Recurrence risk due to germ line mosaicism: Duchenne and Becker muscular dystrophy

The presence of multiple affected offspring from apparently non-carrier parents is caused by germ line mosaicism. Although germ line mosaicism has been reported for many diseases, figures for recurrence risks are known for only a few of them. In X-linked Duchenne and Becker muscular dystrophies (DMD/BMD), the recurrence risk for non-carrier females due to germ line mosaicism has been estimated to be between 14% and 20% (95% confidence interval 3–30) if the risk haplotype is transmitted. In this study, we have analyzed 318 DMD/BMD cases in which the detected mutation was de novo with the aim of obtaining a better estimate of the 'true' number of germ line mosaics and a more precise recurrence risk. This knowledge is essential for genetic counseling. Our data indicate a recurrence risk of 8.6% (4.8–12.2) if the risk haplotype is transmitted, but there is a remarkable difference between proximal (15.6%) (4.1–27.0) and distal (6.4%) (2.1–10.6) deletions. Overall, most mutations originated in the female. Deletions occur more often on the X chromosome of the maternal grandmother, whereas point mutations occur on the X chromosome of the maternal grandfather. In unhaplotyped de novo DMD/BMD families, the risk of recurrence of the mutation is 4.3%.     

Germinal mosaicism in a Duchenne muscular dystrophy family: implications for genetic counselling

Melis MA, Cau M. Congiu R, Puddu R, Muntoni F, Cao A. Germinal mosaicism in a Duchenne muscular dystrophy family: implications for genetic counselling.
Clin Genet 1993: 43: 247–249. © Munksgaard, 1993
In this study we describe a three-generation family in which two siblings were affected by Duchenne muscular dystrophy (DMD). Immunohisto-chemical analysis of muscle dystrophin and haplotype analysis of the DMD locus revealed that the X chromosome carrying the DMD gene was transmitted from the healthy maternal grandfather to his three daughters. including the proband's mother. These findings indicate that the grandfather was a germinal mosaic for the DMD gene. The definition of the carrier status in two possible carriers led us to give accurate genetic counselling and to prevent the birth of an affected boy. The results of this study demonstrate the usefulness of haplotype analysis and immuno-histochemical muscle dystrophin studies to detect hidden germinal mosaicism and to improve genetic counselling.     

Mandibulofacial dysostosis in Hutterite sibs: a possible recessive trait

We report on two sisters with mandibulofacial dysostosis (MFD). Both parents were examined carefully by clinical, radiographic, audiologic, and cephalometric methods. Neither showed evidence of the MFD gene. Photographs of three grandparents and examination of one disclosed no evidence of MFD. The parents are from the Hutterite Brethren and are consanguineous. Examination of the literature on MFD disclosed a number of other families with affected sibs and apparently normal parents. These families raise the possibility of an autosomal recessive form of MFD or some other explanation such as germinal mosaicism, chromosome rearrangement, or delayed mutation. For our family, the recurrence risk is probably 25%, but since germinal mosaicism cannot be excluded, it could be as high as 50%.     

X-linked mental retardation: transmission of the trait by an apparently unaffected male.

A pedigree is presented in which an apparently unaffected man transmitted the gene for X-linked mental retardation to at least four of his 12 daughters. None of his 12 sons was mentally retarded. These findings may be explained by a somatic mutation and germinal mosaicism in the father or by a half chromatid mutation in maternal gametes.     

X-linked mental retardation: Transmission of the trait by an apparently unaffected male

A pedigree is presented in which an apparently unaffected man transmitted the gene for X-linked mental retardation to at least four of his 12 daughters. None of his 12 sons was mentally retarded. These findings may be explained by a somatic mutation and germinal mosaicism in the father or by a half chromatid mutation in maternal gametes.     

Recurrence of perinatal lethal osteogenesis imperfecta in sibships: Parsing the risk between parental mosaicism for dominant mutations and autosomal recessive inheritance

