Poly‐Q aggregate trap interference with DNA repair genes in Huntington disease cells (736.10)

Abstract

Huntington’s disease (HD)–an autosomal dominant neurodegenerative disease –is characterized by expansion of a (CAG)9‐34 trinucleotide sequence found in exon 1 of the human Huntingtin (HTT) gene. Posttranslational modifications of an expanded mutant Huntingtin protein (mHtt) renders polar poly‐glutamine (Poly‐Q) fragments that aggregate and interfere with cellular proteins. Poly‐Q aggregate traps (PQATs) hijack transcriptional regulators including cyclic AMP‐responsive element‐binding protein (CREB), CREB‐binding protein (CREB‐BP/CBP), transcription factor Sp1 and TATA‐box‐binding protein. CAG repeats form hairpin structures that typically trigger DNA replication fork stalling and are recognized by DNA repair enzymes. However, HD cells appear to have defective repair pathways in the absence of any known repair gene mutation. We hypothesized that PQATs are responsible for decreased transcriptional regulation of essential DNA repair genes, facilitating the characteristic trinucleotide expansion. Here we examined mRNA levels of cultured HD‐affected and unaffected fibroblasts from related individuals utilizing a RT2 Profiler PCR Array for human DNA repair genes. Our findings reveal down‐regulation of several DNA repair genes involved in base‐excision repair (BER), mismatch repair (MMR) and double‐strand break repair. Examination of their respective promoters revealed these genes have binding sites for the transcription factors Sp1, CREB and CREB‐BP/CBP. Thus, PQATs may contribute to decreased transcriptional regulation of multiple DNA repair genes. We suggest that diminished mRNA levels of an interconnected complex of repair genes involved in sensing a stalled replication fork as well as correcting a hairpin–rather than a single repair enzyme–contribute to repeat expansion.