X-linked myotubular myopathy arises from pathogenic variants in MTM1, a gene that encodes myotubularin, a phosphoinositide phosphatase essential for maintaining the structural integrity of muscle fibers and organizing the triads that couple electrical signals to contraction. In its canonical form, the disorder strikes male infants with profound hypotonia and respiratory failure so severe that survival beyond the first months was historically rare. But over the past decade, large cohort studies and deep-phenotyping efforts have steadily widened the clinical lens, revealing that a meaningful subset of heterozygous females—women carrying one mutated copy of the gene on one of their two X chromosomes—develop myopathy ranging from mild limb-girdle weakness to profound, progressive disability.

The Chinese patient at the center of the new report tells a striking story of lifelong, insidious decline. Her family noticed generalized hypotonia at birth, along with poor head control and torticollis. She did not walk independently until age four, and even then her gait remained unsteady, punctuated by recurrent falls. Toe-walking appeared at age seven and gradually hardened into bilateral talipes equinovarus, a fixed deformity of the feet. Overhead arm elevation became impaired, her eyelids would not close completely, chewing grew weak, and her face developed a visible asymmetry, with the right side smaller than the left. During adolescence her motor function deteriorated steadily: she needed assistance to walk between ages fifteen and eighteen, became wheelchair-bound at nineteen, and lost the ability to stand unsupported a year before admission to the hospital.

Neurological examination at admission painted a picture of severe, asymmetric weakness concentrated in the axial and limb-girdle muscles. On the Medical Research Council scale, neck flexion scored a mere Grade 1, while proximal upper limb strength hovered around Grade 3 bilaterally. Iliopsoas strength, critical for hip flexion, was Grade 1 on both sides. Prominent involvement of the extraocular, facial, and bulbar muscles was evident, including mild restriction of bilateral eye abduction, incomplete eyelid closure, rightward tongue deviation, a high-arched palate, and masticatory weakness. Deep tendon reflexes were diminished to absent across all limbs. Notably, her serum creatine kinase—a workhorse marker of muscle damage—was entirely normal at 69 U/L, a reminder that normal CK levels do not exclude serious structural muscle disease.

Genetic testing solved the mystery. Whole-exome sequencing, performed with an Agilent SureSelect Human All Exome V6 kit on an Illumina HiSeq X platform, uncovered a heterozygous insertion in MTM1: c.1266_1267insATGGTGATAAAAACCATTCA, a 20-base-pair insertion on the X chromosome that produces a frameshift, p.(Gly423MetfsTer48). The insertion scrambles the reading frame starting at codon 423 and introduces a premature stop signal after 48 aberrant amino acids, predicted to truncate the C-terminal functional domain of myotubularin or trigger nonsense-mediated mRNA decay—either outcome eliminating the protein’s function. Sanger sequencing confirmed the variant in the proband and found it absent from the peripheral blood of her father, mother, and younger brother, consistent with a de novo event arising in her germline.

Under the 2015 American College of Medical Genetics and Genomics classification framework, the variant was classified as pathogenic on three independent grounds. The frameshift itself satisfies the PVS1 criterion for predicted loss of function; the variant is entirely absent from the gnomAD, ClinVar, and HGMD databases, satisfying PM2; and the confirmed de novo origin in an affected individual, with both parents and the unaffected brother testing negative, satisfies PS2. Together, these lines of evidence make a strong case that this single insertion is sufficient to explain the patient’s two decades of progressive weakness.

The diagnostic workup also revealed how extensively the disease had remodeled her body. Pulmonary function testing demonstrated moderate-to-severe restrictive ventilatory impairment, with forced vital capacity at just 53.12 percent of predicted, alongside small airway dysfunction and reduced diffusion capacity—findings that establish respiratory muscle involvement as a central feature of her disease rather than a late complication. Electromyography showed a myopathic pattern, while nerve conduction studies suggested an axonal sensorimotor peripheral neuropathy predominantly affecting the lower limbs. Muscle MRI documented chronic myopathic change with marked atrophy and extensive fatty replacement: the deltoid, biceps, and triceps of the left arm were severely infiltrated with fat, the thighs showed asymmetric involvement far worse on the left, and in the calves the left tibialis anterior and peroneal muscles were nearly completely replaced by fat while the soleus and gastrocnemius were only mildly affected.

Muscle biopsy added the histological signature. Stained sections revealed marked variation in fiber size with numerous centrally located nuclei—the hallmark of centronuclear myopathy—along with endomysial hyperplasia and compensatory hypertrophic fibers, though the necklace-fiber pattern sometimes seen in MTM1-related disease was absent. Transmission electron microscopy showed largely preserved myofibrillar architecture with focal sarcomeric disruption and accumulations of lipid droplets beneath the sarcolemma and between myofibrils, without nemaline rods, tubular aggregates, inflammation, or significant fibrosis. This dissociation between widespread radiological involvement and clinically dominant unilateral weakness is thought to arise from skewed X-chromosome inactivation: the proportion of cells expressing the mutant MTM1 copy varies independently across muscle groups, producing patchy functional impairment even when imaging shows diffuse degeneration.

