Unequal crossing over between CYP11B2 and CYP11B1 causes 11 β -hydroxylase deficiency in a consanguineous family

The p.W651fsX666 mutation on COL10A1 results in impaired trimerization of normal collagen X to induce Schmid type Metaphyseal chondrodysplasia

Haploinsufficiency resulting from the degradation of mutant Collagen Type X Alpha 1 Chain (COL10A1) mRNA by nonsense-mediated decay (NMD) has been attributed to the pathogenesis of Schmid-type metaphyseal chondrodysplasia (SMCD) in cases involving nonsense mutations. However, this mechanism does not fully explain the complexity of SMCD. In this study, we identified a c.1951_1952 InsT (p.W651 fsX666) mutation in exon 3 of COL10A1 that is associated with chondrodysplasia phenotypes in a two-generation family with SMCD. The mRNA decay of the mutant COL10A1 (named as affected E666X-COL10A1) is caused by the p.W651fsX666 mutation, which also disrupts the trimerization of normal collagen X. However, the mutant mRNA decay of affected exogenous E666X-COL10A1, as well as the complete degradation of E666X-COL10A1 mRNA in the proband, is not significantly induced by the W651fsX666 mutation. In vitro trimerization analyses results indicate that the trimerization of normal collagen X and wild-type collagen X are disrupted by W651fsX666 and E666X-collagen X mutations, respectively, suggesting that the mutant allele collagen X may impose a dominant-negative effect on the normal collagen X. Our results are the first to reveal that the impaired trimerization of normal collagen X is caused by the W651fsX666 mutation and a dominant-negative effect on the normal allele collagen X exerted by the mutant allele collagen X, causing impaired trimerization of collagen X, which will interpret the phenotype variability among the affected individuals in the pedigree with metaphyseal chondrodysplasia type Schmid (MCDS) studied.

Rare forms of congenital adrenal hyperplasia

Congenital adrenal hyperplasia (CAH) is a group of autosomal recessive disorders due to pathogenic variants in genes encoding enzymes and cofactors involved in adrenal steroidogenesis. Although 21-hydroxylase, 11β-hydroxylase, 3β-hydroxysteroid dehydrogenase type 2, 17α-hydroxylase/17,20-lyase, P450 oxidoreductase, steroidogenic acute regulatory protein, cholesterol side-chain cleavage enzyme deficiencies are considered within the definition of CAH, the term 'CAH' is often used to refer to '21-hydroxylase deficiency (21OHD)' since 21OHD accounts for approximately 95% of CAH in most populations. The prevalence of the rare forms of CAH varies according to ethnicity and geographical location. In most cases, the biochemical fingerprint of impaired steroidogenesis points to the specific subtypes of CAH, and genetic testing is usually required to confirm the diagnosis. Despite there are significant variations in clinical characteristics and management, most data about the rare CAH forms are extrapolated from 21OHD. This review article aims to collate the currently available data about the diagnosis and the management of rare forms of CAH.

Updates on Mechanisms of Cytochrome P450 Catalysis of Complex Steroid Oxidations

Cytochrome P450 (P450) enzymes dominate steroid metabolism. In general, the simple C-hydroxylation reactions are mechanistically straightforward and are generally agreed to involve a perferryl oxygen species (formally FeO3+). Several of the steroid transformations are more complex and involve C-C bond scission. We initiated mechanistic studies with several of these (i.e., 11A1, 17A1, 19A1, and 51A1) and have now established that the dominant modes of catalysis for P450s 19A1 and 51A1 involve a ferric peroxide anion (i.e., Fe3+O2¯) instead of a perferryl ion complex (FeO3+), as demonstrated with 18O incorporation studies. P450 17A1 is less clear. The indicated P450 reactions all involve sequential oxidations, and we have explored the processivity of these multi-step reactions. P450 19A1 is distributive, i.e., intermediate products dissociate and reassociate, but P450s 11A1 and 51A1 are highly processive. P450 17A1 shows intermediate processivity, as expected from the release of 17-hydroxysteroids for the biosynthesis of key molecules, and P450 19A1 is very distributive. P450 11B2 catalyzes a processive multi-step oxidation process with the complexity of a chemical closure of an intermediate to a locked lactol form.

Chimeric Genes Causing 11β-Hydroxylase Deficiency: Implications in Clinical and Molecular Diagnosis

Deficiency of 11β-hydroxylase (11β-OHD) is the second most common cause of congenital adrenal hyperplasia (CAH), accounting for 0.2-8% of all cases. The disease is transmitted as an autosomal recessive trait and the underlying genetic causes of 11β-OHD are primarily small pathogenic variants affecting the CYP11B1 gene coding the 11β-hydroxylase enzyme. However, special events complicate the molecular diagnosis of 11β-OHD such as an unequal crossing over between the CYP11B2 (coding aldosterone synthase enzyme) and CYP11B1 genes. The resulting allele contains a hybrid gene, with a CYP11B2 5'-end and a CYP11B1 3'-end, where the CYP11B1 gene is under the control of the CYP11B2 promoter and thus not responding to the adrenocorticotropin (ACTH) but to angiotensin II and K+. This leads a reduction of cortisol production in 11β-OHD. In particular, CYP11B2/CYP11B1 chimeric genes can be distinguished into two groups depending on the breakpoint site: chimeras with breakpoint after the exon 5 of CYP11B2 preserve the aldosterone synthase activity, the others with breakpoint before exon 5 lose this function. In the last case, a more severe phenotype is expected. The aim of this review was to explore the setting of CYP11B2/CYP11B1 chimeras in 11β-OHD, performing a careful review of clinical literature cases.

Case Report: A combination of chimeric CYP11B2/CYP11B1 and a novel p.Val68Gly CYP11B1 variant causing 11β-Hydroxylase deficiency in a Chinese patient

Introduction

11β-Hydroxylase deficiency (11β-OHD, OMIM#202010) is the second most common form of congenital adrenal hyperplasia (CAH) caused by pathogenic variants in the CYP11B1 gene. Both single nucleotide variations (SNV)/small insertion and deletion and genomic rearrangements of CYP11B1 are important causes of 11β-OHD. Among these variant types, pathogenic CYP11B2/CYP11B1 chimeras only contribute to a minority of cases. Heterozygote cases (chimera combined with SNV) are very rare, and genetic analysis of these cases can be challenging.

Case presentation

We presented a suspected 11β-OHD female patient with incomplete virilization, adrenal hyperplasia, and hypokalemia hypertension. Whole exome sequencing (WES) revealed that the patient carried both a chimeric CYP11B2/CYP11B1 and a novel missense variant, NM_000497.4: c.203T>G, p.Val68Gly (chr8:143961027) in CYP11B1, which were confirmed by CNVplex and Sanger sequencing, respectively. The patient's manifestations and genetic findings confirmed the diagnosis of 11β-OHD, and oral dexamethasone was administered as a subsequent treatment.

Conclusion

This report showed a rare CYP11B2/CYP11B1 chimera combined with a novel missense variant in a 11β-OHD female patient. The result expands variant spectrum of CYP11B1 and suggests that both chimera and CYP11B1 variant screening should be performed simultaneously in suspected cases of 11β-OHD. To our knowledge, this is the first report about CYP11B2/CYP11B1 chimera detected by WES analysis. WES combined with CNV analysis is an efficient method in the genetic diagnosis of this rare and complex disorder.