Large duplication in MTM1 associated with myotubular myopathy

Abstract

Myotubular myopathy is a subtype of centronuclear myopathy with X-linked inheritance and distinctive clinical and pathologic features. Most boys with myotubular myopathy have MTM1 mutations. In remaining individuals, it is not clear if disease is due to an undetected alteration in MTM1 or mutation of another gene. We describe a boy with myotubular myopathy but without mutation in MTM1 by conventional sequencing. Array-CGH analysis of MTM1 uncovered a large MTM1 duplication. This finding suggests that at least some unresolved cases of myotubular myopathy are due to duplications in MTM1, and that array-CGH should be considered when MTM1 sequencing is unrevealing.

Fig. 1

Pathological findings on muscle biopsy. (A) Hematoxylin and eosin staining of frozen muscle tissue reveals small round myofibers and numerous fibers containing large, centrally placed nuclei. Central areas with clear or basophilic staining are present within many fibers, which contain aggregates of mitochondria and other organelles, and are best visualized on NADH staining (B). Bar = 50 mm.

Fig. 2

Array comparative genomic hybridization (aCGH) results for MTM1. Deletion and duplication testing of the MTM1 gene locus was performed using a custom designed 8 × 60 K array-CGH platform (Agilent Technologies) that contained probes for 45 genes including MTM1. Analysis of genomic DNA of the affected male patient and his mother revealed a duplication of the MTM1 gene involving exons 1, 3–13 and no duplication of exons 2, 14 and 15. Regions of MTM1 copy number gains are highlighted in purple and evidenced by the elevated log2 fluorescence ratios. Each dot shows a log2 ratio of the hybridization signals of patient versus gender-match control for each probe in the MTM1 gene. The positions of the MTM1 exons are indicated in red above each aCGH chart. The x and y axes represent genomic coordinates and the log2 hybridization signals, respectively.

Fig. 3

SNP array results for MTM1 duplication. SNP array was performed to define the proximal end of the MTM1 gene duplication using the Affymetrix CytoScan HD Array (Affymetrix, Santa Clara, CA). Analysis using the ChAS software showed a 356 kb duplication that includes the MTM1 and MAMLD1 genes in the patient’s mother, and was detected as an increase in the weighted log2 ratio and copy number state. Duplicated probes are shown within the green box, while probes in this region with a normal copy number are shown within the blue box.