Predicted Benign and Synonymous Variants in CYP11A1 Cause Primary Adrenal Insufficiency Through Missplicing

Primary adrenal insufficiency (PAI) is a potentially life-threatening condition that can present with nonspecific features and can be difficult to diagnose. We undertook next generation sequencing in a cohort of children and young adults with PAI of unknown etiology from around the world and identified a heterozygous missense variant (rs6161, c.940G>A, p.Glu314Lys) in CYP11A1 in 19 individuals from 13 different families (allele frequency within undiagnosed PAI in our cohort, 0.102 vs 0.0026 in the Genome Aggregation Database; P < 0.0001). Seventeen individuals harbored a second heterozygous rare disruptive variant in CYP11A1 and two had very rare synonymous changes in trans (c.990G>A, Thr330 = ; c.1173C>T, Ser391 =). Although p.Glu314Lys is predicted to be benign and showed no loss-of-function in an Escherichia coli assay system, in silico and in vitro studies revealed that the rs6161/c.940G>A variant, plus the c.990G>A and c.1173C>T changes, affected splicing and that p.Glu314Lys produces a nonfunctional protein in mammalian cells. Taken together, these findings show that compound heterozygosity involving a relatively common and predicted "benign" variant in CYP11A1 is a major contributor to PAI of unknown etiology, especially in European populations. These observations have implications for personalized management and demonstrate how variants that might be overlooked in standard analyses can be pathogenic when combined with other very rare disruptive changes.

Functional characterization of the G162R and D216H genetic variants of human CYP17A1

Cytochrome P450 17A1 (CYP17A1) is a dual-function enzyme catalyzing reactions necessary for cortisol and androgen biosynthesis. CYP17A1 is a validated drug target for prostate cancer as CYP17A1 inhibition significantly reduces circulating androgens and improves survival in castration-resistant prostate cancer. Germline CYP17A1 genetic variants with altered CYP17A1 activity manifesting as various endocrinopathies are extremely rare; however, characterizing these variants provides critical insights into CYP17A1 protein structure and function. By querying the dbSNP online database and publically available data from the 1000 genomes project (http://browser.1000genomes.org), we identified two CYP17A1 nonsynonymous genetic variants with unknown consequences for enzymatic activity and stability. We hypothesized that the resultant amino acid changes would alter CYP17A1 stability or activity. To test this hypothesis, we utilized a HEK-293T cell-based expression system to characterize the functional consequences of two CYP17A1 variants, D216H (rs200063521) and G162R (rs141821705). Cells transiently expressing the D216H variant demonstrate a selective impairment of 16α-hydroxyprogesterone synthesis by 2.1-fold compared to wild-type (WT) CYP17A1, while no effect on 17α-hydroxyprogesterone synthesis was observed. These data suggest that substrate orientations in the active site might be altered with this amino acid substitution. In contrast, the G162R substitution exhibits decreased CYP17A1 protein stability compared to WT with a near 70% reduction in protein levels as determined by immunoblot analysis. This variant is preferentially ubiquitinated and degraded prematurely, with an enzyme half-life calculated to be ∼2.5 h, and proteasome inhibitor treatment recovers G162R protein expression to WT levels. Together, these data provide new insights into CYP17A1 structure-function and stability mechanisms.

A high rate of novel CYP11B1 mutations in Saudi Arabia

Despite ethnic variation, 11 β-hydroxylase deficiency (11β-OHD) has generally been considered the second most common subtype of congenital adrenal hyperplasia (CAH). We report a high rate of novel mutations in this gene (CYP11B1) in patients from Saudi Arabia. We studied 16 patients with 11β-OHD from 8 unrelated families. DNA was isolated from peripheral blood. The 9 exons and exon-intron boundaries of CYP11B1 were PCR-amplified and directly sequenced. The novel mutations were functionally characterized using subcloning, in vitro mutagenesis, cell transfection and 11-deoxycortisol: cortisol conversion assays. Six mutations were found in these 8 unrelated families. Three of these mutations are completely novel and two have just been recently described as novel mutations from the same population. These include a single nucleotide insertion mutation in codon 18 (c.53_54insT) leading to frameshift and truncation in 4 siblings, a novel mutation (c.1343G>C, p.R448P) in 3 unrelated families, a novel mutation (c.1394A>T, p.H465L) in 2 siblings, a novel mutation (c.617G>T, p.G206V) in 1 patient, and a recently described non-sense novel mutation (c.780G>A, p.W260X) in another patient. Out of the 6 mutations described in this report, only one mutation (p.Q356X) was reported previously. In vitro functional testing of the 3 missense and nonsense novel mutations revealed complete loss of the 11 hydroxylase activity. We conclude that 11 β-OHD in Saudi Arabia has a unique genotype with a high rate of novel mutations. The novel p. R448P mutation is the most common mutation in this highly inbred population.

Abstract 1863: Using a cell based model system to characterize CYP17A1 genetic variants

Androgen and estrogen steroid hormones are potent growth stimuli for both prostate and breast cancer. Cytochrome P450 17A1 (CYP17A1), a P450 monooxygenase, is required for the synthesis of these hormones and is the target of abiraterone, a drug approved for the treatment of castration-resistant prostate cancer. CYP17A1 catalyzes the 17α hydroxylation of the C21 steroids pregnenolone and progesterone and has a lyase activity that cleaves the C17-20 bond to produce the sex steroid precursors, dehydroepiandrosterone (DHEA) and androstenedione. We identified twelve nonsynonymous single nucleotide polymorphisms (SNPs) in CYP17A1 and hypothesized that the resultant amino acid changes might alter hydroxylase and lyase activity and thereby impact steroid hormone synthesis. This hypothesis was examined by transiently transfecting HEK293T cells, with either wild-type or polymorphic CYP17A1 constructs. Cells were treated for 3 hours with tritiated ([3H]) progesterone and the production of hydroxylated (OH) metabolites analyzed by HPLC. In cells transfected with wild-type CYP17A1, 8.11% and 55.5% of the [3H]-progesterone was recovered as 16-OH progesterone, and 17-OH progesterone, respectively. Using the ratio of 17-OH progesterone to 16-OH progesterone as a quantitative measure of CYP17A1 enzyme activity, wild-type CYP17A1 yields a ratio of 6.9+/- 0.05. In contrast, the SNP (rs200063521) which causes an aspartic acid to histidine amino acid change at residue 216, results in a 2.2 fold decrease in 16OH-progesterone synthesis compared to wild-type suggesting altered hydroxylase function. CYP17A1 protein expression, as determined by western blot analysis, suggests that a glycine to arginine conversion at residue 162 (rs141821705) results in decreased protein levels. Studies to further characterize the CYP17A1 variants are underway. This work could lead to a better understanding of the role of CYP17A1 SNPs in cancer progression and response to CYP17A1 targeted therapies.

Citation Format: Cameron P. Capper, Jiayan Liu, Lauren McIntosh, José M. Larios, Michael D. Johnson, Paul F. Hollenberg, Richard J. Auchus, James M. Rae. Using a cell based model system to characterize CYP17A1 genetic variants. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1863. doi:10.1158/1538-7445.AM2015-1863