Abnormal sterol metabolism in a patient with Antley‐Bixler syndrome and ambiguous genitalia

Abstract

Antley-Bixler syndrome (ABS) is a rare multiple anomaly syndrome comprising radiohumeral synostosis, bowed femora, fractures of the long bones, premature fusion of the calvarial sutures, severe midface hypoplasia, proptosis, choanal atresia, and, in some, ambiguous genitalia. Of fewer than 40 patients described to date, most have been sporadic, although reports of parental consanguinity and affected sibs of both sexes suggests autosomal recessive inheritance in some families. Known genetic causes among sporadic cases of ABS or ABS-like syndromes are missense mutations in the IgII and IgIII regions of FGFR2, although the assignment of the diagnosis of ABS to such children has been disputed. A third cause of an ABS-like phenotype is early in utero exposure to fluconazole, an inhibitor of lanosterol 14-alpha-demethylase. The fourth proposed cause of ABS is digenic inheritance combining heterozygosity or homozygosity for steroid 21-hydroxylase deficiency with effects from a second gene at an unknown locus. Because fluconazole is a strong inhibitor of lanosterol 14-alpha-demethylase (CYP51), we evaluated sterol metabolism in lymphoblast cell lines from an ABS patient without a known FGFR2 mutation and from a patient with an FGFR2 mutation and ABS-like manifestations. When grown in the absence of cholesterol to stimulate cholesterol biosynthesis, the cells from the ABS patient with ambiguous genitalia but without an FGFR2 mutation accumulated markedly increased levels of lanosterol and dihydrolanosterol. Although the abnormal sterol profile suggested a deficiency of lanosterol 14-alpha-demethylase, mutational analysis of its gene, CYP51, disclosed no obvious pathogenic mutation in any of its 10 exons or exon-intron boundaries. Sterol metabolism in lymphoblasts from the phenotypically unaffected mother was normal. Our results suggest that ABS can occur in a patient with an intrinsic defect of cholesterol biosynthesis at the level of lanosterol 14-alpha-demethylase, although the genetic nature of the deficiency remains to be determined.

Mutant P450 oxidoreductase causes disordered steroidogenesis with and without Antley-Bixler syndrome

“…Cholesterol biosynthetic P450 enzymes were implicated by reports that infants of mothers treated with fluconazole (an antifungal agent that interferes with yeast ergosterol synthesis by inhibiting lanosterol 14α-demethylase) had a pattern of congenital malformations that resembled ABS 3,4 . Mutations in human genes CYP17A1 (encoding P450c17; located at 10q24.3), CYP21A2 (encoding P450c21; located at 6p21.3) and CYP51A1 (encoding lanosterol 14α-demethylase; located at 7q21.2-q21.3) have been sought but not found 2,4,5 . We suggested that defects in P450c17 and P450c21 might be due to mutant POR, the flavoprotein that donates electrons to all microsomal P450 enzymes 6 , but the affected individuals lacked apparent disorders of bile acid synthesis or drug metabolism, which also require P450 enzymes, and knock-out of POR is embryonically lethal in mice 7,8 .…”

“…Sequencing of POR showed that individual 2 was heterozygous with respect to the missense mutation 1475T→A (leading to the amino acid substitution V492E; also found on the maternal allele) but detected no mutations in the exons or first 50 bp of splice donoracceptor sites on the paternal allele. Individual 3 had genital ambiguity, typical findings of ABS and abnormal sterol metabolism, but no steroidal data are available 5 ; he was apparently homozygous with respect to the substitution 859G→C (leading to the amino acid substitution A287P), and his mother was heterozygous (paternal DNA was not available To assess the activities of the POR mutants, we expressed each in Escherichia coli and measured the ability of bacterial membranebound proteins to receive electrons from NADPH and donate them to cytochrome c (see Supplementary Methods online). All five of the missense mutations markedly reduced both the capacity to oxidize NADPH and the catalytic efficiency (V max /K m ) of the reduction of cytochrome c ( Table 1).…”

“…The toxicity of morphogens like retinoids may also be enhanced by POR variants; decreased maternal retinoic acid exposure partially ameliorates the POR-knockout mouse phenotype 8 . POR mutations would account for the decreased lanosterol 14α-demethylase (CYP51) activity described in ABS 5 . The identification of more POR mutations and analysis of correlations between the resulting electron-transfer activity and the clinical phenotype will be of substantial interest.…”

A case of Antley-Bixler syndrome caused by compound heterozygous mutations of the cytochrome P450 oxidoreductase gene

“…Although the pathogenesis of skeletal malformations in POR deficiency is unclear, it has been suggested that cholesterol biosynthesis and POR may be involved in skeletal development. Examples include Smith-Lemli-Opitz syndrome, which shows complex skeletal malformations caused by mutations in the gene encoding 7-dehydrocholesterol reductase [15], and fluconazole-induced ABS, which is caused by inhibition of POR-dependent 14α-demethylase [11].…”

Molecular Basis of Adrenal Insufficiency

“…ABS is characterized by craniosynostosis, radio/ humeral synostosis, ambiguous genitalia, and other congenital anomalies. In addition, some patients with ABS showed abnormal steroid hormone profiles of elevated serum 17-hydroxyprogesterone and low cortisol levels (25)(26)(27). These endocrine findings have strongly suggested that there is a genetic defect of steroid biosynthesis in ABS.…”