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 measure in question is the proton density fat fraction, or PDFF, derived from chemical shift encoding-based water-fat MRI, specifically a six-echo three-dimensional Dixon sequence acquired on a 1.5 Tesla scanner. PDFF expresses the ratio of mobile protons belonging to triglycerides to the total mobile protons from both fat and water within a tissue, yielding a standardised percentage that reflects how much of a muscle has been replaced by fat. Unlike conventional visual scoring, PDFF is largely independent of scanner settings and acquisition parameters, which makes it attractive as a biomarker that could be compared across centres and over time. In many inherited muscle disorders, from dystrophinopathies to limb-girdle muscular dystrophies, quantitative fat mapping has already proven its worth. Mitochondrial myopathies, by contrast, had been something of a blind spot, with only scattered case reports and small series suggesting that limb muscle MRI might be uninformative, particularly in patients whose dominant symptom is progressive external ophthalmoplegia.

The new study set out to change that perception with unusually rigorous methodology. Between May 2023 and May 2025, the researchers imaged 49 adults whose diagnoses rested on pathogenic variants in mitochondrial DNA or in the nuclear genes POLG, TK2 and TWNK. Each participant underwent a dedicated MRI protocol of the pelvis, thighs and lower legs, combining conventional T1-weighted and STIR sequences with quantitative Dixon-based fat fraction mapping of the thighs. Two experienced radiologists, blinded to genotype and clinical data, independently graded the same fifteen muscles using the semi-quantitative Mercuri visual scale, a four-point system that estimates the percentage of muscle volume replaced by fat and connective tissue. In parallel, the team manually drew regions of interest on fat fraction maps at three standardised anatomical levels, quantifying individual muscle fat fractions and summing them into a composite score reflecting the overall structural burden of disease.

The first striking result was the near-perfect agreement between the old visual method and the new quantitative one. Across all analysed muscles, fat fraction values correlated strongly with Mercuri grades, with correlation coefficients exceeding 0.7 and p values below 0.00001, and intraclass correlation coefficients reached 1.00 in the smaller genotypic subgroups. This matters because it anchors the quantitative technique to a well-established clinical standard while offering something the visual scale cannot: a continuous, fine-grained variable capable of detecting subtle change. The cohort itself was deeply characterised. Patients ranged widely in age at onset, from childhood to late adulthood, with a median disease duration of 23 years, and their phenotypes spanned isolated exercise intolerance, pure progressive external ophthalmoplegia, ophthalmoplegia plus additional weakness, and progressive myopathy.

Quantitative imaging revealed a highly selective pattern of fatty replacement that had been underappreciated in mitochondrial disease. The tensor fasciae latae showed the highest median fat fraction at 31 percent, followed by the gluteus maximus at 30 percent, the sartorius at 25 percent and the gracilis at 20 percent. At the other extreme, the quadriceps components were comparatively spared, with the vastus medialis at just 11 percent and the vastus intermedius at 13 percent. These values dwarf published reference ranges for healthy adults of comparable age, in whom quadriceps fat fractions typically sit between 2 and 4 percent, indicating that the fatty infiltration observed reflects genuine disease-related degeneration rather than ordinary ageing.

Crucially, the imaging numbers tracked closely with how patients actually functioned. The summed fat fraction correlated inversely with global muscle strength on the Medical Research Council scale, with a Spearman coefficient of −0.567, and even more strongly with the North Star Ambulatory Assessment, at −0.731. Higher fat burden also predicted longer completion times on the 100-metre run test, with a positive correlation of 0.629, and showed a weaker inverse relationship with six-minute walk distance. Among circulating biomarkers, the strongest association was with serum creatinine, a surrogate of muscle mass, which fell sharply as fat fraction rose, with a coefficient of −0.715. Creatine kinase and growth differentiation factor 15, by contrast, showed no significant relationship with the imaging measure, a biologically coherent finding: creatine kinase reflects ongoing muscle damage, GDF15 signals systemic mitochondrial stress, while fat fraction captures the chronic structural endpoint of fibre loss.

Genotype left a clear fingerprint on the images. Patients with TK2 variants, which impair the replication and maintenance of mitochondrial DNA within muscle nuclei, displayed by far the most severe involvement, with median gluteus maximus fat fractions of 65 percent, semitendinosus values of 54 percent and gracilis values of 44.5 percent, alongside the lowest serum creatinine levels of any group. This quantitative signature confirms and extends earlier qualitative descriptions of late-onset TK2 deficiency as a disorder with a distinctive radiological pattern centred on the pelvic girdle and antero-medial thigh. Patients with single large-scale mitochondrial DNA deletions, who most often presented with pure external ophthalmoplegia and the earliest average age at onset, showed the mildest limb muscle involvement, while POLG and TWNK cases occupied intermediate, overlapping territory.

Perhaps the most clinically provocative finding concerned patients who appeared, by conventional standards, only mildly affected. Those presenting with exercise intolerance but no overt weakness carried significantly more muscle fat than patients with pure external ophthalmoplegia, demonstrating that quantitative MRI can expose subclinical structural damage invisible to manual strength testing. Heatmap-based hierarchical clustering and exploratory multiple correspondence analysis reinforced this picture, arranging patients along a severity gradient that mirrored both phenotype and genotype. Notably, disease duration correlated with none of the outcome measures, suggesting that the extent of fatty replacement is dictated more by the underlying genetic defect than by the simple passage of time, an argument for imaging-based rather than time-based staging in this heterogeneous population.

The authors are careful about limits. The cohort was modest, subgroups were small, the design was cross-sectional, and no prospectively imaged healthy control group was scanned on the same platform, so literature-derived normal values cannot serve as formal thresholds. Multivariable adjustment for age, sex and genotype was not feasible, and volumetric segmentation was impractical without dedicated software. Longitudinal studies will be needed to establish whether fat fraction is sensitive to change and predictive of future decline, the properties that would qualify it as a validated trial endpoint. Yet the direction of travel is unmistakable. As disease-modifying therapies for mitochondrial disorders, particularly nuclear-encoded ones, move toward the clinic, a reproducible imaging biomarker that quantifies structural muscle loss, discriminates genotypes, detects silent involvement and mirrors strength and function is exactly the tool the field has been waiting for. Quantitative muscle MRI, long dismissed as uninformative in mitochondrial disease, may now claim a central place in patient stratification and trial readiness.

Subject of Research: Quantitative muscle MRI fat fraction as an imaging biomarker of disease severity in adult mitochondrial myopathies

Article Title: Quantitative Muscle MRI Fat Fraction as a Biomarker of Disease Severity in Mitochondrial Myopathies

Article References: Bermejo‐Moriñigo, A., Martín‐Jiménez, P., González‐Méndez, V., Bermejo‐Guerrero, L., Ochoa, L. E., Martín‐Arriscado, C., Alcalá‐Galiano, A., Casado‐Pérez, C., Navarro‐Riquelme, M., Garrido‐Moraga, R., González Quintana, A., Blázquez, A., & Domínguez‐González, C. (2026). Quantitative Muscle MRI Fat Fraction as a Biomarker of Disease Severity in Mitochondrial Myopathies. Journal of Cachexia, Sarcopenia and Muscle, 17(5), Article e70380. https://doi.org/10.1002/jcsm.70380

Image Credits: AI Generated

DOI: 10.1002/jcsm.70380

Keywords: mitochondrial myopathy, muscle MRI, fat fraction, PDFF, biomarker, TK2, POLG, TWNK, mtDNA deletion, Mercuri scale, serum creatinine, clinical trials