Mechanisms in Human Genetic Disease Laboratory
Fragile and Unstable Chromosomes in Cancer: Causes and Consequences
Chromosomal fragile sites are non-staining gaps in metaphase chromosomes that are induced to appear under specific cell culture conditions (Richards, 2001a).
There are more than 100 fragile sites in the human genome, distinguished by their frequency in the population and chemistry of induction as well as their physical location. Rare (or inherited) fragile sites are found in less than 5% of individuals and are induced by various chemicals some of which perturb nucleotide levels and others which preferentially bind certain DNA sequences. Common (or constitutive) fragile sites are found in all individuals and are induced by agents that cause replicative stress (e.g. aphidicolin, a DNA polymerase inhibitor).
The study of various rare fragile sites has provided insight into the novel genetic mechanism of repeat expansion termed ‘dynamic mutation’ (Richards, 2001b). All rare fragile sites characterised to date are due to expansion of the longest normal alleles of a polymorphic DNA repeat beyond a particular copy number threshold. The molecular basis of common fragile sites is not yet understood.
Our studies have been aimed at addressing the role of common fragile sites in the DNA instability frequently observed in cancer. These studies have located the FRA16D fragile site within a region of homozygous deletion in cancer cells (Mangelsdorf et al., 2000) and identified a gene (FOR/WWOX) that spans both the FRA16D fragile site and the deleted regions (Ried et al., 2000). Our recent studies have investigated the mutation mechanism that gives rise to the fragile site associated DNA instability in cancer cells (Finnis et al., 2005) and the function of the FOR/WWOX gene using Drosophila as a genetic model system (O’Keefe et al., 2005).
Dynamic Mutation Disorders: Mechanism of Pathogenesis in Neurodegenerative Repeat Expansion Diseases
The term “dynamic mutation” was introduced to distinguish the unique properties of expanding, unstable DNA repeat sequences from other forms of mutation (Richards, 2001b). The past decade has seen dynamic mutations uncovered as the molecular basis for a growing number of human genetic diseases (including Huntington’s Disease) in addition to all of the characterised ‘rare’ chromosomal fragile sites. The common properties of the repeats in different diseases and fragile sites have given insight into this unique form of DNA instability. While the dynamic mutation mechanism explains some of the unusual genetic characteristics of the relevant diseases, unexpected findings have raised new questions and challenged some assumptions about the pathways that lead from mutation to disease (Richards and McLeod 2005). This project is aimed at understanding the molecular mechanisms involved in the pathogenic pathways that lead from expanded repeats to the diseases with which they are associated (McLeod et al., 2005).