
Mother
J
T
A1
0
0
0
CDe/CDe
CDe/cde
CDe/cde
CDe/cde
MsNs Fy(a-)
MsNs Fy(a-)
NsNs Fy(a-)
NsNs Fy(a-)
Kell-neg
Kell-neg
Kell-neg
Kell-neg
-
-
90
90
clinical impression of monozygosity (Table 1). Application of Bayes' theorem gave a posterior probability of monozygosity of 0.982 (Table 2). The patient died at age 28. Her twin sister married and had a son affected with DMD, thus proving that she was a carrier of the disease. This sister never showed signs of muscle disease and thus she has never been investigated; she died later in an automobile accident. The anomalous occurrence of DMD in one of female monozygotic twins could not be accounted for; some years later the formulation of the Lyon hypothesis suggested an explanation (Fraser 1983), as discussed below. DISCUSSION
The clinical picture, muscle biopsy f'mdings and family history of the proposita permitted an unequivocal diagnosis of DMD. The patient was thus a manifesting
TABLE 2 BAYESIAN CALCULATION OF THE PROBABILITY OF MONOZYGOSITY FOR THE TWINS
Character
Dizygosity
Monozygosity
Prior probabilities Conditional probabilities Sex Blood groups
0.67
0.33
0.50
1.00
0.50 0.25 0.50
1.00 1.00 1.00
0.16 0.00335 0.018
0.56 0.1848 0.982
AB0 MN Rh
Dermatoglyphics No difference in TRC Joint probabilities Posterior probabilities
342 carrier. Her apparently monozygous twin sister was also a heterozygote but had no clinical manifestations. How can this discrepancy be explained? The first possibility is that the proposita had a 45,X karyotype or a structural X-chromosome defect. This is very unlikely since normal Barr bodies were seen in her myonuclei and she had normal menses. The second possibility is that she, by chance, inactivated mostly her paternal X-chromosomes and became affected on the basis of uneven lyonization. This hypothesis receives support form the appearance of the muscle biopsy, where large areas of devastated muscle were seen side by side with apparently intact fascicles (Fig. 1). Although uneven X-inactivation is a priori a very rare event, the probability of its occurrence may conceivably be increased by monozygous twinning. X-inactivation occurs early in development, but not before several cell divisions and in a heterogeneous way: it first occurs in the trophectoderm, next in the primitive endoderm, and finally in the inner cell mass which will form the adult somatic cells (Gartler and Riggs 1983). We do not know how many myogenic cells are present in the muscle anlage when lyonization occurs. This knowledge would be essential for calculating the probability of a given level of uneven lyonization by application of the binomial distribution. Also, it is very difficult to interpret in clinical terms the possible results of uneven X-chromosome inactivation in the myonuclei of DMD carriers. Muscle fibers are multinucleated and apparently nuclear territories overlap (Frair and Petersen 1983). Besides, necrosis in DMD usually only affects fiber segments and not the whole muscle fiber (Carpenter and Karpati 1979). For the sake of discussion, let us assume that if75 ~o of the active X-chromosomes in the muscle of a DMD carrier were of maternal origin (thus carrying the abnormal allele) a dystrophic phenotype would ensue. Let us also assume that there are 50 stem cells in the muscle anlage at the time oflyonization. Thus, by application of the binomial distribution we could calculate a probability of 0.0002 of inactivation of 75 ~o or more of paternal X-chromosomes (Table 3). However, the situation would be different in monozygous twins, because of the splitting of the embryonal cell mass in two. This most commonly occurs between 4 and 8 days after conception and results in the emergence
TABLE 3 B I N O M I A L C A L C U L A T I O N O F T H E PROBABILITY OF I N A C T I V A T I O N OF 75% OR M O R E OF P A T E R N A L X C H R O M O S O M E S IN M Y O N U C L E I AS A F U N C T I O N OF T H E N U M B E R OF CELLS IN T H E M U S C L E A N L A G E A T T H E T I M E OF I N A C T I V A T I O N Number of cells in muscle anlage at inactivation
