Somatic mosaicism for a deletion of the dystrophin gene in a carrier of Becker muscular dystrophy

A retrospective cohort study and review of the literature about germline mosaicism in Duchenne/Becker muscular dystrophy prenatal counseling: How to estimate the recurrence risk in clinical settings?

Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) are the most common inherited neuromuscular diseases. Following the identification of a pathogenic causative variant in the DMD gene of a proband, potential carriers can be informed of their risk of having offspring with the disease. Germline mosaicism is a variant that is confined to the gonads that can be transmitted to offspring and is usually reported when a non-carrier of a DMD pathogenic variant has two or more offspring carrying the variant in question. On average, one third of cases are the result of a de novo variant, and as DMD and BMD are prone to germline mosaicism, its inclusion in genetic counseling is mandatory. In this retrospective cohort study, we presented clinical data from an unpublished DMD/BMD cohort of 332 families with incidence of germline mosaicism in families with de novo transmission of 8.1%. This is also the first systematic literature review searching PubMed to provide an accurate assessment of the current literature on germline mosaicism in DMD and BMD, including 17 case reports and 20 original studies. The incidence of documented germline mosaicism in de novo event families ranged from 6.0 to 40%, with a mean of 8.3%. The estimated recurrence risk for mothers of a patient with a proven de novo causal variant ranged from 4.3 to 11%, with a mean of 5.8% for a male fetus. By providing an up-to-date and comprehensive overview of the literature, this review aims to improve our understanding of germline mosaicism in DMD and to promote the development of effective strategies and reliable data for occurrence risk assessment in genetic counseling of de novo event families.

Modeling Duchenne Muscular Dystrophy Cardiomyopathy with Patients' Induced Pluripotent Stem-Cell-Derived Cardiomyocytes

Duchenne muscular dystrophy (DMD) is an X-linked progressive muscle degenerative disease caused by mutations in the dystrophin gene, resulting in death by the end of the third decade of life at the latest. A key aspect of the DMD clinical phenotype is dilated cardiomyopathy, affecting virtually all patients by the end of the second decade of life. Furthermore, despite respiratory complications still being the leading cause of death, with advancements in medical care in recent years, cardiac involvement has become an increasing cause of mortality. Over the years, extensive research has been conducted using different DMD animal models, including the mdx mouse. While these models present certain important similarities to human DMD patients, they also have some differences which pose a challenge to researchers. The development of somatic cell reprograming technology has enabled generation of human induced pluripotent stem cells (hiPSCs) which can be differentiated into different cell types. This technology provides a potentially endless pool of human cells for research. Furthermore, hiPSCs can be generated from patients, thus providing patient-specific cells and enabling research tailored to different mutations. DMD cardiac involvement has been shown in animal models to include changes in gene expression of different proteins, abnormal cellular Ca²⁺ handling, and other aberrations. To gain a better understanding of the disease mechanisms, it is imperative to validate these findings in human cells. Furthermore, with the recent advancements in gene-editing technology, hiPSCs provide a valuable platform for research and development of new therapies including the possibility of regenerative medicine. In this article, we review the DMD cardiac-related research performed so far using human hiPSCs-derived cardiomyocytes (hiPSC-CMs) carrying DMD mutations.

Prenatal diagnosis of Duchenne muscular dystrophy revealed a novel mosaic mutation in Dystrophin gene: a case report

BACKGROUND

Duchenne muscular dystrophies (DMDs) are X-linked recessive neuromuscular disorders with malfunction or absence of the Dystrophin protein. Precise genetic diagnosis is critical for proper planning of patient care and treatment. In this study, we described a Chinese family with mosaic DMD mutations and discussed the best method for prenatal diagnosis and genetic counseling of X-linked familial disorders.

METHODS

We investigated all variants of the whole dystrophin gene using multiple DNA samples isolated from the affected family and identified two variants of the DMD gene in a sick boy and two female carriers by targeted next generation sequencing (TNGS), Sanger sequencing, and haplotype analysis.

RESULTS

We identified the hemizygous mutation c.6794delG (p.G2265Efs*6) of DMD in the sick boy, which was inherited from his mother. Unexpectedly, a novel heterozygous mutation c.6796delA (p.I2266Ffs*5) of the same gene, which was considered to be a de novo variant, was detected from his younger sister instead of his mother by Sanger sequencing. However, further NGS analysis of the mother and her amniotic fluid samples revealed that the mother carried a low-level mosaic c.6796delA mutation.

CONCLUSIONS

We reported two different mutations of the DMD gene in two siblings, including the novel mutation c.6796delA (p.I2266Ffs*5) inherited from the asymptomatic mosaic-carrier mother. This finding has enriched the knowledge of the pathogenesis of DMD. If no mutation is detected in obligate carriers, the administration of intricate STR/NGS/Sanger analysis will provide new ideas on the prenatal diagnosis of DMD.

