Objective: Abnormal corticosteroid-binding globulin (CBG) is an extremely rare condition and only three mutations have been described in four families. The molecular basis of an abnormal CBG in a Brazilian family was studied and correlations between genotype and serum cortisol, cortisol binding capacity (CBC) and CBG levels were determined.
Subjects: All 10 family members, comprising three generations, and nine healthy volunteers were studied.
Measurements: Genomic DNA was extracted from white blood cells from all family members. The human cbg exons 2-5 were amplified by PCR, submitted to automatic sequencing. Cortisol and CBG levels in serum were measured by radioimmunoassay (RIA). CBC in serum was determined using tritiated cortisol and other cortisol binding parameters were calculated through Scatchard analysis.
Results: A missense mutation in exon 5 of cbg (1254G --> A; Asp367Asn), recently described as CBG Lyon, was found in all family members. The proband and one sister were homozygous whereas all other family members, including parents, were heterozygous for this mutation. Cortisol levels in the only two homozygotes were lower than in heterozygotes and both were significantly lower as compared to controls (69 and 182 nmol/l vs. 267 +/- 129 nmol/l vs. 459 +/- 195 nmol/l, respectively, P < 0.05). CBC was decreased in the two homozygotes as compared to heterozygotes and in both groups as compared to controls (< 90 and 114 nmol/l vs. 305.0 +/- 81.4 nmol/l vs. 594.8 +/- 59.5 nmol/l, respectively, P < 0.05). CBG levels were lower in homozygotes as compared to heterozygotes and in both as compared to controls (325 and 375 nmol/l vs. 496.75 +/- 50.75 nmol/l vs. 647.25 +/- 87.50 nmol/l, respectively, P < 0.05).
Conclusions: An abnormal CBG resulting from a missense mutation and known as CBG Lyon was found in this Brazilian kindred. This abnormal CBG has decreased affinity for cortisol and results in low or low normal serum cortisol levels in homozygous and heterozygous subjects. Although relative hypotension and fatigue have recently been associated with CBG deficiency in a family with two CBG mutations (null and Lyon), the two homozygous subjects in this kindred were both normotensive and only the proband presented with fatigue.