RESUMO
XRCC1 is a molecular scaffold protein that assembles multi-protein complexes involved in DNA single-strand break repair. Here we show that biallelic mutations in the human XRCC1 gene are associated with ocular motor apraxia, axonal neuropathy, and progressive cerebellar ataxia. Cells from a patient with mutations in XRCC1 exhibited not only reduced rates of single-strand break repair but also elevated levels of protein ADP-ribosylation. This latter phenotype is recapitulated in a related syndrome caused by mutations in the XRCC1 partner protein PNKP and implicates hyperactivation of poly(ADP-ribose) polymerase/s as a cause of cerebellar ataxia. Indeed, remarkably, genetic deletion of Parp1 rescued normal cerebellar ADP-ribose levels and reduced the loss of cerebellar neurons and ataxia in Xrcc1-defective mice, identifying a molecular mechanism by which endogenous single-strand breaks trigger neuropathology. Collectively, these data establish the importance of XRCC1 protein complexes for normal neurological function and identify PARP1 as a therapeutic target in DNA strand break repair-defective disease.
Assuntos
Ataxia Cerebelar/genética , Proteínas de Ligação a DNA/genética , Proteínas de Ligação a DNA/metabolismo , Mutação , Poli(ADP-Ribose) Polimerase-1/metabolismo , Adenosina Difosfato Ribose/metabolismo , Alelos , Animais , Apraxias/congênito , Apraxias/genética , Ataxia/genética , Axônios/patologia , Ataxia Cerebelar/patologia , Cerebelo/metabolismo , Cerebelo/patologia , Cromatina/metabolismo , Síndrome de Cogan/genética , Quebras de DNA de Cadeia Simples , Reparo do DNA/genética , Enzimas Reparadoras do DNA/genética , Enzimas Reparadoras do DNA/metabolismo , Proteínas de Ligação a DNA/deficiência , Feminino , Humanos , Interneurônios/metabolismo , Interneurônios/patologia , Masculino , Camundongos , Linhagem , Fenótipo , Fosfotransferases (Aceptor do Grupo Álcool)/genética , Fosfotransferases (Aceptor do Grupo Álcool)/metabolismo , Poli(ADP-Ribose) Polimerase-1/deficiência , Poli(ADP-Ribose) Polimerase-1/genética , Proteína 1 Complementadora Cruzada de Reparo de Raio-XRESUMO
Multiple symmetric lipomatosis (MSL) is a mitochondrial disorder with impaired brown fat metabolism that has been associated with MERRF mutations in some, but not all, patients. We studied a sibling pair and an unrelated indiviadual who presented with MSL and neuropathy to determine the genetic etiology of this disorder in patients who did not carry the MSL-associated MERRF mutation. Whole-exome sequencing was performed on the siblings, and a rare, shared homozygous mutation in MFN2 (c.2119C>T: p.R707W) was identified. The mutation was not present in their healthy siblings. In silico programs predict it to be pathogenic, and heterozygous carriers of the MFN2 p.R707W substitution are known to have Charcot-Marie-Tooth (CMT) disease. A third, unrelated patient with multiple symmetrical lipomatosis and neuropathy also harbored the same homozygous mutation and had been previously diagnosed with CMT. Functional studies in patient fibroblasts demonstrate that the p.R707W substitution impairs homotypic (MFN2-MFN2) protein interactions required for normal activity and renders mitochondria prone to perinuclear aggregation. These findings show that homozygous mutations at p.R707W in MFN2 are a novel cause of multiple symmetrical lipomatosis.