PurposeRecurrence of lethal osteogenesis imperfecta in families results from either dominant (parental mosaicism) or recessive inheritance. The proportion of these two mechanisms is not known, and determination of the contribution of each is important to structure genetic counseling for these families.MethodsWe measured the recurrence rate of lethal osteogenesis imperfecta after the birth of an affected infant. We determined the rate of parental mosaicism in a subset of families in which we had identified dominant mutations. In 37 families in which two or more affected infants were born, we identified mutations and determined the proportion that resulted from recessive inheritance.ResultsThe recurrence rate after the first affected pregnancy was 1.3%. The rate of parental mosaicism in families in which a dominant mutation was identified in a first affected child was 16%. In 37 families with two affected infants, 26 had dominant mutations, seven had recessive mutations, and we failed to find mutations in four. The overall recurrence rate for couples after two or more affected infants was 32%; 27% for families with parental mosaicism, 31% for recessive mutations, and 50% for families with no identified mutation.ConclusionsIn most populations, recurrence of lethal osteogenesis imperfecta usually results from parental mosaicism for dominant mutations, but the carrier frequency of recessive forms of osteogenesis imperfecta will alter that proportion. Mutation identification is an important tool to assess risk and facilitate prenatal or Genet Med 2011:13(2):125–130.     

DNA probe analysis for carrier detection and prenatal diagnosis of Duchenne muscular dystrophy: a standard diagnostic procedure.

Thirteen marker loci localised on the short arm of the X chromosome are available for use in genetic studies for Duchenne muscular dystrophy (DMD). This large number of probes detecting about 20 RFLPs encouraged us to set up a standard procedure using a sequence of selected probes and restriction enzymes for the diagnosis of DMD families. The application of DNA probe analysis for carrier detection and prenatal diagnosis, involving 61 pedigrees of both familial and isolated cases, has yielded the following results. Carrier detection using flanking markers was possible in more than 75% of the cases (104 out of 136 females) with a reliability of better than 98%. Prenatal diagnosis was possible in 95% of the cases (65 out of 68 proven carriers or women at risk). Twenty-three prenatal diagnoses were performed on male fetuses; 13 appeared to have a low risk for DMD (less than 1%) and thus the pregnancies continued. Seven have since come to term and the male infants have normal CK levels. The genetic distances of the loci relative to the DMD locus and their order on the short arm of the X chromosome were deduced from our total DMD family material and are not significantly different from those reported earlier. For 754 (DXS84) we found a genetic distance of 5 cM with a lod score of +12.4 and 95% confidence limits between 2 and 12 cM. Similar data were obtained for pERT87 (DXS164), suggesting that in our family material both loci are tightly linked. Multiply informative recombination showed that both 754 and pERT87 map proximal to the DMD mutations in the cases studied. The high frequency of DMD mutations and its relation to the observed instability in this part of the genome will be discussed. Unequal crossing over is proposed as one of the mechanisms contributing to the high mutation frequency.     

Origin of mutation in sporadic cases of severe haemophilia A in Sweden

Many newly diagnosed Swedish severe haemophilia A (HA) patients are sporadic cases. Some genotypically non‐carrier mothers have gone on to have two descendants with the same mutation, presumably because of mosaicism. Aims: To define the origin of mutation in sporadic cases of HA, reveal possible sex‐specific differences in mutagenesis and identify potential mosaics among non‐carrier mothers. Method: Sanger sequencing characterized the mutations and microsatellite haplotyping determined the origin of the X chromosome carrying the mutation in 3 generations of 45 families with sporadic severe HA. Droplet digital polymerase chain reaction (ddPCR) was used in five cases to reveal that mosaicism mutations are not found on conventional DNA sequencing. Results: In 23 out of 45 families, the mother carried the mutation and in 5 out of 28 families, the grandmother was also a carrier. The X chromosome was of grandpaternal origin in 17 out of 23 cases. One of five tested mothers was a mosaic with a mutation frequency of 7%. Conclusion: In 40 out of 45 families, the sporadic case resulted from a mutation in the last two generations. In 82% (23/28), the carrier mothers had a de novo mutation where the X chromosome was of paternal origin in 74% (17/23). ddPCR is a potentially powerful and promising analysis for mosaicism in HA.     

Familial CHARGE syndrome and the CHD7 gene: a recurrent missense mutation, intrafamilial recurrence and variability

CHARGE syndrome is an autosomal dominant condition that is caused by mutations in the CHD7 gene. Few familial cases of this syndrome have been reported and these were characterized by a wide clinical variability. We here report on five CHD7 mutation positive families and comment on their clinical features. We observed somatic and germline mosaicism as well as parent-to-child transmission of non-mosaic CHD7 mutations as causes of familial CHARGE syndrome. In one family with two affected sibs a somatic mutation was identified in lymphocytes of a clinically unaffected parent (2520G > A in exon 8). This is the second report of somatic CHD7 mosaicism in an unaffected parent. In two further families with affected siblings, we could not detect the mutation in parental lymphocytes suggesting germline mosaicism. The previously reported clinical variability was strikingly present in all five families. We find that alterations in CHD7 can result in a very mild phenotype, characterized by only a few minor symptoms of the CHARGE syndrome clinical spectrum. Such a mild phenotype was present in two families that shared the same 6322G > A missense mutation. These two families showed parent-to-child transmission. Phenotypically milder forms of CHARGE syndrome have a higher risk of transmission to multiple family members.     

