CHARACTERIZATION OF POINT MUTATIONS IN PATIENTS WITH X-LINKED ICHTHYOSIS - EFFECTS ON THE STRUCTURE AND FUNCTION OF THE STEROID SULFATASE PROTEIN

X-linked ichthyosis is the result of steroid sulfatase (STS) deficienc y, While most affected individuals have extensive deletions of the STS gene, point mutations have been reported in three patients (1), In th is study, we identify an additional three point mutations and characte rize the effects of all six mutations on STS activity and expression. All six are unique single base pair substitutions, The mutations are l ocated in a 105-amino acid region of the C-terminal half of the polype ptide, Five of the six mutations involve the substitutions of Pro or A rg for Trp(372), Arg for His(444), Tyr for Cys(446), or Leu for Cys(34 1). The other mutation is in a splice junction and results in a frames hift causing premature termination of the polypeptide at residue 427. All the affected residues are conserved to some degree within the sulf atase family, The six mutations were reproduced in normal STS cDNA and transiently expressed in STS-deficient cells. All six mutant vectors direct the expression of STS protein that lacks enzymatic activity, Th e mutant polypeptides show a shift in mobility on SDS-PAGE and resista nce to proteinase K digestion when translated in the presence of dog p ancreas microsomes, indicating glycosylation and normal translocation.