Clinical, Genetic and Functional Characteristics of Three Novel <i>CYP17A1</i> Mutations Causing Combined 17α-Hydroxylase/17,20-Lyase Deficiency

&lt;i&gt;Background:&lt;/i&gt; P450c17 has two distinct activities: 17α-hydroxylase activity and 17,20-lyase activity. Combined 17α-hydroxylase/17,20-lyase deficiency leads to a severe defect in the production of cortisol and sex steroids. In affected males this results in impaired masculinization with ambiguous or female external genitalia. Female patients have normal genitalia but show a lack of pubertal development in adolescence. An increased production of mineralocorticoids often leads to hypertension and hypokalemia in both sexes. &lt;i&gt;Methods:&lt;/i&gt; To better understand the mechanisms of P450c17 deficiency, we studied 2 patients (both 46,XY) with combined 17α-hydroxylase/17,20-lyase deficiency of different severity: one with complete lack of masculinization and one with ambiguous genitalia. &lt;i&gt;Results:&lt;/i&gt; Four mutations were identified by sequencing of the &lt;i&gt;CYP17A1&lt;/i&gt; gene: I332T and A355T in the less severely affected patient; G111S and R440H in the patient with complete lack of masculinization. The three novel mutations were expressed in COS1 cells and all mutant proteins except I332T showed a complete loss of both enzymatic activities. I332T retained some residual 17α-hydroxylase as well as 17,20-lyase activity. &lt;i&gt;Conclusion:&lt;/i&gt; We identified 2 patients with the phenotypical spectrum of P450c17 deficiency. Three novel mutations in the &lt;i&gt;CYP17A1&lt;/i&gt; gene were identified and their functional characterization provided a good phenotype-genotype correlation. The location of the mutated residues in the three-dimensional model of P450c17 gave some additional insights into its structure-function relationship.</jats:p