Uncovering the Role of the MS4A Gene Family in Alzheimer's Disease

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PROJECT SUMMARY Alzheimer’s disease (AD) is a grave neurodegenerative disorder characterized by unrelenting memory loss and deficits in executive function with no effective treatment. Accumulating evidence from genome wide association studies (GWAS) posit that the brain’s innate immune system plays a central role in AD etiology. As the central nervous system’s resident immune cells, microglia have thus emerged as attractive cells to target therapeutically. Such drug targets may lie within the MS4A locus, a region associated with protection from AD, later age-at- onset, and increased levels of sTREM2, a biomarker of microglial activity. This region contains multiple genes within the membrane-spanning 4-domain subfamily A (MS4A) gene cluster, which together encode structurally related transmembrane proteins largely expressed by immune cells whose exact functions are not yet understood. Our work nominated a candidate causal variant within this locus, rs636317, which disrupts an anchor binding site for the chromatin remodeling protein CTCF and is associated with increased expression of MS4A4A and MS4A6A in myeloid cells. This proposal aims to directly test the hypothesis that by modulating MS4A4A and MS4A6A expression via differential CTCF binding, variant rs636317 alters microglial cell function in the context of disease. In AIM 1, I will determine the functional impact of MS4A genes in vitro using CRISPR-edited human induced pluripotent stem cell (iPSC)-derived microglia (iMGL). Given known interactions between MS4A proteins and other immune receptors such as TREM2 and CLEC7A, I will perform targeted functional assays related to immune signaling in iMGLs from two iPSC models: MS4A4A/MS4A6A knockout lines and isogenic lines homozygous for the protective or risk alleles of the candidate causal variant. In AIM 2, I will employ a novel xenotransplantation model involving direct injection of human microglia precursor cells into the mouse brain to evaluate the effect of these genes on cell function in vivo and in the context of disease using 5xFAD chimeric mice. I hypothesize that knocking out MS4A4A and MS4A6A in human microglia promotes protective microglial responses, ameliorating plaque containment and subsequent cognitive decline. Elucidating the function of this gene family and the specific role it plays in AD progression has the potential to greatly impact public health. The proposed research and rigorous training plan outlined here will equip me with the skills needed for a successful future career in neurodegeneration leading an independent research team.