The Importance of Heteroduplexes in Interpreting the Results of PCR-RED Diagnostic Assays: Application to the Analysis of Mutations in the Steroid 21-Hydroxylase Gene in a Case of Congenital Adrenal Hyperplasia.

Background: Congenital adrenal hyperplasia (CAH) due to deficiency of steroid 21- hydroxylase (CYP21) is an autosomal recessive disease that is a major cause of ambiguous genitalia at birth in females. The milder late-onset form of the disease can cause mild virilization in women including hirsutism, infertility, and acne. Characterization of the causative mutations in a patient requires finding mutations on both chromosomes. Methods and Results: We use polymerase chain reaction (PCR) followed by restriction enzyme digestion (RED), among several other methods, to detect mutations in the CYP21 gene. The authors found two different point mutations in a patient with CAH, V281L and Q318X. Did we find two mutations in trans which would account for the disease, or two mutations in cis with a still unknown mutation(s) on the other chromosome? We devised a method to determine the cis/trans nature of the mutations using PCR amplification, digestion with a restriction enzyme (ApaL I) diagnostic for V281L, gel purification of the mutant band which lacks the cutting site, digestion with a second restriction enzyme (Pst I) which detects Q318X, and gel electrophoretic analysis of the products. Although the results initially supported a a cis orientation, consideration of the impact of heteroduplex formation during the PCR on the products of RED showed that the orientation was actually trans. Conclusions: When DNA that is heterozygous for a mutation is amplified using the PCR, four double-stranded DNA products can be formed: the two homoduplexes and two heteroduplexes. The heteroduplex DNA must be considered to ensure correct interpretation of the results of PCR-RED analysis.