P (Xe>~ 75%)
Increased risk due to twinning
25 50 100 200
0.01 0.0002 0.0000003 0.0000000000008
49 × 710 × 368000 ×
343 of two embryonic centers (germinal centers) in one blastocyst (Smith et al. 1976). If the timing of X-chromosome inactivation is determined by the number of cell divisions that have occurred rather than by the number of cells in the inner cell mass, then in monozygous twins, lyonization would occur in a muscle anlage half the normal size, thus considerably increasing the probability of occurrence of uneven lyonization (Table 3). In this case, the occurrence of clinical manifestations in some females who are monozygous twins and heterozygous for an X-linked recessive disease may not be completely accidental. It has been suggested that the tendency for DMD heterozygotes to show clinical manifestations of dystrophy is familial, and that there may be a genetic predisposition to non-random X-inactivation, perhaps by causing X-inactivation to occur earlier (Falc,'io-Conceic,'to et al. 1983). Our case suggests that twinning may do the same. Another possibility is that lyonization could have occurred even before the splitting of embryonic cell mass (Burn et al. 1986); in that case it is likely that there would be some difference, by chance, in the proportion of cells with active paternal or maternal X-chromosomes in the two embryonic centers. Thus, if, for example, one embryonic center ended up with 75 % of its cells with active maternal X-chromosomes, and the other with 25 % of active maternal X-chromosomes, the twin developing from the former embryonic center would have a much higher chance of displaying clinical signs of DMD than the other twin. In fact, by the investigation of somatic cell hybrids derived from fibroblast of a pair of monozygotic twins and a mouse cell line, Burn et al. (1986) obtained suggestive evidence, that the twin that showed DMD had only maternal X-chromosomes, while the phenotypically normal twin had only paternal X-chromosomes. Burn et al. (1986) suggested that this is a result of accidental uneven lyonization occurring before twinning and a subsequent segregation of cells with active maternal or paternal active X-chromosomes in the 2 embryonic centers. Burn et al. (1986) also raised the possibility that such presumed uneven lyonization may also predispose to twinning. Thorough analysis of more female identical twins who are heterozygotes for Duchenne muscular dystrophy or other X-linked recessive traits could, in that respect, be informative. REFERENCES Burn, J., S. Povey, K. Boyd et al. (1986) Duchenne muscular dystrophy in one of monozygotic twin girls. J. Med. Genet., 23: 494-500. Canki, N., B. Dutrillaux and I. Tivader (1979) Dystrophie musculaire de Duchenne chez une petite fille porteuse d'une transloeation (~'X;3) (p21 ;q13) de novo. Ann. Genet., 22: 35-39. Carpenter, S. and G. Karpati (1979) Duchenne muscular dystrophy: plasma membrane loss initiates muscle cell necrosis unless it is repaired, Brain, 102: 147-161. Chutkow, J.G., J.A. Edwards, R.R. Heffner and J.J. Czyrny (1986) Monozygotic twins discordant for the clinical manifestations of Duehenne muscular dystrophy. Neurology, Sul~l. 1, 36:113 (Abstract). Davies, K.E. (1985) Molecular genetics of the human X-chromosome. J. Med. Genet., 22: 243-249. Emery, A.E.H. (1981) Duchenne muscular dystrophy -genetic aspects, carrier detection and antenatal diagnosis. Brit. Med. Bull., 36: 117-122. Falcao-Conceiciio, D., M. de C. G. Pereira, M.M. Goncalves and M. L. Baptista (1983) Familial occurrence of heterozygous manifestations in X-linked muscular dystrophies. Rev. Brasil Genet., 6: 527-538. Friar, P.M. and A.C. Petersen (1983) The nuclear-cytoplasmic relationship in "mosaic" skeletal muscle fibers from mouse chimaeras. Exp. Cell Res., 145: 167-178.
