Making (anti-) sense out of huntingtin levels in Huntington disease.

Melvin M Evers,Barry A Pepers,Menno H Schut,Martine J Van Belzen,Melek Atalar,Richard Lm Faull,Willeke Mc Van Roon-Mom,Raymund Ac Roos

doi:10.1186/s13024-015-0018-7

Abstract

BackgroundHuntington disease (HD) is an autosomal dominant neurodegenerative disorder, characterized by motor, psychiatric and cognitive symptoms. HD is caused by a CAG repeat expansion in the first exon of the HTT gene, resulting in an expanded polyglutamine tract at the N-terminus of the huntingtin protein. Typical disease onset is around mid-life (adult-onset HD) whereas onset below 21 years is classified as juvenile HD. While much research has been done on the underlying HD disease mechanisms, little is known about regulation and expression levels of huntingtin RNA and protein.ResultsIn this study we used 15 human post-mortem HD brain samples to investigate the expression of wild-type and mutant huntingtin mRNA and protein. In adult-onset HD brain samples, there was a small but significantly lower expression of mutant huntingtin mRNA compared to wild-type huntingtin mRNA, while wild-type and mutant huntingtin protein expression levels did not differ significantly. Juvenile HD subjects did show a lower expression of mutant huntingtin protein compared to wild-type huntingtin protein. Our results in HD brain and fibroblasts suggest that protein aggregation does not affect levels of huntingtin RNA and protein. Additionally, we did not find any evidence for a reduced expression of huntingtin antisense in fibroblasts derived from a homozygous HD patient.ConclusionsWe found small differences in allelic huntingtin mRNA levels in adult-onset HD brain, with significantly lower mutant huntingtin mRNA levels. Wild-type and mutant huntingtin protein were not significantly different in adult-onset HD brain samples. Conversely, in juvenile HD brain samples mutant huntingtin protein levels were lower compared with wild-type huntingtin, showing subtle differences between juvenile HD and adult-onset HD. Since most HD model systems harbor juvenile repeat expansions, our results suggest caution with the interpretation of huntingtin mRNA and protein studies using HD cell and animal models with such long repeats. Furthermore, our huntingtin antisense results in homozygous HD cells do not support reduced huntingtin antisense expression due to an expanded CAG repeat.

Highlights

Huntington disease (HD) is an autosomal dominant neurodegenerative disorder, characterized by motor, psychiatric and cognitive symptoms
No significant difference (P = 0.7642) between wild-type and mutant HTT mRNA expression was observed in adult-onset HD patient-derived fibroblasts
Since most HD model systems harbor juvenile repeat expansions, our results suggest caution with the interpretation of htt mRNA and protein studies using HD cell and animal models with such long repeats

Summary

Introduction

Huntington disease (HD) is an autosomal dominant neurodegenerative disorder, characterized by motor, psychiatric and cognitive symptoms. HD is caused by a CAG repeat expansion in the first exon of the HTT gene, resulting in an expanded polyglutamine tract at the N-terminus of the huntingtin protein. Typical disease onset is around mid-life (adult-onset HD) whereas onset below 21 years is classified as juvenile HD. While much research has been done on the underlying HD disease mechanisms, little is known about regulation and expression levels of huntingtin RNA and protein. HD is caused by a CAG repeat expansion in the first exon of the HTT gene on chromosome 4p16, resulting in an expanded polyglutamine (polyQ) tract at the N-terminus of the huntingtin (htt) protein. The mean disease onset lies between 30 and 50 years of age (adult-onset HD). HD patients carrying more than 50 CAGs will have a disease onset typically below 21 years of age (juvenile HD) [1]. The major neuropathology in HD occurs in the striatum and cerebral cortex but degeneration is seen throughout the brain as the disease progresses [4] and insoluble protein aggregates in the nucleus and cytoplasm of cells are a hallmark of the disease [5]

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