MLPA analysis for the detection of deletions, duplications and complex rearrangements in the dystrophin gene: potential and pitfalls

Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) are common X-chromosomal recessive disorders caused by mutations in the dystrophin gene. Using the novel multiplex ligation-dependent probe amplification (MLPA) method we performed retrospective and prospective analyses in a total of 193 individuals. Deletions or duplications were identified in 14 out of 90 families previously tested negative by multiplex PCR or FISH analysis. Partially incorrect results were subsequently identified in two families: the loss of exon 38 signal in one case was due to a p.Q1802X nonsense mutation, whilst in another patient an apparent deletion of exon 37 (coinciding with a duplication of exons 46-53) was caused by a p.R1735C polymorphism. In one case we found a complex rearrangement involving a duplication of two regions: dupEX45-48 and dupEX54-55. We conclude that MLPA is a highly sensitive and rapid alternative to multiplex PCR. It can be used on blood samples, chorionic villi and paraffin-embedded tissue. The ease of detection of duplications and the application for female carrier analysis are clearly the main advantages of the method. However, apparent single exon deletions detected by MLPA should be checked by an independent method. Complex rearrangements such as double mutations on the same allele are rare.

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.

The clinical and molecular genetic approach to Duchenne and Becker muscular dystrophy: an updated protocol

Detection of large rearrangements in the dystrophin gene in Duchenne and Becker muscular dystrophy is possible in about 65-70% of patients by Southern blotting or multiplex PCR. Subsequently, carrier detection is possible by assessing the intensity of relevant bands, but preferably by a non-quantitative test method. Detection of microlesions in Duchenne and Becker muscular dystrophy is currently under way. Single strand conformational analysis, heteroduplex analysis, and the protein truncation test are mostly used for this purpose. In this paper we review the available methods for detection of large and small mutations in patients and in carriers and propose a systematic approach for genetic analysis and genetic counselling of DMD and BMD families, including prenatal and preimplantation diagnosis.

Germline and somatic mosaicism in a female carrier of Hunter disease

Carrier detection in a mucopolysaccharidosis type II family (Hunter disease) allowed the identification of germline and somatic mosaicism in the patient's mother: the R443X mutation was found in a varying proportion in tested tissue (7% in leucocytes, lymphocytes, and lymphoblastoid cells, and 22% in fibroblasts). The proband's sister carries the at risk allele (determined by haplotype analysis), but not the mutation. In sporadic cases of X linked diseases, germline mosaicism of the proband's mother is difficult to exclude and should be considered in genetic counselling.

Mosaicism for del(17)(p11.2p11.2) underlying the Smith-Magenis syndrome

Smith-Magenis syndrome (SMS) is a multiple congenital anomalies/mental retardation syndrome associated with deletion of band p11.2 of chromosome 17. The deletion is typically detected by high-resolution cytogenetic analysis of chromosomes from peripheral lymphocytes. Fluorescence in situ hybridization (FISH) has been previously used to rule out apparent mosaicism for del(17)(p11.2p11.2) indicated by routine cytogenetics. We now report mosaicism for del(17)(p11.2p11.2) in a child with SMS. The mosaicism had gone undetected during previous routine cytogenetic analysis. FISH analysis of peripheral lymphocytes as well as immortalized lymphoblasts using markers from 17p11.2 revealed that approximately 60% of cells carried the deletion. To our knowledge, this is the first case of SMS associated with mosaicism for del(17)(p11.2p11.2).

Polymerase chain reaction fiber analysis and somatic mosaicism in autopsied tissue from a man with Duchenne muscular dystrophy

Single muscle fibers, obtained at autopsy from a 22-year-old man with Duchenne muscular dystrophy were examined immunocytochemically and also using polymerase chain reaction (PCR). Dystrophin-positive cells were widespread in skeletal, cardiac, smooth muscle, and in brain cells. PCR and Southern blot analyses of DNA from peripheral blood lymphocytes revealed a deletion of exon 45 in the dystrophin gene. With PCR of single fibers, three bands corresponding to exons 44, 45, and 47 were present in the normal control muscle fibers and dystrophin-positive fibers from the patient, while only two bands, exons 44 and 47, were observed in dystrophin-negative fibers. Therefore, in this patient, the genotype of dystrophin-positive fibers differed from that of the dystrophin-negative fibers, possibly because of a somatic mosaicism for deletion in the dystrophin gene. A mutation of the dystrophin gene may have occurred in one cell at an early stage of ontogenesis.

Somatic mosaicism for a DMD gene deletion

Mosaicism is a mixed state, with two cell populations of different genetic origins caused by a cell mutation occurring after fertilization. In the present case, DNA analysis of lymphocytes led to a DMD diagnosis before death. Postmortem immunocytochemical and DNA analysis showed somatic mosaicism. At age 18 years, blood lymphocyte DNA analysis showed a DMD gene deletion, upstream from exon 7 to the 5' end containing both muscle and brain promoters. As the patient's mother and elder sister had no deletions, he was considered to have a new mutation. Immunocytochemical studies of postmortem tissues showed that dystrophin was absent from the tongue, deltoid, intercostal, psoas and rectus femoris muscles, but there was a mix of dystrophin-positive and negative fibers in the rectus abdominis, cardiac, temporalis and sternocleidomastoid muscles. All diaphragm cells were dystrophin positive. Polymerase chain reaction (PCR) amplification from all tissues except the temporalis and sternocleidomastoid muscles, diaphragm and kidney, in which no deletion was found, showed the deletion from at least exon 6 to the 5' end containing both muscle and brain promoters. In this case, a genomic deletion of the DMD gene contributed to the formation of tissues derived from both ectoderm and endoderm, and cells of mesodermal origin showed genotypic and phenotypic heterogeneity. Our results indicate a mutation of the present case may have occurred just before the period of germ layer formation.