GENOMICSti,279-286 (1988) Characterization of the Human HEXB Gene Encoding Lysosomal &Hexosaminidase’ K. NEo-rE,*,t B. BAPAT,* A. DUMBRILLE-ROSS,* C. TROXEL,$ 5. M. SCHUSTER,+ D. 1. MAHURAN,**§ AND R. A. GwwEL*at*2 *Research Institute, Hospital for Sick Children, Toronto, Ontario, Canada MSG 1X8; tDepartment of Medical Genetics, University of Toronto, Ontario, Canada MS5 7A8; *Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588; and § Department of Clinical Biochemistry, University of Toronto, Ontario, Canada MSG 1L5 ReceivedJune8,1988;revisedAugustl, 1988 The lysosomal enzyme @-hexosaminidase A contains a- and &subunits that are encoded by the HEXA and HEXB genes, respectively. The human HEXB gene has been isolated and characterized. It is 46 kb long and is split into 14 exons. Of the 13 introns, 12 inter- rupt the coding sequences at homologous positions in the HEXA and HEXB genes. The 6’ flanking region contains the functional HEXB gene promoter. While a fine-structure analysis has yet to be done, we note that the sequence is GC rich and has several GC boxes and one CAAT box. There are also sequences related or identical to a progesterone response element and an AP- 1 binding motif. 8 leea Academic PIM. I~C. INTRODUCI’ION Lysosomal &hexosaminidase occurs in two major forms, hexosaminidase A and hexosaminidase B (Ma- huran et al., 1985). Hexosaminidase A is composed of one a-subunit and one @subunit, while hexosaminidase B is composed of two /3-subunits. The a-subunit is made up of a single a-polypeptide that is encoded by the HEXA gene residing on chromosome 15 (Gilbert et al., 1975). The j%subunit contains two nonidentical poly- peptides that are derived from the cleavage of a /3-poly- peptide (pre-/3-propolypeptide) (Mahuran et al., 1985). It is encoded by the HEXB gene residing on chromo- some 5 (Lalley et aZ., 1974). Mutation in the HEXA or HEXB gene leads to Tay-Sachs or Sandhoff disease, respectively (O’Brien, 1983). We and others have cloned the cDNAs encoding the a- and B-subunits (O’Dowd et aZ., 1985; Korneluk et al., 1986; Myerowitz et al., 1985), and these have been used to investigate the molecular defects in Tay-Sachs and Sandhoff dis- ’ Sequence data from this article have been deposited with the EMBL/GenBank Data Libraries under Accession No. 503065. ’ To whom correspondence should be addressed at Research In- stitute, Hospital for Sick Children, Toronto, Ontario, Canada M5G 1X8. eases (O’Dowd et al., 1986; Myerowitz and Hogikyan, 1987), including the identification of a splice mutation in the Ashkenazi infantile form of Tay-Sachs disease (Arpaia et aZ., 1988). Molecular cloning of the cDNAs has also given insight into the structural similarities of the a- and fl-subunits, suggesting that the HEXA and HEXB genes have evolved from a common ances- tral gene (Korneluk et al., 1986). &Hexosaminidase has been found in all mammalian tissues surveyed. The enzyme activity varies in differ- ent tissues, with higher activities in kidney, brain, and liver and at least 10 times basal levels in epididymis (Conchie and Findlay, 1959; Stirling, unpublished re- sults). Interestingly, brain and spleen contain 75% hexosaminidase A, whereas epididymis and kidney ap- pear to contain nearly all hexosaminidase B. These findings suggest that the HEXA and HEXB genes are regulated by independent mechanisms. The activity of &hexosaminidase may also be under hormonal regu- lation. For example, @-hexosaminidase activity in- creases during pregnancy in humans and other species (Isaksson et al., 1984; Rahi and Srivistava, 1983). This has been particularly relevant in carrier screening for Tay-Sachs disease where the relative proportions of hexosaminidase A and hexosaminidase B are altered in the 8em of pregnant women (Iaaksson et cd., 1984). Furthermore, it has been shown that in gilt uterus, the secretion of @-hexosaminidase increases 300-fold after treatment with progesterone (Rahi and Srivistava, 1983). In order to gain a molecular understanding of the expression and differential tissue distribution of /3- hexosaminidase, we have cloned and determined the detailed structure of the HEXB gene and its promoter. We note highly related structure8 of the HEXA (Proia and Soravia, 1987) and HEXB genes but complete dis- similarity of their promoters. The similar locations of the HEXA and HEXB gene intron-exon junctions have recently been reported by Proia (1988). 279 033&7543/Q3 $3.00 Copyright Q 1938 by Academic Press, Inc. All rights of reproduction in any form reserved.