The case carries practical implications beyond the genetics. Because MTM1 is expressed not only in muscle but also in peripheral nerves and spinal neurons, pathogenic variants can disrupt phosphoinositide metabolism and axonal transport, driving primary axonal degeneration—explaining the patient’s areflexia and neuropathy despite her lack of sensory complaints. The authors argue that routine nerve conduction testing should be arranged for symptomatic female carriers. Equally important is respiratory surveillance: consistent with American College of Chest Physicians guidance for neuromuscular weakness, the findings support serial pulmonary function testing and timely initiation of noninvasive ventilation when indicated, since respiratory decline is easily under-recognized in slowly progressive phenotypes.

Finally, the report sounds a cautionary note for genetic counseling. Although the variant’s absence from parental blood supports a de novo origin, parental germline mosaicism or low-level mosaicism cannot be excluded without high-depth or multi-tissue testing, and recent genome-wide phasing studies show that standard testing can miss such mosaicism with real consequences for recurrence-risk estimates. The authors also acknowledge limitations, including a single pulmonary function assessment without longitudinal follow-up and the absence of standardized MRI scoring. Even so, the case expands the known mutational spectrum of MTM1 and reinforces a message increasingly echoed across neuromuscular medicine: women presenting with asymmetric limb-girdle weakness, craniofacial and ocular involvement, and restrictive lung disease—even with normal creatine kinase—deserve MTM1 on the differential diagnosis, and gene replacement therapies now in development make precise molecular diagnosis more consequential than ever.

Subject of Research: A novel de novo MTM1 frameshift variant causing X-linked myotubular myopathy in a symptomatic female

Article Title: A Novel De Novo MTM1 Insertion Frameshift Variant Causes X‐Linked Myotubular Myopathy in a Chinese Female

Article References: Chen, L., Bao, Y., & Liu, G. (2026). A Novel De Novo MTM1 Insertion Frameshift Variant Causes X‐Linked Myotubular Myopathy in a Chinese Female. Molecular Genetics & Genomic Medicine, 14(10), Article e70318. https://doi.org/10.1002/mgg3.70318

Image Credits: AI Generated

DOI: 10.1002/mgg3.70318

Keywords: MTM1, X-linked myotubular myopathy, centronuclear myopathy, frameshift variant, de novo mutation, myotubularin, skewed X-inactivation, muscle MRI, restrictive ventilatory impairment, whole-exome sequencing, genetic counseling, female carriers

The research, led by Çağrı Doğan of the Department of Medical Genetics at Ordu University Training and Research Hospital together with pediatric neurologist Seren Aydın of Ondokuz Mayıs University Hospital, examined every consecutive individual tested for DMD variants at their centre between January 2022 and February 2026. Forty-seven individuals passed through the diagnostic pipeline, 29 female and 18 male, with a median age of 12.8 years. The team applied two complementary molecular techniques: multiplex ligation-dependent probe amplification, or MLPA, a copy-number detection method available for 45 of the 47 participants, and whole-gene sequencing, performed in 16 individuals and run in parallel with MLPA in 13. All variants were described against the reference transcript NM_004006.3 and classified according to the 2015 ACMG/AMP standards for sequence variants and the 2020 ACMG/ClinGen standards for copy-number variants.

The headline result is deceptively simple: pathogenic variants were identified in 19 of the 47 individuals. Ten of these were probands, the first affected or index individuals in their families to receive a molecular diagnosis, and nine were heterozygous female carriers discovered through cascade screening, the systematic testing of relatives once a familial variant is known. In index testing, the diagnostic yield was 30.3 percent, or ten of 33 individuals tested, with a 95 percent confidence interval of 17.4 to 47.3 percent. Once a familial variant was established, cascade screening detected carriers in nine of 14 relatives tested, a carrier-detection rate of 64.3 percent with a confidence interval of 38.8 to 83.7 percent. Those figures underline a point that geneticists have long emphasised: identifying one patient can protect an entire extended family.

The variant spectrum itself conformed closely to what has been reported across Türkiye and worldwide. Among the nine families with characterised variants, seven carried deletions and two carried single-nucleotide sequence variants: a nonsense change, NM_004006.3:c.701C>G producing p.(Ser234Ter), and a canonical splice-site variant, c.2803+1G>T, which disrupts the invariant guanine at the first position of an intron and is predicted to derail RNA splicing. Strikingly, six of the nine families carried deletions within the so-called distal hotspot spanning exons 45 to 55, a known deletion-prone region of this extraordinarily large gene. Three of the deletions were in-frame, meaning the reading frame of the dystrophin protein was preserved, and four were out-of-frame, consistent with the classical reading-frame rule that links out-of-frame disruptions to the severe Duchenne phenotype and in-frame disruptions to the milder Becker form. Each deletion was cross-verified against public resources, the Leiden Open Variation Database and the DOVE deletion-out-of-frame evaluation tool.

