ClinVar

Description

The m.14709T>C variant in MT-TE has been reported in at least 39 affected individuals from 20 kindreds (PMIDs: 7726155, 7726154, 9353617, 10392369, 11437868, 15048886, 15880407, 15607216, 17161635, 17891417, 20045353, 26469001, 26885883, 29941710, 32313153, 33304817, 32948797; web story: https://www.chop.edu/stories/mitochondrial-disease-noras-story; PS4). Age of onset ranged from shortly after birth to 50s, although there are some reports of decreased fetal movements and hydrops fetalis. Features in affected individuals include myopathy, diabetes, pulmonary hypertension, ataxia, neuropathy, global developmental delay, dysarthria, hearing loss, hypertrophic cardiomyopathy, and dilated cardiomyopathy. Furthermore, this variant has been reported in some individuals with mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS)/Leigh syndrome spectrum overlap and myoclonic epilepsy with ragged red fibers (MERRF). Muscle biopsies in affected individuals showed ragged red fibers, COX deficiency, and reduced respiratory chain enzyme activities. The variant was present at variable levels in affected and unaffected individuals. Some healthy family members had the variant present at homoplasmy, however heteroplasmy levels were generally higher in more severely affected individuals. The variant was present at 78% to homoplasmy in muscle and was variable among tissues from the same individual. There are no reported de novo occurrences to our knowledge. The variant segregates with manifestations in several families (PMIDs: 7726155, 7726154, 9353617, 10392369, 11437868, 15607216, 17161635, 17891417, 20045353; PP1_moderate). The computational predictor MitoTIP suggests this variant is pathogenic (56.2 percentile) and HmtVAR predicts it to be pathogenic score of 0.9 (PP3). There is one occurrence in the GenBank population database, and the variant is absent in the Helix dataset and gnomAD v.3.1.2. Although there is one occurrence, the frequency is still low (PM2_supporting). Single fiber testing showed higher levels of heteroplasmy in COX-negative fibers (97.4 ± 3.5%; n=16) than in COX-positive fibers (89.6 ± 9.3%; n=15; P<0.004; PMID: 7726154), and cybrid studies, Northern blot analyses, and protein synthesis assays further support the functional impact of this variant (PMIDs: 15048886, 7726155, 12393175; PS3_supporting). In summary, this variant meets criteria to be classified as likely pathogenic for primary mitochondrial disease inherited in a mitochondrial manner. This classification was approved by the NICHD/NINDS U24 ClinGen Mitochondrial Disease Variant Curation Expert Panel on January 23, 2023. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PS4, PP1_moderate, PM2_supporting, PP3, PS3_supporting.