Within the non-pathogenic range of CAGs, the HTT gene encodes for a protein (HTT) which is implicated in multiple fundamental physiological processes. It is currently unknown whether HD pathogenesis occurs at the DNA, RNA, or protein levels. Huntington’s disease (HD) is a neurological genetic disorder caused by an anomalous expansion of a >35 CAG triplet repeats in the huntingtin gene ( HTT) that leads to an abnormally elongated polyglutamine (polyQ) tract. We propose that these changes in HTT polyQ length contribute to evolutionary fitness including potentially to the development of a more complex nervous system. We conclude that during evolution natural selection promotes the conservation and purity of the CAG-encoded polyQ tract and that small increases in its physiological length influence neural functions of HTT. By combining genome-edited mouse embryonic stem cells and cell assays, we show that small variations in HTT polyQ lengths significantly correlate with cells’ neurogenic potential and with changes in the gene transcription network governing neuronal function. For natural selection to operate, the polyQ must perform a function. We show that the poly-glutamine (polyQ) tract encoded by CAGs in the huntingtin protein (HTT) is under purifying selection and subjected to stronger selective pressures than CAG-encoded polyQ tracts in other proteins. Here, we investigated the function and evolution of the physiological CAG tract in the HTT gene. While pathologically expanded CAG repeats are the focus of extensive investigations, non-pathogenic CAG tracts in protein-coding genes are less well characterized. Huntington’s disease is caused by a pathologically long (>35) CAG repeat located in the first exon of the Huntingtin gene ( HTT).
0 kommentar(er)
