Somatic mosaicism for a DMD gene deletion

A Comprehensive Analysis of 2013 Dystrophinopathies in China: A Report From National Rare Disease Center

Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) are X-linked recessive neuromuscular disorders caused by mutations in DMD. A high-quality database of DMD/BMD is essential not only for clinical practice but also for fundamental research. Here, we aimed to build the largest Chinese national dystrophinopathy database using the National Rare Diseases Registry System of China. Peking Union Medical College Hospital (PUMCH) was the National Rare Diseases Center of China. This research involved 2013 patients with dystrophinopathies, whose diagnoses were confirmed; they were registered and followed up at PUMCH from March 2011 to December 2018. Family history, clinical signs, and treatment data were reported for patients with DMD and BMD at different rates. All six serum biochemical indexes could accurately distinguish between DMD and BMD patients. Copy number variations were the most frequent mutation type (79.2% in DMD and 84.3% in BMD), of which large deletions accounted for 88.4 and 88.6%, large duplications accounted for 11.6 and 11.4% in DMD and BMD, respectively. An exon deletion hotspot, located in exons 45-54, was observed in DMD, and intron 44 was the most frequent deletion starting point (26.5%). Duplication and single nucleotide variations appeared to be uniformly distributed among all exons. Eleven patients were identified to have ultrarare mutation types. Eleven other patients suffered from two separate mutations simultaneously, some of which may have taken place via dependent mechanisms. Thus, we have established the largest hospital-based Chinese dystrophinopathy database via the National Rare Diseases Registry System. This study provides valuable information for further diagnostic and therapeutic studies of dystrophinopathy.

Isolated cardiomyopathy caused by a DMD nonsense mutation in somatic mosaicism: genetic normalization in skeletal muscle

X-linked dilated cardiomyopathy is a pure cardiac dystrophinopathy phenotype mainly caused by DMD mutations that present a specific transcription effect in cardiac tissue. We report a 26-year-old male who presented with severe dilated cardiomyopathy and high creatine kinase. The patient did not complain of skeletal muscle weakness. A muscle biopsy showed mild dystrophic changes and a low proportion of dystrophin-negative fibres. A molecular study identified a nonsense DMD mutation (p.Arg2098X) in somatic mosaicism. The ratio of mutant versus normal allele in blood and skeletal muscle suggests selective pressure against mutant muscle cells, a process known as genetic normalization. We hypothesize that this process may have mitigated skeletal muscle symptoms in this patient. This is the second report of a DMD somatic mosaic with evidence of genetic normalization in muscle. Somatic DMD mutations should be considered in patients presenting with idiopathic dilated cardiomyopathy.

Somatic mosaicism for Duchenne dystrophy: evidence for genetic normalization mitigating muscle symptoms

We describe a young adult male presenting with cardiac failure necessitating cardiac transplantation 7 months after presentation. Skeletal muscle biopsy showed mosaic immunostaining for dystrophin. DNA studies showed somatic mosaicism for a nonsense mutation in the dystrophin gene (Arg2905X). The frequency of normal versus mutant genes were determined in blood/DNA (50:50), muscle/DNA (80:20) and muscle/mRNA (90:10). These data are consistent with genetic normalization processes that may biochemically rescue skeletal muscle in male somatic mosaic patients mitigating muscle symptoms (gradual loss of dystrophin-negative skeletal muscle tissue replaced by dystrophin-positive stem cells). To our knowledge, this is only the second reported case of a clinically ascertained patient showing somatic mosaicism for Duchenne muscular dystrophy (DMD). We hypothesize that many somatic mosaic males for DMD exist, yet they are not detected clinically due to genetic normalization. Somatic mosaicism for DMD should be considered in acute heart failure with dilated cardiomyopathy, as genetic normalization in heart is unlikely to occur.

Somatic mosaicism of a point mutation in the dystrophin gene in a patient presenting with an asymmetrical muscle weakness and contractures

We describe a patient with somatic mosaicism of a point mutation in the dystrophin gene causing benign muscular dystrophy with an unusual asymmetrical distribution of muscle weakness and contractures. To our knowledge this is the first patient with asymmetrical weakness and contractures in an ambulatory patient with a dystrophinopathy.

