The presence of fetal DNA in maternal plasma has allowed the development of strategies for noninvasive prenatal diagnosis (1). However, because fetal DNA in maternal plasma circulates among a background of maternal DNA, strategies for noninvasive prenatal diagnosis with applications of fetal DNA in maternal plasma have been confined to the detection of autosomal dominant, paternally inherited genetic traits, such as fetal gender (for sex-linked disorders) (1), rhesus D (2, 3), myotonic dystrophy (4), and achondroplasia (5). Using congenital adrenal hyperplasia (CAH) as a model system, we present a strategy for the noninvasive prenatal exclusion of an autosomal recessive condition through the detection of fetal DNA in maternal plasma. The approach described in this study may potentially be applicable to other autosomal recessive conditions. More than 90% of cases of CAH are a result of deficiency of 21-hydroxylase, an enzyme of the adrenal gland involved in the synthesis of glucocorticoids and mineralocorticoids. 21-Hydroxylase is encoded by CYP21, a MHC class III gene located on chromosome 6p21.3. Most mutations causing 21-hydroxylase deficiency are caused by either gene deletions or gene conversions, whereby deleterious mutations are transferred from the nearby pseudogene, CYP21P, which shares 98% homology with CYP21 (6). Consequent to profound deficiency or the complete absence of activity of 21-hydroxylase, severe forms of CAH manifest as salt-wasting attributable to impaired synthesis of mineralocorticoids and glucocorticoids. In addition, the excess buildup of metabolic precursors causes excessive androgen production, leading to virilization of female fetuses (6). Hence, dexamethasone therapy is customarily prescribed prenatally to prevent in utero virilization of an affected female fetus through suppression of the excessive production of androgens (7). This type of treatment is not indicated for a male fetus. In the antenatal management of CAH, fetal DNA in maternal plasma has been reported to be useful for the stratification of antenatal dexamethasone therapy through noninvasive fetal gender determination (8) because such therapy would be unnecessary for male fetuses. However, it would be ideal to further limit the dexamethasone therapy only to female fetuses known to be affected by CAH because the use of antenatal dexamethasone is not without complications (6). Consequently, this study at tempted to develop a noninvasive approach that allows the effective in utero exclusion of CAH. The strategy proposed is derived from the reasoning that the presence of a wild-type paternally inherited CYP21 allele in maternal plasma would infer that the fetus had inherited the nonmutated paternal allele and thus would not manifest CAH.
