RESUMEN
BACKGROUND: Given the etiologic heterogeneity of disease classification using clinical phenomenology, we employed contemporary criteria to classify variants associated with myoclonic epilepsy with ragged-red fibers (MERRF) syndrome and to assess the strength of evidence of gene-disease associations. Standardized approaches are used to clarify the definition of MERRF, which is essential for patient diagnosis, patient classification, and clinical trial design. METHODS: Systematic literature and database search with application of standardized assessment of gene-disease relationships using modified Smith criteria and of variants reported to be associated with MERRF using modified Yarham criteria. RESULTS: Review of available evidence supports a gene-disease association for two MT-tRNAs and for POLG. Using modified Smith criteria, definitive evidence of a MERRF gene-disease association is identified for MT-TK. Strong gene-disease evidence is present for MT-TL1 and POLG. Functional assays that directly associate variants with oxidative phosphorylation impairment were critical to mtDNA variant classification. In silico analysis was of limited utility to the assessment of individual MT-tRNA variants. With the use of contemporary classification criteria, several mtDNA variants previously reported as pathogenic or possibly pathogenic are reclassified as neutral variants. CONCLUSIONS: MERRF is primarily an MT-TK disease, with pathogenic variants in this gene accounting for ~90% of MERRF patients. Although MERRF is phenotypically and genotypically heterogeneous, myoclonic epilepsy is the clinical feature that distinguishes MERRF from other categories of mitochondrial disorders. Given its low frequency in mitochondrial disorders, myoclonic epilepsy is not explained simply by an impairment of cellular energetics. Although MERRF phenocopies can occur in other genes, additional data are needed to establish a MERRF disease-gene association. This approach to MERRF emphasizes standardized classification rather than clinical phenomenology, thus improving patient diagnosis and clinical trial design.
Asunto(s)
Síndrome MERRF , Humanos , Síndrome MERRF/clasificación , Síndrome MERRF/diagnóstico , Síndrome MERRF/genética , Síndrome MERRF/fisiopatologíaAsunto(s)
Epilepsias Mioclónicas/clasificación , Síndrome MERRF/clasificación , Encéfalo/patología , Epilepsias Mioclónicas/genética , Epilepsias Mioclónicas/patología , Humanos , Japón , Síndrome MERRF/genética , Síndrome MERRF/patología , Pigmentos Biológicos/análisis , Células de Purkinje/patologíaRESUMEN
The classifications of hereditary ataxias (HA) proposed from 1907 to 1984 are reviewed. An analysis is provided of the possible variables in the classification of HA, including inheritance, known metabolic or other cause, localization of pathological lesions, clinical signs, natural history, epidemiology, diagnostic tools. Harding's classification is assumed to be the best clinical tool to support molecular genetics studies. However, we suggest the inclusion of Late Onset Recessive Cerebellar Ataxias in Harding's classification. Some exceptions must be considered for the diagnostic criteria of Friedreich's disease. Early Onset Cerebellar Ataxia with retained tendon reflexes (EOCA) is probably a heterogeneous entity.