Regulation of the Huntingtin Interacting Protein 1 (HIP1) Gene.

Abstract

Huntingtin Interacting Protein (HIP1) is a multi-domain clathrin binding protein thought to play a role in receptor-mediated endocytosis. HIP1 contains an adaptor AP180 N-terminal homology (ANTH) inositol lipid-binding domain, specific to endocytic proteins, as well as clathrin- , AP2- and actin-binding motifs. Loss of Hip1 and its only known mammalian relative Hip1-related (Hip1r) leads to spinal defects, testicular degeneration, cataracts, adult weight loss, and early death in mice, indicating that the Hip1 family is necessary for the maintenance of normal adult tissues. There are conflicting reports concerning the role of HIP1 in cellular survival and proliferation. Our laboratory has found that HIP1 promotes cellular survival and transformation and therefore is involved in supporting tumorigenesis. Others have suggested that HIP1 plays a role in promoting apoptosis and this activity is the cause of neuronal cell death in Huntington’s disease patients. This conflict raises the possibility that multiple isoforms of HIP1 exist and these isoforms have contrasting activities. Indeed, I have observed multiple isoforms of HIP1 using many different anti-HIP1 antibodies and western blot analysis in both mouse and human tissues. To understand the regulation of HIP1 we have isolated its promoter and characterized it. We have also evaluated the gene for alternative transcripts. The HIP1 gene is a complex 31-exon gene that spans approximately 250 Kb of DNA. This complexity is highlighted by our first attempt to generate a conditional knockout allele of Hip1. Unexpectedly, we found expression of a mutant HIP1 protein in select tissues (lung, brain) due to use of a cryptic splice site. Additionally, we have identified and characterized two alternative Hip1 mRNA transcripts that comprise distinct first exons, designated Hip1a and Hip1b. These transcripts are present in human and mouse tissues and may encode for two HIP1 isoforms with distinct cellular functions. The findings described in this thesis provide a foundation for expanding our current understanding of HIP1 gene regulation and aberrant expression of HIP1 in cancer.