Carrier detection of Duchenne/Becker muscular dystrophy by analysis of STRs loci linked to the gene of dystrophin in Venezuelan families

The Duchenne/Becker Muscular Dystrophy (DMD/BMD) is an X linked recessive lethal disease. The female carrier will transmit the disease gene to half of her sons and half of her daughters; half of the daughters will be carriers, while half will be normal. Half of the sons will be normal and, on average, half will have the disease. It is of particular relevance to be able to detect carrier status among female relatives of the patients for genetic counseling and prenatal diagnosis. The method of Short Tandem Repeat (STR) sequence polymorphism analysis can determine haplotype at normal status or at risk status and, to establish genetic linkage between the mutated gene and the segregated haplotype. We have analyzed 105 members from 15 unrelated Venezuelan families with one or more siblings affected with DMD/DMB and 7 unrelated males. Of the 105, 37 were male (26 affected and 11 normal) and 68 were female. STR sequences (STR44, STR45, STR49, STR50, STR3'DYS) of the gene of the Dystrophin were amplified by polymerase chain reaction (PCR) to analyze allelic polymorphism in the families. Five of the 15 families (33%) had a deletion of one or several of the exons. Of the 68 females, 27 (39.7%) were carriers, 27 (39.7%) were non-carriers and in 14 cases (20.58%) it was not possible to reach a definitive diagnosis. The definitive diagnosis could be established in 79% of the females. This analysis also shows that the mutation occurred on the grandpaternal X chromosome in one family. Hemizygocity was detected and carrier status ascertained in the mother of other patient and in one family we were able to do prenatal diagnosis. The germinal mosaicism could not be excluded in 3 patients.     

Mutations in von Recklinghausen neurofibromatosis: An hypothesis

Von Recklinghausen neurofibromatosis or neurofibromatosis type I (NF1) is relatively frequent (1/3,000 livebirths) autosomal dominant condition. Some unusual aspects are noted in this disorder: new mutations are frequent and almost all are of paternal origin without parental age effect. The recurrence of NF1 among children of healthy parents is rare as opposed to other dominant disorders. I propose that in NF1 (1) new mutations occur often in somatic cells or in late germinal cells, however, they occur very rarely in early germinal cells leadding to germinal mosaicism and (2) the individual with somatic mosaicism presents symptoms of the disease. Therefore, an NF1 patient with an apparent new mutation is often a somatic mosaic for the mutation and if the mosaic is alo present in germinal cells some of his children will be affected. This hypotesis may explain the unusual aspects of mutation in NF1. © 1993 Wiley-Liss, Inc.     

Different mechanisms cause imprinting defects at the IGF2/H19 locus in Beckwith-Wiedemann syndrome and Wilms' tumour

The parent of origin-dependent expression of the IGF2 and H19 genes is controlled by the imprinting centre 1 (IC1) consisting in a methylation-sensitive chromatin insulator. Deletions removing part of IC1 have been found in patients affected by the overgrowth- and tumour-associated Beckwith-Wiedemann syndrome (BWS). These mutations result in the hypermethylation of the remaining IC1 region, loss of IGF2/H19 imprinting and fully penetrant BWS phenotype when maternally transmitted. We now report that 12 additional cases with IC1 hypermethylation have a similar clinical phenotype but showed neither a detectable deletion nor other mutation in the local vicinity. Likewise, no IC1 deletion was detected in 40 sporadic non-syndromic Wilms' tumours. A detailed analysis of the BWS patients showed that the hypermethylation variably affected the IC1 region and was generally mosaic. We observed that all these cases were sporadic and in at least two families affected and unaffected members shared the same maternal IC1 allele but not the abnormal maternal chromosome epigenotype. Furthermore, the chromosome with the imprinting defect derived from either the maternal grandfather or maternal grandmother. Overall, these results indicate that methylation-imprinting defects at the IGF2-H19 locus can result from inherited mutations of the IC and have high recurrence risk or arise independently from the sequence context and generally not transmitted to the progeny. Despite these differences, the epigenetic abnormalities are usually present in the patients in the mosaic form and probably acquired by post-zygotic de novo methylation. Distinguishing between these two groups of cases is important for genetic counselling.     

