A double mutation in a patient with X-linked myotubular myopathy

Expanding the MTM1 mutational spectrum: novel variants including the first multi-exonic duplication and development of a locus-specific database

Myotubular myopathy (MIM#310400), the X-linked form of Centronuclear myopathy (CNM) is mainly characterized by neonatal hypotonia and inability to maintain unassisted respiration. The MTM1 gene, responsible for this disease, encodes myotubularin - a lipidic phosphatase involved in vesicle trafficking regulation and maturation. Recently, it was shown that myotubularin interacts with desmin, being a major regulator of intermediate filaments. We report the development of a locus-specific database for MTM1 using the Leiden Open Variation database software (http://www.lovd.nl/MTM1), with data collated for 474 mutations identified in 472 patients (by June 2012). Among the entries are a total of 25 new mutations, including a large deletion encompassing introns 2-15. During database implementation it was noticed that no large duplications had been reported. We tested a group of eight uncharacterized CNM patients for this specific type of mutation, by multiple ligation-dependent probe amplification (MLPA) analysis. A large duplication spanning exons 1-5 was identified in a boy with a mild phenotype, with results pointing toward possible somatic mosaicism. Further characterization revealed that this duplication causes an in-frame deletion at the mRNA level (r.343_444del). Results obtained with a next generation sequencing approach suggested that the duplication extends into the neighboring MAMLD1 gene and subsequent cDNA analysis detected the presence of a MTM1/MAMLD1 fusion transcript. A complex rearrangement involving the duplication of exon 10 has since been reported, with detection also enabled by MLPA analysis. It is thus conceivable that large duplications in MTM1 may account for a number of CNM cases that have remained genetically unresolved.

Congenital myopathies at their molecular dawning

The introduction and application of molecular techniques have commenced to influence and alter the nosology of congenital myopathies. Long-known entities such as nemaline myopathies, core diseases, and desmin-related myopathies have now been found to be caused by unequivocal mutations. Several of these mutations and their genes have been identified by analyzing aggregates of proteins within muscle fibers as a morphological hallmark as in desminopathy and actinopathy, the latter a subtype among the nemaline myopathies. Immunohistochemistry has played a crucial role in recognizing this new group of protein aggregate myopathies within the spectrum of congenital myopathies. It is to be expected that other congenital myopathies marked by inclusion bodies may turn out to be such protein aggregate myopathies, depending on analysis of individual proteins within these protein aggregates and their association with putative gene mutations.

Congenital myopathies and related disorders

PURPOSE OF REVIEW

Considerable progress has been made in molecular genetic research and in identifying the underlying pathogenesis of congenital myopathies, with implications for genetic counseling. Therefore an overview of such advances in the last two years is most timely and relevant for a more precise delineation of these disorders.

RECENT FINDINGS

New mutations have been described on the ryanodine receptor gene, including the carboxyl-terminus region, and experimental models developed to explain their role in central core disease. Phenotype-genotype correlations for nemaline myopathy have improved our understanding of those related to gene mutations. In multi-minicore disease, collaborative studies support genetic heterogeneity and autosomal-recessive inheritance. Research on X-linked myotubular myopathies has revealed a high percentage of mothers of sporadic cases as carriers. Although not initially included within the congenital myopathies, desmin-related or myofibrillar myopathies are described here because they are closely related to other congenital myopathies with intracytoplasmic inclusions. Western blot for myotubularin and desmin has been proposed as a useful diagnostic test for both X-linked myotubular myopathy and desmin-related myopathy, and in-vitro and mouse models for the latter have provided insights into its pathogenesis. Several entities still await genetic characterization. Here we focus on clinical features, inheritance, and molecular genetics.

SUMMARY

Advances in immunohistochemistry and molecular genetics in congenital muscular dystrophies have enriched our knowledge of this heterogeneous group of disorders, leading to more accurate classification and differentiation between the various congenital myopathies.