Clinical and molecular spectrum of somatic mosaicism in androgen insensitivity syndrome

We recently found that postzygotic de novo mutations occur at the expected high rate of an X-linked recessive mutation in androgen insensitivity syndrome. The resulting somatic mosaicism can be an important molecular determinant of in vivo androgen action caused by expression of the wild-type androgen receptor (AR). However, the clinical relevance of this previously underestimated genetic condition in androgen insensitivity syndrome has not been investigated in detail as yet. Here, we present the clinical and molecular spectrum of somatic mosaicism considering all five patients with mosaic androgen insensitivity syndrome, whom we have identified since 1993: Patient 1 (predominantly female, clitoromegaly), 172 TTA(Leu)/TGA(Stop); patient 2 (ambiguous), 596 GCC(Ala)/ACC(Thr); patient 3 (ambiguous), 733 CAG(Gln)/ CAT(His); patient 4 (completely female), 774 CGC(Arg)/TGC (Cys); and patient 5 (ambiguous), 866 GTG(Val)/ATG(Met). Serum sex hormone binding globulin response to stanozolol, usually correlating well with in vivo AR function, was inconclusive for assessment of the phenotypes in all tested mosaic individuals. An unexpectedly strong virilization occurred in patients 1, 3, and 5 compared with phenotypes as published with corresponding inherited mutations and compared with the markedly impaired transactivation caused by the mutant ARs in cotransfection experiments. Only the prepubertal virilization of patients 2 and 4 matched appropriately with transactivation studies (patient 4) or the literature (patients 2 and 4). However, partial pubertal virilization in patient 4 caused by increasing serum androgens and subsequent activation of the wild-type AR could not be excluded. We conclude that somatic mosaicism is of particular clinical relevance in androgen insensitivity syndrome. The possibility of functionally relevant expression of the wild-type AR needs to be considered in all mosaic individuals, and treatment should be adjusted accordingly.

Somatic mosaicism: a common cause of classic disease in tumor-prone syndromes? Lessons from type 2 neurofibromatosis

Blood samples from 125 families with classic type 2 neurofibromatosis with bilateral vestibular schwannomas were analyzed for mutations in the NF2 gene. Causative mutations were identified in 52 families. In five families, the first affected individual in the family (the index case) was a mosaic for a disease-causing mutation. Only one of nine children from the three mosaic cases with children are affected. Four of these nine children inherited the allele associated with the disease-causing mutation yet did not inherit the mutation. NF2 mutations were identified in only 27/79 (34%) of sporadic cases, compared with 25/46 (54%) of familial cases (P<.05). In 48 families in which a mutation has not been identified, the index cases have had 125 children, of whom only 29 are affected with NF2 and of whom only a further 21 cases would be predicted to be affected by use of life curves. The 50/125 (40%) of cases is significantly less than the 50% expected eventually to develop NF2 (P<.05). Somatic mosaicism is likely to be a common cause of classic NF2 and may well account for a low detection rate for mutations in sporadic cases. Degrees of gonosomal mosaicism mean that recurrence risks may well be <50% in the index case when a mutation is not identified in lymphocyte DNA.

The relative levels of different types of beta-mRNA and beta-globin in BFU-E derived colonies from patients with beta chain variants; further evidence for somatic mosaicism in the Hb Costa Rica carrier [beta 77(EF1)His-->Arg]

We have identified and quantitated the different types of mRNA in single BFU-E derived colonies from Hb S and Hb Atlanta [beta 75 (E19)Leu-->Pro] heterozygotes and observed that the normal and mutated mRNAs were present in equal quantities. Similar studies for the different protein products gave less accurate data because high performance liquid chromatography methods were not sensitive enough for the analysis of a single colony, and as many as five colonies needed to be combined. The level of Hb S (approximately 40%) was the same as in red cell lysates of the Hb S heterozygote, while that of the unstable beta-Atlanta chain was lower than expected from the values observed in red cells. Similar studies for a carrier of the stable Hb Costa Rica [beta 77(EF1) His-->Arg] which was reported to be the result of a somatic cell mutation (1) gave quite different data. Dot-blot analysis with 32P-labeled probes and allele specific amplification methodology identified numerous colonies with beta A-mRNA only, while 12-15% of the colonies contained both beta A- and beta-Costa Rica-mRNA. This limited distribution of the beta-Costa Rica-mRNA was confirmed by hemoglobin analysis with anion exchange high performance liquid chromatography. These results are considered to provide additional and convincing evidence for a somatic mosaicism for the CAC-->CGC mutation at codon 77 of the beta gene which occurred during the development of the embryo and results in the presence of only some 6-8% of the abnormal Hb Costa Rica in the circulating red cells.