The gene products involved in mammalian mitochondrial DNA (mtDNA) maintenance and organization remain largely unknown. We report here a novel mitochondrial protein, Twinkle, with structural similarity to phage T7 gene 4 primase/helicase and other hexameric ring helicases. Twinkle colocalizes with mtDNA in mitochondrial nucleoids. Screening of the gene encoding Twinkle in individuals with autosomal dominant progressive external ophthalmoplegia (adPEO), associated with multiple mtDNA deletions, identified 11 different coding-region mutations co-segregating with the disorder in 12 adPEO pedigrees of various ethnic origins. The mutations cluster in a region of the protein proposed to be involved in subunit interactions. The function of Twinkle is inferred to be critical for lifetime maintenance of human mtDNA integrity.
Congenital myasthenic syndromes are a group of rare genetic disorders that compromise neuromuscular transmission. A subset of these disorders, the slow-channel congenital myasthenic syndrome (SCCMS), is dominantly inherited and has been shown to involve mutations within the muscle acetylcholine receptor (AChR). We have identified three new SCCMS mutations and a further familial case of the alpha G153S mutation. Single channel recordings from wild-type and mutant human AChR expressed in Xenopus oocytes demonstrate that each mutation prolongs channel activation episodes. The novel mutations alpha V156M, alpha T254I and alpha S269I are in different functional domains of the AChR alpha subunit. Whereas alpha T254I is in the pore-lining region, like five of six previously reported SCCMS mutations, alpha S269I and alpha V156M are in extracellular domains. alpha S269I lies within the short extracellular sequence between M2 and M3, and identifies a new region of muscle AChR involved in ACh binding/channel gating. alpha V156M, although located close to alpha G153S which has been shown to increase ACh binding affinity, appears to alter channel function through a different molecular mechanism. Our results demonstrate heterogeneity in the SCCMS, indicate new regions of the AChR involved in ACh binding/channel gating and highlight the potential role of mutations outside the pore-lining regions in altering channel function in other ion channel disorders.
SCCMS mutations may show a recessive inheritance pattern and variable penetrance. A diagnosis of SCCMS should not be ruled out in cases of CMS with an apparent recessive inheritance pattern.
Congenital myasthenic syndrome comprises a heterogeneous group of inherited disorders of neuromuscular transmission. Acetylcholine receptor (AChR) deficiency is the most common form of congenital myasthenic syndrome and in most cases results from mutations within the coding region of the AChR ε subunit. However, studies in mice have established that synapse‐specific expression of AChR is dependent on a sequence contained within the AChR‐subunit promoter regions, termed an N‐box. We describe a consanguineous family in which 2 of 7 siblings had clinical and electromyographic features consistent with AChR deficiency. Muscle biopsy demonstrated low AChR numbers, establishing the disorder as postsynaptic. Single‐strand conformational polymorphism analysis identified an abnormal conformer in the AChR ε‐subunit gene promoter of the patients. DNA sequence and restriction endonuclease analysis shows that the disorder cosegregates with recessive inheritance of a single point mutation, a transition (C→T) in the N‐box of the ε‐subunit promoter. Analysis of an intercostal biopsy from 1 of the patients showed a dramatic reduction in ε‐subunit mRNA levels compared with disease and normal controls. This is the first evidence in humans that an N‐box mutation can lead to disruption of ε‐subunit transcription, resulting in the loss of adult AChR synthesis and the clinical phenotype of AChR‐deficiency congenital myasthenic syndrome. Ann Neurol 1999;45:439–443
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