IDENTIFICATION OF AN ITGA7 VARIANT ASSOCIATED WITH ALZHEIMER’S DISEASE AND MULTIPLE OTHER NEURODEGENERATIVE DISEASES

Abstract

The genetic causes of mixed Alzheimer's Disease (AD) and Dementia with Lewy Bodies (DLB) pathology remain unclear. In the current study we searched for exonic variants contributing to dual pathology in younger patients. We analyzed whole exome sequencing data collected through Stanford University and the Alzheimer's Disease Sequencing Project (ADSP). High impact (missense or nonsense) variants found in autopsy-confirmed mixed AD/DLB cases (age < 75 years, n=143) were screened against older, autopsy-confirmed pure AD controls (age > 85 years, n=274) and clinically-defined healthy controls (age > 80 years, n=4572). Variants seen in at least two dual path cases and none of the older, pure AD controls or healthy controls were examined further. For the remaining variants, we searched other neurodegenerative exome databases to determine their specificity to mixed AD/DLB pathology. For our top variant we performed immunohistochemical staining of autopsy-confirmed AD/DLB mixed pathology brain tissue. Finally, we explored potential functional effects of the variant by examining gene expression in post-mortem temporal cortex RNA-seq data from AD cases (n=82) and controls (n=80). The initial screening process yielded eleven variants present in at least two dual pathology cases and none of the healthy controls or older pure AD cases. Of these, we focused on rs76938320-A (NP_001138468.1:p. Ser700Leu, MAF=0.0002) on exon 18 of ITGA7 because of its predicted probable damaging effect using in silico prediction tools, its presence in three younger AD cases (age < 80 years) in ADSP, its high frequency in the ALS exome database, and the finding of a homozygous carrier with TDP43 and tau pathology. ITGA7 transcript isoforms that contain exon 18 show significantly higher expression in AD temporal cortex tissue when compared to controls (P < 0.001) in the Mayo Clinic Transcriptomics data set. Immunohistochemical staining of AD, AD/DLB, DLB, and vascular dementia brain tissue suggest that AD and AD/DLB tissue express higher levels of ITGA7 than vascular controls and pure DLB. Convergent data from exome sequencing, immunohistochemical stains, and transcriptomic analysis suggest that this ITGA7 variant may predispose not only to mixed AD/DLB pathology but to the development of multiple proteinopathies. Normalized FPKM values of the three ITGA7 transcript isoforms encoding rs76938420 are significantly higher in AD cases (n = 82) than in controls (n = 80).ITGA7 transcript expression values in post-mortem brain tissue were corrected for subject age at death, gender, RIN, source, and cell type markers. *** P < 0.001. ITGA7 neuronal cytoplasmic immunoreactivity in post-mortem brain tissue. Mixed DLB/AD and AD cases demonstrate stronger neuronal cytoplasmic immunoreactivity for ITGA7 when compared to vascular dementia and DLB cases. ITGA7 immunoreactivity is strongest within neurons containing neurofibrillary tangles.