Cellular Models of a Monogenic Small Vessel Disease of the Brain

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Project Summary VCID (Vascular contributions to cognitive impairment and dementia) is increasingly recognized as both a primary cause of dementia and as a secondary contributor to dementia, as in the setting of Alzheimer’s pathology. VCID most commonly progresses from accumulated injury due to small vessel disease of the brain (SVD). While several risk factors are associated with SVD, the mechanisms that drive its initiation and progression have not been well characterized. Retinal Vasculopathy with Cerebral Leukoencephalopathy and Systemic Manifestations (RVCL-S) is a rare monogenic late onset condition that bears striking clinical and pathological similarities to small vessel disease of the brain. Our goal is to facilitate the development of new methods for the prevention and treatment of sporadic SVD through an investigation of RVCL-S. RVCL-S is caused by dominant mutations in the autosomal gene TREX1 which encodes the major intracellular 3’ DNA exonuclease. TREX1 protein harbors two major functional domains: the amino-terminal domain is the exonuclease while the carboxy-terminal domain positions the protein in the endoplasmic reticulum (ER) membrane, extending the DNAse domain into the cytosol where it plays a critical role in dampening the innate immune response to cytosolic self-DNA. RVCL-S is caused by fully penetrant frameshift mutations in TREX1 gene, each producing a protein that carries a functional exonuclease domain but lacks the domain that anchors the complete protein to the ER membrane resulting in atypical localization of TREX1 exonuclease throughout the cytosol and the nucleus. Importantly, mutations that truncate TREX1 within the exonuclease coding region, resulting in proteins lacking both DNase activity and the ER membrane tether, are not pathogenic in the heterozygous state, suggesting that RVCL-S is caused by a toxic gain-of-function of the untethered exonuclease. We propose that aberrant activity of TREX1 exonuclease in the cell nucleus interferes with DNA replication/repair as well as the activity of closely coordinated cell cycle regulatory signaling proteins. Our long- term objectives are to identify the critical detrimental effects of the mutant exonuclease and to develop strategies to abate these effects. To that end we propose the following specific aims: 1. Examine the effects of the RVCL-S genotype on the DNA damage response and cell cycle regulation. 2. Employ RNA interference to suppress expression of the RVCL-S associated TREX1 frameshift mutant. Impact: Successful completion of this study will provide 1) strategies for disease prevention; 2) biomarkers for disease monitoring; and 3) novel therapeutic approaches that will be applicable to RVCL-S and very likely applicable to sporadic SVD.