Novel Splicing Mutation in MTM1 Leading to Two Abnormal Transcripts Causes Severe Myotubular Myopathy

X-linked myotubular myopathy (XLMTM) is a severe form of centronuclear myopathy, characterized by generalized weakness and respiratory insufficiency, associated with pathogenic variants in the MTM1 gene. NGS targeted sequencing on the DNA of a three-month-old child affected by XLMTM identified the novel hemizygous MTM1 c.1261-5T>G intronic variant, which interferes with the normal splicing process, generating two different abnormal transcripts simultaneously expressed in the patient's muscular cells. The first aberrant transcript, induced by the activation of a cryptic splice site in intron 11, includes four intronic nucleotides upstream of exon 12, resulting in a shift in the transcript reading frame and introducing a new premature stop codon in the catalytic domain of the protein (p.Arg421SerfsTer7). The second aberrant MTM1 transcript, due to the lack of recognition of the 3 ' acceptor splice site of intron 11 from the spliceosome complex, leads to the complete skipping of exon 12. We expanded the genotypic spectrum of XLMTM underlying the importance of intron-exons boundaries sequencing in male patients affected by XLMTM.