Thesis (Ph.D.)--Memorial University of Newfoundland, 2005. / Bibliography: leaves 370-404.

Chorea-acanthocytosis (ChAc) is an autosomal recessive neurological disorder whose characteristic features include hyperkinetic movements and abnormal red blood cell morphology. The disorder shares features with Huntington's disease and McLeod syndrome (MLS), and can sometimes be difficult to distinguish clinically from the latter. In 1997, ChAc was linked to a 6-cM region on chromosome 9q21-22. A novel gene, >em>CHAC, was identified in the critical region. CHAC (now renamed VPS13A) encodes a large protein called chorein, with a yeast homologue implicated in protein sorting. In this study, all 73 exons plus flanking intronic sequence in VPS13A were screened for mutations in 83 unrelated ChAc patients. We identified 88 different VPS13A mutations in 72 probands, comprising six deletions of entire exons, 22 nonsense, 36 frameshift, 19 splice-site and five missense mutations. This disorder therefore shows substantial allelic heterogeneity: however, evidence for common inheritance of the EX70_73del mutation in four French Canadian pedigrees indicates a possible founder effect in this population. Expression of VPS13A appears to be ubiquitous, as determined by tissue-specific analysis of mRNA and chorein distribution. However, chorein expression was markedly reduced or undetectable in lymphoblasts, fibroblasts and erythrocyte membranes from 14 ChAc patients. In contrast, MLS cells showed chorein expression similar to control levels, suggesting that loss of chorein expression is a diagnostic feature of ChAc. Yeast two-hybrid analysis of six different -600 amino-acid chorein fragments was used to screen a human brain cDNA library for proteins that may interact with chorein. One fragment interacted weakly with constructs derived from transcription factor NF-κB, putative protein phosphatase PP2Cη and TAB2, a protein implicated in the mitogen-activated kinase cascade. Although exogenously expressed chorein and TAB2 did not appear to colocalise, co-immunoprecipitation experiments supported an interaction between the two proteins, suggesting an avenue for future research into chorein function.

Paradoxically enhanced cognitive processes in neurological disorders provide vital clues to understanding neural function. However, what determines whether the neurological damage is impairing or enhancing is unclear. Here we use the performance of patients with two disorders of the striatum to dissociate mechanisms underlying cognitive enhancement and impairment resulting from damage to the same system. In a two-choice decision task, Huntington\'s disease patients were faster and less error prone than controls, yet a patient with the rare condition of benign hereditary chorea (BHC) was both slower and more error prone. EEG recordings confirmed significant differences in neural processing between the groups. Analysis of a computational model revealed that the common loss of connectivity between striatal neurons in BHC and Huntington\'s disease impairs response selection, but the increased sensitivity of NMDA receptors in Huntington\'s disease potentially enhances response selection. Crucially the model shows that there is a critical threshold for increased sensitivity: below that threshold, impaired response selection results. Our data and model thus predict that specific striatal malfunctions can contribute to either impaired or enhanced selection, and provide clues to solving the paradox of how Huntington\'s disease can lead to both impaired and enhanced cognitive processes.