344 Fraser, F.C. (1963) Taking the family history. Am. J. Med., 34: 585-593. Gartler, S.M. and A.D. Riggs (1983) Mammalian X-chromosome inactivation. Annu. Rev. Genet., 17: 155-190. Gomez, M.R., A.G. Engel, G. Dewald etal. (1977) Failure of inactivation of Ducherme dystrophy X-chromosome in one of female identical twins. Neurology, 27: 537-541. Hazama, R., M. Tsujihata, M. Mori and K. Mori (1979) Muscular dystrophy in six young girls. Neurology, 29:1486-1491. Moser, H. and A. E. H. Emery (1974) The manifesting carrier in Duchenne muscular dystrophy. Clin. Genet., 5: 271-284. Pena, S., G. Karpati, S. Carpenter and F.G. Fraser (1982) The clinical consequences of X-chromosomal inactivation: Duchenne muscular dystrophy in one of monozygotic twins. Neurology, 32: A83. Smith, D.W., C. Bartlett and L.M. Harrah (1976) Monozygous twinning and the Duhamel anomaly (imperforate anus to syrenomelia): a nonrandom association between two aberrations in morphogenesis. Birth Defects, Orig. Art. Set. XII, 5: 53-63. Verellen, C., M. Freund, R. Meyer, C. Laterre, B. Scholberg and J. Frederic (1977) Progressive muscular dystrophy of the Duchenne type in a young girl associated with an aberration of chromosome X. International Congress Series, Excerpta Medica, Amsterdam, pp. 426-442.
337
JNS 02837
The clinical consequences of X-chromosome inactivation" Duchenne muscular dystrophy in one of monozygotic twins Sergio D.J. Pena l, George Karpati 2, Stirling Carpenter 2 and F. Clarke Fraser 3 1Department of Biochemistry, Universidade Federal de Minas Gerais, Belo Horizonte (Brazil); 2Department of Neurology and Neurosurgery, McGill University, and Montreal Neurological Institute, Montreal and 3Departments of Pediatrics, Biology and Center for Human Genetics, McGill University, Montreal (Canada) (Received 5 January, 1987) (Revision, received 10 February, 1987) (Accepted 10 February, 1987)
SUMMARY
We have ascertained retrospectively a female patient, one of identical twins, who was diagnosed at age 23 years as having Duchenne muscular dystrophy (DMD). A muscle biopsy at that time showed a pattern in which large areas of destroyed muscle fibers replaced with adipose tissue were interspersed with normal-appearing muscle fascicles. The visualization of Barr bodies in the muscle biopsy, plus the patient's normal menstrual history served to rule out Turner's syndrome. The clinical expression of DMD in only one of monozygotic twins is strongly suggestive of uneven lyonization, with an excess of paternally derived X-chromosomes being inactivated in the patient. This view is supported by the appearance of the muscle biopsy. Twinning may conceivably predispose to uneven lyonization by reducing the size of the muscle cell anlage at the time of X-chromosome inactivation. Alternatively, lyonization may occur before the splitting of the embryonic mass, and by chance, the two embryonic centers could end up with a significantly different proportion of active maternal and paternal X-chromosomes.
Key words: Duchenne dystrophy; Monozygous twins; X-chromosome inactivation
Supported by the Medical Research Council of Canada, The Muscular Dystrophy Association of Canada and the Killam Fund of the Montreal Neurological Institute. Correspondence and reprint requests to: Dr. George Karpati, 3801 University Street, Montreal (Quebec), Canada, H3A 2B4. 0022-510X/87/$03.50 © 1987 Elsevier Science Publishers B.V. (Biomedical Division)
338 INTRODUCTION
It is well established that during early embryonic development one of the two X-chromosomes in all female somatic cells is inactivated, a phenomenon known as lyonization (Gartler and Riggs 1983). The inactivation is random and clonal; either the paternal or the maternal X-chromosome can be inactivated in a given cell, but once one or the other has been inactivated, such differentiation will be maintained in all the progeny of that cell. The inactivated X-chromosome becomes heterochromatic and can be detected microscopically in the interphase nucleus as the sex chromatin or Barr body. Duchenne muscular dystrophy is an X-linked recessive disease and as such, characteristically affects boys. There are, however, numerous reports of affected girls with bona fide disease. Homozygosityfor the Duchenne gene is