Human 18 kDa phosphotyrosine protein phosphatase (ACP1) polymorphism: studies of rare variants provide evidence that substitutions within or near alternatively spliced exons affect splicing result

The mammalian low molecular weight phosphotyrosine protein phosphatase is e xpressed as two distinct isoforms. The human 'fast' and 'slow' isoforms dif fer only in the sequence of an internal segment of 34 residues, and the ACP 1 gene contains two adjacent exons (E3F and E3S) which encode these segment s. We have previously suggested that the fast and slow isoforms are generat ed by mutually exclusive pre-mRNA splicing of E3F and E3S. The common allel es ACP1*A, *B and *C express the fast and slow isoforms in differ ent ratio s. The *A and *C alleles differ from :*B by C --> T transitions in E3S and E3F respectively. To test the idea that the fast:slow ratio is determined b y nucleoticle substitutions in the E3F-I3F-E3S region, four groups of rare ACP1 variants with unusual fast:slow ratios and the rare *E and *R alleles, expressing fast:slow ratios similar to *C and *B, respectively, were analy sed. Gene segments of the 12-13S region were amplified by PCR and analysed by SSCP and variant hands were excised and sequenced. For each of the rare isozymic variants one of six different nucleotide substitutions in E3F (nts + 42, + 85, + 109, + 110), 13F (nt + 1) and 13S (nt+8) was observed. The * E and *R alleles showed C and B sequence, respectively, in accordance with the fast:slow ratio. The results support the hypothesis that the fast:slow ratio is constitutive.