GENE CONVERSION IN THE CYP11B2 GENE ENCODING P450C11AS IS ASSOCIATED WITH, BUT DOES NOT CAUSE, THE SYNDROME OF CORTICOSTERONE METHYLOXIDASE-II DEFICIENCY

Cytochrome P450c11AS (aldosterone synthase) has 11 beta-hydroxylase, 1 8-hydroxylase, and 18-oxidase activities and is expressed solely in th e adrenal tons glomerulosa. Corticosterone methyloxidase II (CMOII) de ficiency denotes a rare disorder of adrenal steroidogenesis in which o nly the 18-oxidase activity of P450c11AS is disrupted, while the 11 be ta-hydroxylase and 18-hydroxylase activities persist. Such patients ha ve elevated serum concentrations of corticosterone and 18-hydroxycorti costerone and very low or unmeasurable concentrations of aldosterone, often resulting in a clinical salt-losing crisis in infancy. One pair of point mutations, Arg(181)-->Trp and Val(386)-->Ala, has been previo usly characterized to cause this disorder in an inbred Iranian Jewish population. We have sought mutations causing CMOII deficiency in outbr ed populations. In three of four unrelated P450c11AS alleles from two unrelated patients with CMOII deficiency, we found a gene conversion e vent in which exons 3 and 4 of the CYP11B2 gene encoding P450c11AS wer e changed to the sequence of the nearby CYP11B1 gene, which encodes th e related enzyme P450c11 beta. This conversion resulted in a mutant P4 50c11AS protein carrying three changes: Asp(141)-->Glu, Lys(151)-->Asn , and Ile(248)-->Thr. We built seven vectors expressing P450c11AS carr ying each mutation singly, each of the three possible pairs of mutatio ns, and the triple mutation as found in the proband. The activities of both the normal P450c11AS and the various mutants in transfected nons teroidogenic COS-1 cells were very low, but their activities in steroi dogenic MA-10 and JEG-3 cells were 10- to 20-fold higher. In these sys tems all of the mutants retained normal 18-oxidase activity, indicatin g that the detected gene conversion event is associated with but does not cause CMOII deficiency. None of the four CYP11B2 alleles in these two patients bore other identifiable mutations. These patients might h ave mutations in the promoters or other noncoding regions, or mutation s in genes other than CYP11B2 may cause the syndrome of CMOII deficien cy.