a theoretical possibility, but unlikely on statistical grounds. Most affected females can be shown to have a sex chromosome abnormality such as a 45,X karyotype or a structurally abnormal X-chromosome. Recently, there have been several reports of affected girls with X-autosomal translocations with breakpoints on Xp21. The Duchenne locus has consequently been assigned to that region of the short arm of the X-chromosome (Canki et al. 1979; Verellen et al. 1977). This assignment has been confirmed by linkage of the disease to 11 different sites of DNA polymorphism, all of which map to the same region of the X-chromosome (Davies 1985). Approximately 8 ~o of the female heterozygotes for DMD show some clinical manifestations, which range from mild hypertrophy of the calf muscles to proximal muscle weakness and wasting (Emery 1981). It has been proposed that excess random inactivation of the X-chromosome carrying the normal allele might explain such clinical manifestations in DMD carder females (Hazama et al. 1979; Moser and Emery 1974). This hypothesis was supported by the reports by Gomez et al. (1977) of severe clinical manifestations in one of monozygous female twins carders for DMD. In 1982, we reported in abstract a second such patient (Pena et al. 1982). Two further patients have subsequently been reported (Bum et al. 1986; Chutkow et al. 1986). In the present paper we provide a full report of our patient and discuss possible mechanisms for the association of twinning and uneven X-chromosome inactivation. PATIENTS AND RESULTS
The case was brought to our attention again during a genetic counselling session in 1981 and was studied retrospectively from medical records. The proposita was hospitalized in the Montreal Neurological Hospital in 1954, at the age of 23 years. She first showed calf hypertrophy and proximal muscle weakness at the age of 6 years, progressing steadily and leading to loss of ambulation in the mid-teens. She had menarche at age 14 and then regular menstrual periods. At age 23 she presented with severe muscle weakness and wasting. Paraffin sections of a muscle biopsy from the gastrocnemius showed muscle fiber necrosis with phagocytosis and numerous hypercontracted fibers as well as atrophic and hypertrophic fibers in many randomly scattered fascicles. Large areas of muscle were replaced by fat cells. However,
339
A
B Fig. 1. A: 'Mosaic' appearance of the muscle biopsy where several well-preserved fascicles are interspersed with large areas in which destroyed muscle fibers were replaced by adipocytes. Hematoxylin-eosin, × 100. B: Higher power photomicrograph illustrates the practically normal appearance of a large muscle fascicle. Hematoxylin-eosin, × 350.
340 there were also several normal appearing muscle fascicles which gave a peculiar "mosaic" appearance to the specimen (Fig. 1). The myonuclei exhibited Barr bodies (Fig. 2). The family history disclosed several cases of Duchenne muscular dystrophy in the family, including one maternal uncle and two of the patient's brothers (Fig. 3). The patient had a healthy twin sister, to whom she was completely similar during infancy and early childhood. Blood group studies and dermatoglyphic analysis supported the
Fig. 2. In muscle cell nuclei Barr bodies are seen adjacent to the nuclear membrane (arrows). Hematoxylin-eosin, x 600.
11
I!Z •
•
DuchenneMuscularDystrophy(DMD)
(~) Carrierfor DMD
Fig. 3. Pedigree. The proposita (1II-8) is indicated by an arrow.
341 TABLE 1 BLOOD GROUP AND DERMATOGLYPHICS FINDINGS ON THE PROPOSITA (J), HER TWIN SISTER (T) AND THEIR PARENTS
Blood groups
Dermatoglyphics Total ridge count (TRC)
Father
Mother
J
T
A1
0
0
0
CDe/CDe
CDe/cde
CDe/cde
CDe/cde
MsNs Fy(a-)
MsNs Fy(a-)
NsNs Fy(a-)
NsNs Fy(a-)
Kell-neg
Kell-neg
Kell-neg
Kell-neg
-
-
90
90
clinical impression of monozygosity (Table 1). Application of Bayes' theorem gave a posterior probability of monozygosity of 0.982 (Table 2). The patient died at age 28. Her twin sister married and had a son affected with DMD, thus proving that she was a carrier of the disease. This sister never showed signs of muscle disease and thus she has never been investigated; she died later in an automobile accident. The anomalous occurrence of DMD in one of female monozygotic twins could not be accounted for; some years later the formulation of the Lyon hypothesis suggested an explanation (Fraser 1983), as discussed below. DISCUSSION
