Human iPSC‐microglia expressing TREM2 p.R47H risk variant for Alzheimer’s disease do not show a loss‐of‐function phenotype

Abstract

AbstractBackgroundThe most common TREM2 variant associated with increased risk to develop Alzheimer’s disease (AD) is p.R47H. This variant has been classified as loss‐of‐function, mostly by comparing p.R47H with the common variant in TREM2 over‐expressing cell lines. Whether this holds true when TREM2 is expressed at physiological levels in a disease‐relevant cell type such as human microglia, has not been fully characterized.MethodWe established and thoroughly characterized human iPSC‐derived microglia endogenously expressing TREM2 to assess the effect of p.R47H on TREM2 cellular distribution, TREM2 signaling and microglial functioning. For this study microglia generated from a set of isogenic gene‐edited iPSC lines homozygous for TREM2 knockout (KO) and TREM2 p.R47H were evaluated and compared to microglia from the parental control iPSC line. In addition, microglia generated from iPSC lines obtained from several p.R47H carriers were compared to microglia from iPSC lines obtained from several healthy control non‐carriers. SNP genotyping confirmed the presence of R47H.ResultThe p.R47H and common variant TREM2 were present at similar levels at the cell surface of iPSC‐derived microglia, whereas no expression of TREM2 was detected in the TREM2 KO microglia. Microarray profiling indicated that the microglial transcriptome is very similar between the common variant and p.R47H yet distinct from TREM2 KO microglia. Biochemical evaluation of TREM2 signaling indicated no reduction in TREM2 downstream signaling activation in the p.R47H variant expressing microglia. Similarly, soluble TREM2 levels were increased in culture medium of p.R47H microglia. Finally, analysis of microglial functioning including phagocytosis of artificial and disease‐relevant substrates showed no difference between the p.R47H variant and the common variant, in contrast to TREM2 KO microglia.ConclusionWe conclude that in cultured human iPSC‐derived microglia, the p.R47H variant shows no evidence of a loss‐of‐function mechanism. Future experiments will determine whether this is also the case in microglia in their natural brain environment. Our long‐term goal is to better understand TREM2 biology to support the development of novel therapeutics for AD and other neurodegenerative diseases characterized by TREM2‐mediated neuroinflammation.