804 Expression of a RNK-TRYP-2-homologue serinprotease mRNA in the mouse brain

Abstract

Two genes, HEXA and HEXB, encode the α- and β-subunits, respectively, of human β-hexosaminidase. In the mouse, the corresponding genes are termed Hexa and Hexb. The subunits dimerize to yield three isozymes, β-hexosaminidase A (αβ), B (ββ), and S (αα), that have the capacity to degrade a variety of substrates containing β-linked N-acetylglucosamine and N-acetylgalactosamine residues. Mutations in the HEXA or HEXB gene resulting in a β-hexosaminidase deficiency cause Tay-Sachs or Sandhoff disease, respectively. As a prelude to the creation of mouse models of these lysosomal storage diseases, we have characterized the molecular biology of the mouse β-hexosaminidase system. Protein sequences derived from the cloned Hexa and Hexb cDNAs were 55% identical to each other and were also very similar to the cognate human sequences: 84% sequence identity with human HEXA and 75% with HEXB. The mouse hexosaminidase subunits, when expressed in HeLa cells from the cDNAs, displayed specificity toward synthetic substrates similar to the human subunits. The Hexa and Hexb genes were 25 and 22 kb in length, respectively. Each gene was divided into 14 exons, with the positions of introns precisely matching those of the corresponding human genes. The 5′ flanking regions of the mouse genes demonstrated promoter activity as ascertained by their ability to drive chloramphenicol acetyltransferase gene expression in transfected NIH 3T3 cells. The sequences of these regulatory regions were G + C-rich in the 200 bp upstream of the respective initiator ATGs. Several putative promoter elements were present, including Sp1, AP2, CAAT, and TATA motifs. The widespread tissue distribution of the Hexa and Hexb transcripts conformed to the housekeeping role of the enzyme. However, the expression pattern of the genes was not coincident, implying tissue-specific differences in promoter function.