Proven germline mosaicism in a father of two children with CHARGE syndrome

CHARGE syndrome is an autosomal dominant malformation syndrome caused by mutations in the CHD7 gene. The majority of cases are sporadic and only few familial cases have been reported. In these families, mosaicism in one parent, as well as parent- to-child transmission of a CHD7 mutation, has been described. In some further cases, germline mosaicism has been suggested. Here, we report the first case in which germline mosaicism could be demonstrated in a father of two affected children with CHARGE syndrome. The truncating mutation c.7302dupA in exon 34 of the CHD7 gene was found in both affected children but was not detected in parental lymphocytes. However, in DNA extracted from the father's spermatozoa, the c.7302dupA mutation could be identified. Furthermore, mutation analysis of DNA isolated from 59 single spermatozoa revealed that the c.7302dupA mutation occurs in 16 spermatozoa, confirming germline mosaicism in the father of the affected children. This result has a high impact for genetic counselling of the family and for their recurrence risk in further pregnancies.     

X-linked adrenoleukodystrophy: ABCD1 de novo mutations and mosaicism

X-linked adrenoleukodystrophy (X-ALD) is a progressive peroxisomal disorder affecting adrenal glands, testes and myelin stability that is caused by mutations in the ABCD1 (NM_000033) gene. Males with X-ALD may be diagnosed by the demonstration of elevated very long chain fatty acid (VLCFA) levels in plasma. In contrast, only 80% of female carriers have elevated plasma VLCFA; therefore targeted mutation analysis is the most effective means for carrier detection. Amongst 489 X-ALD families tested at Kennedy Krieger Institute, we identified 20 cases in which the ABCD1 mutation was de novo in the index case, indicating that the mutation arose in the maternal germ line and supporting a new mutation rate of at least 4.1% for this group. In addition, we identified 10 cases in which a de novo mutation arose in the mother or the grandmother of the index case. In two of these cases studies indicated that the mothers were low level gonosomal mosaics. In a third case biochemical, molecular and pedigree analysis indicated the mother was a gonadal mosaic. To the best of our knowledge mosaicism has not been previously reported in X-ALD. In addition, we identified one pedigree in which the maternal grandfather was mosaic for the familial ABCD1 mutation. Less than 1% of our patient population had evidence of gonadal or gonosomal mosaicism, suggesting it is a rare occurrence for this gene and its associated disorders. However, the residual maternal risk for having additional ovum carrying the mutant allele identified in an index case that appears to have a de novo mutation is at least 13%.     

Mutation analysis and prenatal diagnosis for 50 pedigrees affected with Duchenne/Becker muscular dystrophyCSCD

Objective To establish individualized prenatal diagnosis program for families affected with Duchenne/Becker muscular dystrophy (DMD/BMD) and different clinical background using a variety of methods. Methods Multiplex ligation-dependent probe amplification (MLPA) was performed on 50 patients suspected for DMD/BMD. For single exon deletions of the DMD gene, PCR was used for validating the results. For those without any deletion or duplication, Sanger sequencing was used to screen for DMD gene mutations in the children and their mothers. Prenatal genetic testing was provided to female carriers using chorionic villus, amniocentesis or cord blood samples. To ensure the accuracy of diagnosis, all prenatal specimens were also subjected to linkage analysis. Results Among the 50 patients with DMD/ BMD, 23 harbored large deletions, 11 only had single exon deletions, 10 harbored duplications, and 5 had small scare mutations. No mutation was detected in one family. For 37 women undergoing prenatal diagnosis, 10 fetuses were identified as affected males, 6 were female carriers, while 21 were not found to carry any mutation. Testing of creatine kinase was consistent with the results of prenatal diagnosis. For a patient harboring exon 51 deletion, the same mutation was found in a fetus but not in their mother. The proband and fetus had inherited the same haplotype, which suggested that the mother probably has germline mosaicism for the mutation. Conclusion Application of individualized methods for analyzing pregnant women with different clinical background can minimize the risk for giving birth to further children affected with DMD/BMD. © 2018 West China University of Medical Sciences. All rights reserved.     

Duchenne/Becker muscular dystrophy in the family: have potential carriers been tested at a molecular level?

