Functional Assessment of Human 21st Chromosome Orthologs in Caenorhabditis elegans

Abstract

Down syndrome (DS) is a neurodevelopmental disorder caused by having an extra copy of the 21st chromosome. Every person with DS exhibits muscle weakness and ID, though the precise genotype-phenotype is not fully understood. Little is known about the individual genes that make up the human 21st chromosome. In attempting to fill this gap, I have worked with the laboratory of Dr. Jon Pierce to systematically characterize the genes of the 21st chromosome for a potential role in the nervous system or muscle function. To this end, the Pierce lab uses the nematode Caenorhabditis elegans to study orthologs, or equivalent genes, from the human 21st chromosome. The general approach is to identify genes conserved between human and C. elegans and conduct broad, unbiased screens to identify potentially important genes. C. elegans can be a useful model for taking this approach to DS, as it has been used to discover the function of hundreds of genes in the nervous system and muscle in humans and shares orthologs with many of the protein-coding genes on our 21st chromosome. We first conducted a loss-of-function screen, taking the C. elegans genes and systematically knocking them down to observe phenotypic defects. The C. elegans loss-of-function mutants underwent a series of behavioral and pharmacological assays to evaluate phenotypic defects. The loss-of-function screen resulted in the identification of 10 genes thought to be critical for synaptic or motor function. Several of these genes were not previously identified for a contribution to DS. We have since begun a gain-of-function screen using similar behavioral tests to empirically determine which genes are problematic when overexpressed. Preliminary results implicate a few genes in disrupting neuron-to-muscle circuitry. I have also verified transcriptional overexpression in mutants relative to control strains. I aim to uncover the roles of the individual genes involved in DS-associated phenotypes. Establishing which genes contribute to DS could clarify the mechanism for the neuromuscular symptoms associated with DS. Doing so can provide a foundation for other DS researchers and inform efforts toward a more precise mechanism of action. The hope is that these genes might later serve as therapeutic targets for alleviating neuromuscular symptoms in people with DS.