The study also captured a subtler and less frequently discussed facet of dystrophinopathy: female carriers with biochemical evidence of muscle damage. Four heterozygous women, three of whom were probands in their own right rather than relatives of an affected boy, presented with unexplained hyperCKemia, with creatine kinase values ranging from 776 to 2,816 U/L, but no documented muscle weakness. The one carrier who underwent complete evaluation had normal strength and a normal echocardiogram. This pattern reflects the stochastic nature of X-chromosome inactivation, the process by which female cells randomly silence one X chromosome. When inactivation happens to favour the chromosome carrying the mutant DMD allele, enough dystrophin-deficient muscle fibres can exist to leak creatine kinase into the blood, occasionally producing overt disease in women, including cardiomyopathy, decades before anyone suspects a condition classically associated with boys.

Two of the deletions were apparently de novo, arising fresh in the affected child rather than being inherited from a carrier mother, a well-recognised phenomenon in DMD given the enormous size of the gene and its high spontaneous mutation rate. Equally consequential for treatment planning, none of the probands in this series carried a variant amenable to an approved exon-skipping oligonucleotide. These antisense drugs, including eteplirsen, golodirsen, viltolarsen and casimersen, work by masking specific exons during RNA splicing so that the reading frame is restored and a shorter but partly functional dystrophin protein can be made. Eligibility depends entirely on the exact exon boundaries of the deletion or point mutation, which is why a precise molecular diagnosis is not merely academic but the gateway to genotype-matched therapy.

Technically, the study illustrates the standard two-step architecture of DMD molecular diagnostics. MLPA, first described by Schouten and colleagues in 2002, uses pairs of probes that ligate only when bound to their target sequences and are then amplified, allowing relative quantification of all 79 exons of the DMD gene in a single reaction; it remains the most efficient way to detect the exon deletions and duplications that account for the majority of pathogenic DMD variants. When MLPA is negative, sequencing of the gene captures the point mutations, small insertions and deletions, and splice-site changes that copy-number assays cannot see. Running both methods in parallel, as this centre did in 13 cases, accelerates turnaround. The authors’ adherence to Human Genome Variation Society nomenclature and to consensus ACMG classification frameworks ensures that their variants can be compared directly with entries in international databases such as the TREAT-NMD global mutation registry, which has catalogued more than 7,000 DMD mutations.

The regional significance of the work lies in establishing a baseline. Previous Turkish series, including a 2019 analysis of 260 dystrophinopathy patients and later single-centre surveys, had sketched the national mutation spectrum, but the Central-Eastern Black Sea provinces had remained a blank spot on that map. The new data suggest that the local spectrum is broadly congruent with national and global patterns, dominated by deletions concentrated in the distal hotspot. That congruence matters for genetic counselling: it means clinicians in the region can reasonably apply the established genotype-phenotype correlations and exon-skipping eligibility frameworks without needing region-specific correction, while still sequencing each family’s variant to confirm its exact boundaries.

The authors are careful about the limits of what such a small series can prove. With only ten probands, the reported proportions are descriptive rather than prevalence estimates, and genotype-based phenotype assignment in individual patients requires prospective confirmation with longitudinal follow-up. The retrospective, single-centre design also means the cohort reflects referral patterns and test availability at one hospital rather than systematic population screening. Still, the cascade-screening outcome, nine additional heterozygotes identified from fourteen relatives, is a concrete demonstration of why molecular diagnosis should trigger immediate family testing. Every carrier identified can be offered cardiac surveillance, reproductive counselling including prenatal and preimplantation genetic options, and the knowledge that ultimately saves the next generation of boys in the family from a delayed diagnosis.

For a disease in which early corticosteroid treatment, standardised cardiac and respiratory care, and emerging gene-targeted therapies all depend on catching patients early, the message from Türkiye’s Black Sea coast is clear: find the variant in one child, and the laboratory work has only just begun. The first regional map of DMD variants is now drawn, and its most important lines may be the family trees radiating outward from each proband.

Subject of Research: Mutational spectrum of DMD gene variants and cascade screening for Duchenne and Becker muscular dystrophy in Türkiye

Article Title: Mutational Spectrum of DMD Gene Variants and Cascade Screening Outcomes in the Central-Eastern Black Sea Region of Türkiye: A Single-Centre Retrospective Study

Article References: Doğan, Ç., & Aydın, S. (2026). Mutational Spectrum of DMD Gene Variants and Cascade Screening Outcomes in the Central-Eastern Black Sea Region of Türkiye: A Single-Centre Retrospective Study. Molecular Biology Reports, 53(1), Article 1600. https://doi.org/10.1007/s11033-026-12785-9

Image Credits: AI Generated

DOI: 10.1007/s11033-026-12785-9

Keywords: Duchenne muscular dystrophy, DMD gene, Becker muscular dystrophy, cascade screening, MLPA, female carriers, exon deletions, genetic testing, dystrophinopathy, hyperCKemia, Türkiye, X-chromosome inactivation