The clinical picture, muscle biopsy f'mdings and family history of the proposita permitted an unequivocal diagnosis of DMD. The patient was thus a manifesting
TABLE 2 BAYESIAN CALCULATION OF THE PROBABILITY OF MONOZYGOSITY FOR THE TWINS
Character
Dizygosity
Monozygosity
Prior probabilities Conditional probabilities Sex Blood groups
0.67
0.33
0.50
1.00
0.50 0.25 0.50
1.00 1.00 1.00
0.16 0.00335 0.018
0.56 0.1848 0.982
AB0 MN Rh
Dermatoglyphics No difference in TRC Joint probabilities Posterior probabilities
342 carrier. Her apparently monozygous twin sister was also a heterozygote but had no clinical manifestations. How can this discrepancy be explained? The first possibility is that the proposita had a 45,X karyotype or a structural X-chromosome defect. This is very unlikely since normal Barr bodies were seen in her myonuclei and she had normal menses. The second possibility is that she, by chance, inactivated mostly her paternal X-chromosomes and became affected on the basis of uneven lyonization. This hypothesis receives support form the appearance of the muscle biopsy, where large areas of devastated muscle were seen side by side with apparently intact fascicles (Fig. 1). Although uneven X-inactivation is a priori a very rare event, the probability of its occurrence may conceivably be increased by monozygous twinning. X-inactivation occurs early in development, but not before several cell divisions and in a heterogeneous way: it first occurs in the trophectoderm, next in the primitive endoderm, and finally in the inner cell mass which will form the adult somatic cells (Gartler and Riggs 1983). We do not know how many myogenic cells are present in the muscle anlage when lyonization occurs. This knowledge would be essential for calculating the probability of a given level of uneven lyonization by application of the binomial distribution. Also, it is very difficult to interpret in clinical terms the possible results of uneven X-chromosome inactivation in the myonuclei of DMD carriers. Muscle fibers are multinucleated and apparently nuclear territories overlap (Frair and Petersen 1983). Besides, necrosis in DMD usually only affects fiber segments and not the whole muscle fiber (Carpenter and Karpati 1979). For the sake of discussion, let us assume that if75 ~o of the active X-chromosomes in the muscle of a DMD carrier were of maternal origin (thus carrying the abnormal allele) a dystrophic phenotype would ensue. Let us also assume that there are 50 stem cells in the muscle anlage at the time oflyonization. Thus, by application of the binomial distribution we could calculate a probability of 0.0002 of inactivation of 75 ~o or more of paternal X-chromosomes (Table 3). However, the situation would be different in monozygous twins, because of the splitting of the embryonal cell mass in two. This most commonly occurs between 4 and 8 days after conception and results in the emergence
TABLE 3 B I N O M I A L C A L C U L A T I O N O F T H E PROBABILITY OF I N A C T I V A T I O N OF 75% OR M O R E OF P A T E R N A L X C H R O M O S O M E S IN M Y O N U C L E I AS A F U N C T I O N OF T H E N U M B E R OF CELLS IN T H E M U S C L E A N L A G E A T T H E T I M E OF I N A C T I V A T I O N Number of cells in muscle anlage at inactivation
P (Xe>~ 75%)
Increased risk due to twinning
25 50 100 200
0.01 0.0002 0.0000003 0.0000000000008
49 × 710 × 368000 ×
343 of two embryonic centers (germinal centers) in one blastocyst (Smith et al. 1976). If the timing of X-chromosome inactivation is determined by the number of cell divisions that have occurred rather than by the number of cells in the inner cell mass, then in monozygous twins, lyonization would occur in a muscle anlage half the normal size, thus considerably increasing the probability of occurrence of uneven lyonization (Table 3). In this case, the occurrence of clinical manifestations in some females who are monozygous twins and heterozygous for an X-linked recessive disease may not be completely accidental. It has been suggested that the tendency for DMD heterozygotes to show clinical manifestations