ELEVATED LEVELS OF MOSAIC ANEUPLOIDY IN HUNTINGTON’S DISEASE

Abstract

Huntington disease (HD) is an autosomal dominant neurodegenerative disorder caused by a CAG trinucleotide repeat expansion mutation in the huntingtin (HTT) gene. The HTT gene encodes the huntingtin protein, which has been shown to localize to spindle poles and is required for proper mitotic spindle orientation, chromosome segregation, and cell cycle regulation. Mutations in the HTT gene lead to the production of a mature huntingtin protein with an expended polyglutamine region (>35 repeats) at its N terminus. Previous studies have shown that mouse models of HD exhibit defects in microtubule function and in cell cycle regulation. Like many other neurodegenerative disorders, HD is a progressive condition that is characterized by decline in cognitive and executive functions. Over the years, studies from our laboratory and others have shown elevated levels of aneuploidy in patients with different neurodegenerative disorders and their animal and cell models (i.e., Alzheimer's disease [AD] and frontotemporal lobar degeneration [FTLD]) that are characterized by cognitive deficits. To investigate whether chromosome missegregation and aneuploidy may serve as a common mechanistic defect among many neurodegenerative disorders, we proceeded to analyze the levels of chromosomal aneuploidy in brain tissue and fibroblasts from HD patients and from age-matched healthy controls. Single cell suspensions were prepared from brain tissue and from cultured fibroblast cells from HD patients and healthy controls and were processed for fluorescence in situ hybridization (FISH) analysis using probes for human chromosomes 12 and 21 and NeuN immunostaining of brain tissue to identify neurons. FISH analyses of brain tissue and fibroblast cells revealed a statistically significantly higher level of mosaic aneuploidy among brain cells and fibroblasts from HD patients relative to those from healthy control subjects. Taken together, these data from HD patients, combined with previous results from both AD and FTLD patients showing increased levels of aneuploidy and correlated cell death, provide strong evidence of genomic instability as a shared defect in neurodegenerative disease pathogenesis and dementia. These findings emphasize the need to determine the underlying mechanism(s) by which aneuploidy arises in neurodegenerative diseases and to identify new approaches to prevent it.