ABSTRACT Dementias lead to disability and dependency and are the leading cause of death among all diseases. While Alzheimer's disease is the most common form of dementia, genetically inherited dementias are epidemiologically relevant and may provide insight into the pathogenic pathways and therapeutic targets for common dementias. Here, we focus on an autosomal recessive microgliopathy, known as Nasu-Hakola Disease (NHD), which is caused by rare homozygous loss-of-function mutations in either TREM2 or TYROBP (encoding DAP12). In contrast to AD, which manifests with aging, NHD progresses quickly, often resulting in death by the fifth decade. We recently examined specimens of brain occipital cortical tissues from three rare DAP12-deficient patients with NHD by single nucleus RNA-seq. This analysis showed that lack of DAP12 is associated with a unique transcriptome microglia profile indicative of enhanced RUNX1/2, STAT3 and TGFβ signaling pathways that mediate tissue repair functions. We postulate that DAP12 attenuates the RUNX, STAT3 and/or TGFβ pathways. In the absence of DAP12 (or TREM2), unrestricted signaling of these pathways may cause maladaptive responses of microglia to stimuli normally received from the brain microenvironment, leading to exaggerated tissue repair reactions, widespread tissue alterations, microvascular alterations, and neuronal loss. In this grant, we aim to investigate whether the lack of DAP12 in microglia leads to heightened signaling of RUNX/STAT3 in vitro. Furthermore, we will explore whether excessive RUNX/STAT3 signaling in microglia, independent of DAP12 influence, disrupts the expression of genes responsible for microglial functions, including phagocytosis, cytokine production, and wound repair. Ultimately, we seek to understand how these mechanisms contribute to brain pathology. In Aim 1, we will investigate the impact of DAP12 deficiency on the expression of RUNX1/2 in human iPSC-derived microglia (iMGL). Regardless of whether DAP12 directly regulates these factors, we will explore the consequences of RUNX1/2 overexpression on transcriptome and functionality of iMGL. Additionally, we will assess the repercussions of altered microglial expression of RUNX in vivo using a mouse model where RUNX2 is selectively induced in microglia. In Aim 2, we will focus on STAT3, which drives macrophage responses to IL-6 family cytokines and IL-10. Our preliminary data show that NHD microglia exhibit a transcriptional signature resembling that of IL-10–STAT3-stimulated macrophages. We also found that DAP12- deficient macrophages contain more phosphorylated and total STAT3 than do wild-type macrophages. We will investigate whether lack of DAP12 increases STAT3 activation in human iMGLs and delineate the molecular and functional changes induced by a gain-of-function mutant of STAT3 in human iMGLs. Furthermore, we will evaluate the impact of STAT3 gain-of-function on glial phenotype and brain pathology using in an vivo knock-in mouse...