Duchenne muscular dystrophy (DMD) is the most common inherited neuromuscular disease. After identification of the mutation in the index patient, family members can be reliably investigated. Carriers should be informed about their risk of having offspring with the disease and about their own risk for cardiomyopathy for which regular cardiac surveillance is recommended. In a small country like the Netherlands with well-organized genetic services, one would expect that most DMD families are adequately informed about the above mentioned risks for carriers. We have investigated whether women at risk had been tested at a molecular level. In the national Duchenne/Becker database 311 DMD and 99 Becker muscular dystrophy (BMD) patients had been registered up to 1 July 2009. These patients were asked to give information about the number of sisters and maternal aunts of the DMD/BMD patient and anything that was known about their genetic status and that of the mother. This information was compared with the information known at the genetic laboratory. Thirty-five of 104 adult sisters/maternal aunts of DMD patients with a 50% risk of being a carrier and 45 of 148 adult women with a 4.3% risk because of germ line mosaicism for DMD had not been tested by DNA analysis. Our study indicates that about one third of the potential carriers have not been tested. Given the possible far-reaching clinical consequences of being a carrier, further studies are needed to investigate the reasons why potential female carriers have not been tested.     

An explanation for another familial case of Rett syndrome: maternal germline mosaicism

Rett syndrome (RTT; OMIM#312750) is a severe neurodevelopmental disorder that affects mainly girls. It has an estimated incidence of 1:10,000-15,000 females. Mutations in the X-linked gene methyl CpG-binding protein 2 (MECP2) have been found in most patients. The most accepted explanation for the sex bias is that the Rett mutation in sporadic cases has its origin in the paternal germline X chromosome and can thus only be transmitted to females. The majority of cases are sporadic (99.5%) but some familial cases have been described. These cases can either be explained by germline mosaicism or by asymptomatic carrier mothers with skewing of X-inactivation towards the wild-type MECP2 allele. We describe one of the few familial cases of RTT in which a maternal germline mosaicism is the most likely explanation. The mutation p.Arg270fs (c.808delC) was identified in both a girl with classical RTT and her brother who had the severe neurological phenotype usually described in males. The mutation was absent in DNA extracted from blood of both parents. These type of events must be taken into consideration in the genetic counselling of families after the diagnosis of a first case of RTT in a female or a MECP2 mutation in a male.     

Parental gonadal but not somatic mosaicism leading to de novo NFIX variants shared by two brothers with Malan syndrome

The importance of gonadal mosaicism in families with apparently de novo mutations is being increasingly recognized. We report on two affected brothers initially suggestive of X‐linked or autosomal recessive inheritance. Malan syndrome due to shared NFIX variants was diagnosed in the brothers using exome sequencing. The boys shared the same paternal but not maternal haplotype around NFIX, and deep amplicon sequencing showed ~7% of the variant in paternal sperm but not in paternal blood and saliva. We performed review of previous cases of gonadal mosaicism, which suggests that the phenomenon is not uncommon. Gonadal mosaicism is often not accompanied by somatic mosaicism in tissues routinely used for testing, and if both types of mosaicism are present, the frequency of the variant in sperm is often higher than in somatic cells. In families with shared apparently de novo variants without evidence of parental somatic mosaicism, the transmitting parent may be determined through haplotyping of exome variants. Gonadal mosaicism has important consequences for recurrence risks and should be considered in genetic counseling in families with de novo variants.     

Mutation frequencies of X-linked mental retardation genes in families from the EuroMRX consortium

The EuroMRX family cohort consists of about 400 families with non-syndromic and 200 families with syndromic X-linked mental retardation (XLMR). After exclusion of Fragile X (Fra X) syndrome, probands from these families were tested for mutations in the coding sequence of 90 known and candidate XLMR genes. In total, 73 causative mutations were identified in 21 genes. For 42% of the families with obligate female carriers, the mental retardation phenotype could be explained by a mutation. There was no difference between families with (lod score >2) or without (lod score <2) significant linkage to the X chromosome. For families with two to five affected brothers (brother pair=BP families) only 17% of the MR could be explained. This is significantly lower (P=0.0067) than in families with obligate carrier females and indicates that the MR in about 40% (17/42) of the BP families is due to a single genetic defect on the X chromosome. The mutation frequency of XLMR genes in BP families is lower than can be expected on basis of the male to female ratio of patients with MR or observed recurrence risks. This might be explained by genetic risk factors on the X chromosome, resulting in a more complex etiology in a substantial portion of XLMR patients. The EuroMRX effort is the first attempt to unravel the molecular basis of cognitive dysfunction by large-scale approaches in a large patient cohort. Our results show that it is now possible to identify 42% of the genetic defects in non-syndromic and syndromic XLMR families with obligate female carriers.