Duchenne muscular dystrophy and myotonic dystrophy in the same patient

Case Report: Whole-Exome Sequencing With MLPA Revealed Variants in Two Genes in a Patient With Combined Manifestations of Spinal Muscular Atrophy and Duchenne Muscular Dystrophy

Spinal muscular atrophy (SMA) and Duchenne muscular dystrophy (DMD) are two common kinds of neuromuscular disorders sharing various similarities in clinical manifestations. SMA is an autosomal recessive genetic disorder that results from biallelic mutations of the survival motor neuron 1 gene (SMN1; OMIM 600354) on the 5q13 chromosome. DMD is an X-linked disorder caused by defects in the DMD gene (OMIM 300377) on the X chromosome. Here, for the first time, we report a case from a Chinese family who present with clinical manifestations of both two diseases, including poor motor development and progressive muscle weakness. We identified a homozygous deletion in exons 7 and 8 of the SMN1 gene and a deletion in exon 50 of the DMD gene by whole-exome sequencing (WES) and multiplex ligation-dependent probe amplification (MLPA). This case expands our understanding of diagnosis for synchronous SMA and DMD and highlights the importance of various genetic testing methods, including WES, in differential diagnosis of neuromuscular diseases.

Disease severity in dominant Emery Dreifuss is increased by mutations in both emerin and desmin proteins

Individuals with the same genetic disorder often show remarkable differences in clinical severity, a finding generally attributed to the genetic background. We identified two patients with genetically proven Emery-Dreifuss muscular dystrophy (EDMD) who followed an unusual course and had uncommon clinicopathological findings. We hypothesized digenic inheritance and looked for additional molecular explanations. Mutations in additional separate genes were identified in both patients. The first patient was a member of a family with molecularly proven X-linked EDMD. However, the clinical features were unusually severe for this condition in the propositus: he presented at 2.5 years with severe proximal weakness and markedly elevated serum creatine kinase. Muscle weakness rapidly progressed, leading to loss of independent ambulation by the age of 12. In addition, the patient developed cardiac conduction system disease requiring pacing at the age of 11 and severe dilated cardiomyopathy in the early teens. Despite pacing, he had several syncopal episodes attributed to ventricular dysrhythmias. As these resemble the cardiac features of patients with the autosomal dominant variant of EDMD, we examined the lamin A/C gene, identifying a de-novo mutation in the propositus. The second patient had a cardioskeletal myopathy, similar to his mother who had died more than 20 years previously. Because of the dominant family history, a laminopathy was suspected and a mutation in exon 11 of the LMNA gene was identified. This mutation, however, was not present in his mother, but instead, surprisingly, was identified in his virtually asymptomatic father. Unusual accumulations of desmin found in the cardiac muscle of the propositus prompted us to examine the desmin gene in this patient, and in so doing, we identified a desmin mutation, in addition to the LMNA mutation in the propositus. These cases suggest that separate mutations in related proteins that are believed to interact, or that represent different parts of a presumed functional pathway, may synergistically contribute to disease severity in autosomal dominant EDMD. Furthermore, digenic inheritance may well contribute to the clinical severity of many other neuromuscular disorders.

Myotonic ADR-MDX mutant mice show less severe muscular dystrophy than MDX mice

In Duchenne muscular dystrophy (DMD) and its murine model, the dystrophic mouse (MDX), the skeletal musculature lacks dystrophin. The presumed function of this cytoskeletal protein is to protect the sarcolemma against mechanical stress during muscle activity. To test this hypothesis in vivo, we bred a double mutant mouse that combines two genetic defects: the dystrophin-deficiency of the MDX mouse and the Cl- channel myotonia of the arrested development of righting response (ADR) mouse. We hypothesized that high mechanical muscle activity would aggravate muscular dystrophy in double mutant ADR-MDX mice. On the contrary, ADR-MDX mice showed fewer signs of muscle fiber necrosis and fibrosis than MDX mice at all ages. Plasma creatine kinase levels were slightly increased in ADR-MDX, but significantly lower when compared to MDX mice. Sections of ADR-MDX muscle showed a uniform pattern of oxidative muscle fibers. Similar findings have been obtained in dystrophin-positive ADR mice, they result from a complete fiber-type IIB to IIA transformation in myotonic muscle. Our results suggest that small, oxidative fibers of myotonic mice are less sensitive to dystrophin deficiency. Therefore, ADR-MDX mice develop less severe muscular dystrophy than MDX mice do, although their muscles are continually stressed. The new ADR-MDX double mutant mouse is the first animal model combining both a dystrophinopathy and a channelopathy. The results presented here give new insights into the pathomechanism of muscular dystrophy and may be helpful for the therapeutic management of DMD.