Functional Studies in AD Candidate Genes

The laboratory aims to understand the functional consequences of modifications in expression levels of genes and specific isoforms implicated in Alzhemier's disease.

One gene pair of particular interest is leucine rich repeat transmembrane neuronal 3 (LRRTM3) and alpha 3 catenin (CTNNA3). LRRTM3 is a member of a synaptic protein family and is a nested gene within CTNNA3, residing at the linkage peak for late-onset Alzheimer's disease (LOAD) risk and plasma amyloid-beta levels. In vitro knockdown of LRRTM3 has previously shown to decrease secreted amyloid-beta, although the mechanism of this is unclear.

The research findings of this project included:

  • In SH-SY5Y cells that are overexpressing amyloid precursor protein (APP) and transiently transfected with LRRTM3 alone or with beta-secretase 1 (BACE1), LRRTM3 co-localizes with both APP and BACE1 in early endosomes where BACE1 processing of APP occurs
  • LRRTM3 co-localizes with APP in primary neuronal cultures from transgenic mice expressing mutant APP (Tg2576 mice) transduced with LRRTM3-expressing adeno-associated virus
  • LRRTM3 co-immunoprecipitates with both endogenous APP and overexpressed BACE1 in human embryonic kidney 293 cells (HEK293T) transfected with LRRTM3
  • SH-SY5Y cells with knockdown of LRRTM3 had lower BACE1 and higher CTNNA3 mRNA levels, but no change in APP
  • Brain mRNA levels of LRRTM3 showed significant correlations with BACE1, CTNNA3 and APP in 400 humans, but not in LRRTM3 knockout mice

Finally, after assessment, the lab studied 69 single nucleotide polymorphisms (SNPs) within and flanking LRRTM3 in a LOAD case-control series and identified a multilocus genotype within a linkage disequilibrium block encompassing 5' untranslated region (5' UTR) intron 1 of LRRTM3 that associates with Alzheimer's disease risk.

Collectively, these results suggest that protein interactions between LRRTM3, APP and BACE1, as well as complex associations between mRNA levels of LRRTM3, CTNNA3, APP and BACE1 in humans might influence APP metabolism and ultimately risk of Alzheimer's disease.

The lab is currently utilizing gene expression, immunohistochemistry and animal behavior approaches to understand the functional consequences of loss of LRRTM3 in a knockout mouse model to further these in vitro findings.