Molecular genetic analysis

Point mutations, deletions, and duplications
occur in the CYP21 gene. The deletions and duplications
result from misalignment of the homologous
chromatids during meiosis and unequal
crossing-over. Deletions occur in about
20–25% of patientswith classic 21-hydroxylase
deficiency. Duplications have no clinical consequences.
Deletions and duplications can be
easily detected by Southern blot analysis. The
most frequent type of deletion is loss of a 30 kb
region including the 3' part of the CYP21P pseudogene,
the entire C4B gene, and the 5' part of
the CYP21 gene. The resulting fusion gene of
CYP21P and CYP21 carries a TaqI restriction site
in the 5' region of CYP21P that is not present in
the CYP21 gene. Therefore, the fusion gene has a
characteristic 3.2 kb TaqI fragment. This distinguishes
the rearrangement from the normal
CYP21 gene, which has a characteristic 3.7 kb
fragment. In the example shown, the CYP21
gene (21-OHB) is represented by a 3.7 kb DNA
fragment, the pseudogene CYP21P (21-OHA) by
a 3.2 kb fragment after TaqI digestion (1). Thus,
the normal pattern is a 3.7 kb and a 3.2 kb fragment
(2). Homozygous deletion of either of the
genes may be apparent by lack of either of the
two fragments (3, 4). A heterozygous deletion
shows reduced intensity (5) and a duplication
shows increased intensity (6).