Expanding the genetics of microcephalic primordial dwarfism
Murray, Jennie Elaine
Body mass varies considerably between different mammals and this variation is largely accounted for by a difference in total cell number rather than individual cell size. Insights into mechanisms regulating growth can therefore be gained by understanding what governs total cell number at any one point. In addition, control of cell proliferation and programmed cell death is fundamental to other areas of research such as cancer and stem cell research. Microcephalic Primordial Dwarfism (MPD) is a group of rare Mendelian human disorders in which there is an extreme global failure of growth with affected individuals often only reaching a height of around one metre in adulthood. To date, all identified disease genes follow an autosomal recessive mode of inheritance and encode key regulators of the cell cycle, where mutations impact on overall cell number and result in a substantially reduced body size. MPD therefore provides a valuable model for examining genetic and cellular mechanisms that impact on growth. The overall aims of this thesis were to identify novel disease causing genes, as well as provide further characterisation of known disease causing genes, through the analysis of whole exome sequencing (WES) within a large cohort of MPD patients. Following the design and implementation of an analytical bioinformatics pipeline, deleterious mutations were identified in multiple disease genes including LIG4 and XRCC4. Both genes encode components of the non-homologous end joining machinery, a DNA repair mechanism not previously implicated in MPD. Additionally, the pathogenicity of novel mutations in subunits of a protein complex involved in chromosome segregation was assessed using patient-derived cells. These findings demonstrate WES can be successfully implemented to identify known and novel disease causing genes within a large heterogeneous cohort of patients, expanding the phenotype of known disorders and improving diagnosis as well as providing novel insights into intrinsic cellular mechanisms critical to growth.