Book Description

"Genetically determined leukoencephalopathies (gLEs) are a group of rare heritable white matter disorders primarily affecting children, for which there remains many unsolved genetic cases. Identifying the molecular cause of these neurodegenerative diseases is essential as it allows the patients and their families to know the disease course, gain access to therapeutic options if available and obtain genetic counselling to make reproductive decisions.The first section of this thesis focuses on identifying the genetic cause of a cohort of patients with unsolved gLEs using whole exome sequencing (WES). The analysis was performed on 18 patients, 12 of which were second or third analyses, in accordance with the American College of Medical Genetics and Genomics (ACMG) Guidelines. We found a de novo pathogenic variant in DYNC1H1 in two twin siblings, a strong candidate gene ABHD16A in 4 cases from two families, and variants of unknown significance in other genes in another 5 cases. We thereby report a solved rate of approximately 27% (5/18), wherein three of the second analyses, and both third analyses have strong candidate genes. Our solved rate greatly exceeds the anticipated 5-11% rate published in literature for other re-analyses cohorts. In the second part of this thesis, we explore the functional defects caused by biallelic variants identified in the LSM7 (NM_016199) gene, which encodes an RNA-binding protein that is part of two complexes playing a role in either RNA splicing or mRNA decay. Following a clinical WES, a homozygous variant in LSM7 at position c.121G>A; p.Asp41Asn was identified in a patient (GB114.0) with a leukodystrophy. Through GeneMatcher, another individual with a different homozygous variant in LSM7 at position c.206G>C; p.Arg69Pro was found. Structural data predicted that the mutant residues would result in the loss of key intra- and inter-molecular interactions. Affinity purification of wildtype and disease-associated LSM7 variants from human cells confirmed that both variants lead to a defect in assembly of both LSM complexes. Molecular investigations of the p.Asp41Asn variant showed decreased levels of LSM7 mRNA and protein compared to healthy controls. We also found that knockout of Lsm7 in zebrafish embryos led to a defect in nervous system development, supporting a role for LSM7 mutations in neurodegeneration. This thesis demonstrated that WES is an invaluable tool to identify the molecular causes of rare genetic diseases and that re-analysis of existing WES data should be considered before WGS and RNA sequencing. We describe a novel ultra rare neurodevelopmental and neurodegenerative disorder caused by biallelic pathogenic variants in LSM7 and shed light on the disease pathogenesis. We are hopeful that our work will provide the foundation for future investigations into this rare human disease and lead to the development of potential therapies"--