Case report
X-linked myotubular myopathy due to a complex rearrangement involving a duplication of MTM1 exon 10

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Abstract

X-linked myotubular myopathy is a predominantly severe congenital myopathy with central nuclei on muscle biopsy due to mutations in the MTM1 gene encoding myotubularin. We report a boy with typical features of X-linked myotubular myopathy. Sequencing of the MTM1 gene did not reveal any causative mutations. Subsequent MLPA analysis identified a duplication of MTM1 exon 10 both in the patient and his mother. Additional quantitative fluorescent PCR and long-range PCR revealed an additional large deletion (2536 bp) within intron 10, 143 bp downstream of exon 10, and confirmed the duplication of exon 10. Our findings suggest that complex rearrangements have to be considered in typically affected males with X-linked myotubular myopathy.

Introduction

X-linked centronuclear (“myotubular”) myopathy (XLMTM) is a predominantly severe congenital myopathy in males characterized by numerous central nuclei on muscle biopsy (for review, [1]). XLMTM is due to hemizygous mutations in the MTM1 gene on chromosome Xq28 [2] encoding myotubularin, a dual-specificity 3-phosphoinositide phosphatase with an important role in the regulation of signalling pathways involved in growth and differentiation. Dominant mutations in the dynamin 2 (DNM2) gene [3] as well as recessive mutations in the amphiphysin 2 (BIN1) [4] and skeletal muscle ryanodine receptor (RYR1) gene [5], respectively, have been implicated in autosomal forms of centronuclear myopathy (CNM), in the majority of patients associated with milder clinical features and easily distinguishable from XLMTM. However, there is clear overlap with some patients harbouring MTM1 mutations, especially those at the milder end of the clinical spectrum.

Molecular genetic analysis of the MTM1 gene is now widely available as a routine diagnostic service and disease-causing mutations have been identified in more than 400 patients [2], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Maternal carrier state is confirmed in around 85% of affected families [8], [14] and germ cell mosaicism has been reported in several instances [8], [15], [16]. Causative MTM1 mutations include deletions/insertions, nonsense, missense and splice mutations, with approximately equal distribution of the specific mutation classes [8], [14]. Although three substitutions account for 15% of all MTM1 mutations [7], most MTM1 mutations are private or have been reported in few families only. MTM1 mutations localize most frequently (in descending order) to exons 12, 4, 11, 8 and 9 [8], [9], [10], [14], [17], [18], [19], [20], [21] but there are no clear mutational hotspots.

Although routine MTM1 molecular analysis, typically involving sequencing of all exons with inclusion of exon–intron boundaries, will detect the majority of causative MTM1 mutations, more complex rearrangements such as duplications will remain undetected applying this approach [22]. Here we report a patient with typical clinical and histopathologic features of XLMTM due to a complex rearrangement involving a duplication of MTM1.

Section snippets

Case report

This premature male infant (35 + 1 weeks gestation) presented shortly after birth with severe hypotonia and respiratory insufficiency. In the family history he was the only affected member of a healthy non-consanguineous Caucasian couple who also had two healthy daughters and one healthy son. Three years later another healthy daughter was born. There was no family history of neuromuscular or neurological disorders; his mother had eight brothers who were all healthy. On examination he was

Molecular genetic studies

A DNA sample from the patient was received and screened for mutations in the MTM1 and, subsequently, BIN1, DNM2 and RYR1 genes by routine DNA sequencing, all of which were negative. Haplotype analysis of the patient and his healthy brother showed different haplotypes around the MTM1 gene. Because of clinical and pathological features highly suggestive of XLMTM, MLPA analysis of the MTM1 and MTMR1 genes was then performed (SALSA MLPA kit P309-A1 MTM1, MRC, Holland) revealing a duplication of MTM1

Discussion

Here we reported a male infant with characteristic clinical and histopathologic findings of X-linked myotubular myopathy (XLMTM) in the context of a complex genetic background involving duplication of exon 10 of the myotubularin (MTM1) gene.

Currently no specific mechanism is hypothesised for the complex rearrangement identified in this family. Non-allelic homologous recombination (NAHR) between low copy repeats (LCRs) is proposed to be a frequent mechanism for recurrent rearrangements and

Acknowledgements

Part of this work was supported by a grant from the Guy’s and St. Thomas’ Charitable Foundation to H.J. (Grant No. 070404). The support of the National Commissioning Group (NCG) of the United Kingdom to the Dubowitz Neuromuscular Centre and Guy’s Hospital is gratefully acknowledged. F.M. is supported by the Great Ormond Street Children’s Hospital Charity.

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    1

    Current address: Regional Molecular Genetics Laboratory, Great Ormond Street Hospital, London, UK.

    2

    These authors equally contributed to this work.

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