of dystrophy is familial, and that there may be a genetic predisposition to non-random X-inactivation, perhaps by causing X-inactivation to occur earlier (Falc,'io-Conceic,'to et al. 1983). Our case suggests that twinning may do the same. Another possibility is that lyonization could have occurred even before the splitting of embryonic cell mass (Burn et al. 1986); in that case it is likely that there would be some difference, by chance, in the proportion of cells with active paternal or maternal X-chromosomes in the two embryonic centers. Thus, if, for example, one embryonic center ended up with 75 % of its cells with active maternal X-chromosomes, and the other with 25 % of active maternal X-chromosomes, the twin developing from the former embryonic center would have a much higher chance of displaying clinical signs of DMD than the other twin. In fact, by the investigation of somatic cell hybrids derived from fibroblast of a pair of monozygotic twins and a mouse cell line, Burn et al. (1986) obtained suggestive evidence, that the twin that showed DMD had only maternal X-chromosomes, while the phenotypically normal twin had only paternal X-chromosomes. Burn et al. (1986) suggested that this is a result of accidental uneven lyonization occurring before twinning and a subsequent segregation of cells with active maternal or paternal active X-chromosomes in the 2 embryonic centers. Burn et al. (1986) also raised the possibility that such presumed uneven lyonization may also predispose to twinning. Thorough analysis of more female identical twins who are heterozygotes for Duchenne muscular dystrophy or other X-linked recessive traits could, in that respect, be informative. REFERENCES Burn, J., S. Povey, K. Boyd et al. (1986) Duchenne muscular dystrophy in one of monozygotic twin girls. J. Med. Genet., 23: 494-500. Canki, N., B. Dutrillaux and I. Tivader (1979) Dystrophie musculaire de Duchenne chez une petite fille porteuse d'une transloeation (~'X;3) (p21 ;q13) de novo. Ann. Genet., 22: 35-39. Carpenter, S. and G. Karpati (1979) Duchenne muscular dystrophy: plasma membrane loss initiates muscle cell necrosis unless it is repaired, Brain, 102: 147-161. Chutkow, J.G., J.A. Edwards, R.R. Heffner and J.J. Czyrny (1986) Monozygotic twins discordant for the clinical manifestations of Duehenne muscular dystrophy. Neurology, Sul~l. 1, 36:113 (Abstract). Davies, K.E. (1985) Molecular genetics of the human X-chromosome. J. Med. Genet., 22: 243-249. Emery, A.E.H. (1981) Duchenne muscular dystrophy -genetic aspects, carrier detection and antenatal diagnosis. Brit. Med. Bull., 36: 117-122. Falcao-Conceiciio, D., M. de C. G. Pereira, M.M. Goncalves and M. L. Baptista (1983) Familial occurrence of heterozygous manifestations in X-linked muscular dystrophies. Rev. Brasil Genet., 6: 527-538. Friar, P.M. and A.C. Petersen (1983) The nuclear-cytoplasmic relationship in "mosaic" skeletal muscle fibers from mouse chimaeras. Exp. Cell Res., 145: 167-178.
344 Fraser, F.C. (1963) Taking the family history. Am. J. Med., 34: 585-593. Gartler, S.M. and A.D. Riggs (1983) Mammalian X-chromosome inactivation. Annu. Rev. Genet., 17: 155-190. Gomez, M.R., A.G. Engel, G. Dewald etal. (1977) Failure of inactivation of Ducherme dystrophy X-chromosome in one of female identical twins. Neurology, 27: 537-541. Hazama, R., M. Tsujihata, M. Mori and K. Mori (1979) Muscular dystrophy in six young girls. Neurology, 29:1486-1491. Moser, H. and A. E. H. Emery (1974) The manifesting carrier in Duchenne muscular dystrophy. Clin. Genet., 5: 271-284. Pena, S., G. Karpati, S. Carpenter and F.G. Fraser (1982) The clinical consequences of X-chromosomal inactivation: Duchenne muscular dystrophy in one of monozygotic twins. Neurology, 32: A83. Smith, D.W., C. Bartlett and L.M. Harrah (1976) Monozygous twinning and the Duhamel anomaly (imperforate anus to syrenomelia): a nonrandom association between two aberrations in morphogenesis. Birth Defects, Orig. Art. Set. XII, 5: 53-63. Verellen, C., M. Freund, R. Meyer, C. Laterre, B. Scholberg and J. Frederic (1977) Progressive muscular dystrophy of the Duchenne type in a young girl associated with an aberration of chromosome X. International Congress Series, Excerpta Medica, Amsterdam, pp. 426-442.