GENETICS OF SALIVARY PROTEIN POLYMORPHISMS

Human salivary PRPs are determined by six closely linked genes on chro mosome 12p13.2. The many PRPs show complex electrophoretic patterns th at differ between individuals and reflect numerous genetic polymorphis ms. Frequent length and null polymorphisms are common among PRPs. Comm on themes emerge as a background for these PRP polymorphisms. First, p osttranslational proteolysis occurs with double-banded patterns among acidic PRPs and the generation of numerous basic PRPs derived from pre cursor proteins. Specific mutations may interfere with proteolysis, pr eventing generation of double-banded acidic PRPs (as with the Pa prote in) or of small basic PRPs from precursor proteins (as with Pm protein s). Second, single cysteine substitutions in PRPs (Pa from PRH1 and Gl 8 from PRB3) may lead to disulfide bonded homodimers as well as heter odimers with salivary peroxidase. Third, frequent homologous and unequ al crossing-over within the PRP gene cluster leads to frequent protein size-variants (intragenic events as with the GI protein variants) and the generation of the PRB2/1 fusion gene (intergenic event) with dele tion of the PRB1 coding region and absence of multiple PRB1 coded prot eins (Ps, Pm, Pe) in PRB2/1 homozygotes. Fourth, null mutations may al so be produced (as with PsO and Gl 0) by single nucleotide changes.