Novel De Novo MTM1 Frameshift Variant Drives Severe Muscle Disease in a Young Chinese Woman

A rare genetic detective story unfolding in China is reshaping how scientists think about a disease long considered an affliction of infant boys. Researchers at the Second Affiliated Hospital of Zhejiang University School of Medicine have identified a brand-new mutation in the MTM1 gene in a 24-year-old woman whose muscles have been slowly failing since birth. The finding, published in Molecular Genetics & Genomic Medicine, marks the first report of this specific variant and adds compelling evidence that X-linked myotubular myopathy, classically a devastating neonatal condition in males, can emerge in females through de novo mutations with no family history whatsoever.

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