Ehlers-Danlos Syndrome: Molecular and Clinical Characterization of TNXA/TNXB Chimeras in Congenital Adrenal Hyperplasia

Long-read sequencing resolves the clinically relevant CYP21A2 locus, supporting a new clinical test for Congenital Adrenal Hyperplasia

Congenital Adrenal Hyperplasia (CAH), one of the most common inherited disorders, is caused by defects in adrenal steroidogenesis. It is potentially lethal if untreated and is associated with multiple comorbidities, including fertility issues, obesity, insulin resistance, and dyslipidemia. CAH can result from variants in multiple genes, but the most frequent cause is deletions and conversions in the segmentally duplicated RCCX module, which contains the CYP21A2 gene and a pseudogene. The molecular genetic test to identify pathogenic alleles is cumbersome, incomplete, and available from a limited number of laboratories. It requires testing parents for accurate interpretation, leading to healthcare inequity. Less severe forms are frequently misdiagnosed, and phenotype/genotype correlations incompletely understood. We explored whether emerging technologies could be leveraged to identify all pathogenic alleles of CAH, including phasing in proband-only cases. We targeted long-read sequencing outputs that would be practical in a clinical laboratory setting. Both HiFi-based and nanopore-based whole-genome long-read sequencing datasets could be mined to accurately identify pathogenic single-nucleotide variants, full gene deletions, fusions creating non-functional hybrids between the gene and pseudogene ("30-kb deletion"), as well as count the number of RCCX modules and phase the resulting multimodular haplotypes. On the Hi-Fi data set of 6 samples, the PacBio Paraphase tool was able to distinguish nine different mono-, bi-, and tri-modular haplotypes, as well as the 30-kb and whole gene deletions. To do the same on the ONT-Nanopore dataset, we designed a tool, Parakit, which creates an enriched local pangenome to represent known haplotype assemblies and map ClinVar pathogenic variants and fusions onto them. With few labels in the region, optical genome mapping was not able to reliably resolve module counts or fusions, although designing a tool to mine the dataset specifically for this region may allow doing so in the future. Both sequencing techniques yielded congruent results, matching clinically identified variants, and offered additional information above the clinical test, including phasing, count of RCCX modules, and status of the other module genes, all of which may be of clinical relevance. Thus long-read sequencing could be used to identify variants causing multiple forms of CAH in a single test.

Long-Read Sequencing Solves Complex Structure of CYP21A2 in a Large 21-Hydroxylase Deficiency Cohort

CONTEXT

Genetic testing for 21-hydroxylase deficiency (21-OHD) is always challenging. The current approaches of short-read sequencing and multiplex ligation-dependent probe amplification (MLPA) are insufficient for the detection of chimeric genes or complicated variants from multiple copies. Recently developed long-read sequencing (LRS) can solve this problem.

OBJECTIVE

To investigate the clinical utility of LRS in precision diagnosis of 21-OHD.

METHODS

In the cohort of 832 patients with 21-OHD, the current approaches provided the precise molecular diagnosis for 81.7% (680/832) of cases. LRS was performed to solve the remaining 144 cases with complex chimeric variants and 8 cases with variants from multiple copies. Clinical manifestations in patients with continuous deletions of CYP21A2 extending to TNXB (namely CAH-X) were further evaluated.

RESULTS

Using LRS in combination with previous genetic test results, a total of 16.9% (281/1664) CYP21A1P/CYP21A2 or TNXA/TNXB chimeric alleles were identified in 832 patients, with CYP21A1P/CYP21A2 accounting for 10.4% and TNXA/TNXB for 6.5%. The top 3 common chimeras were CYP21 CH-1, TNX CH-1, and TNX CH-2, accounting for 77.2% (217/281) of all chimeric alleles. The 8 patients with variants on multiple copies of CYP21A2 were accurately identified with LRS. The prevalence of CAH-X in our cohort was 12.1%, and a high frequency of connective tissue-related symptoms was observed in CAH-X patients.

CONCLUSION

LRS can detect all types of CYP21A2 variants, including complex chimeras and pathogenic variants on multiple copies in patients with 21-OHD, which could be utilized as a first-tier routine test for the precision diagnosis and categorization of congenital adrenal hyperplasia.

Comparison of long-read sequencing and MLPA combined with long-PCR sequencing of CYP21A2 mutations in patients with 21-OHD

Background

21-Hydroxylase deficiency (21-OHD) is caused by mutations in the CYP21A2 gene. Due to the complex structure and the high genetic heterogeneity of the CYP21A2 gene, genetic testing for 21-OHD is currently facing challenges. Moreover, there are no comparative studies on detecting CYP21A2 mutations by both second-generation sequencing and long-read sequencing (LRS, also known as third-generation sequencing).

Objective

To detect CYP21A2 variations in 21-OHD patients using targeted capture with LRS method based on the PacBio (Pacific Biosciences) Sequel II platform.

Methods

A total of 67 patients with 21-OHD were admitted in Wuhan Children's Hospital. The full sequence of CYP21A2 gene was analyzed by targeted capture combined with LRS based on the PacBio Sequel II platform. The results were compared with those of long-polymerase chain reaction (Long-PCR) combined with multiplex ligation probe amplification (MLPA) detection. Based on the in vitro study of 21-hydroxylase activity of common mutations, the patient genotypes were divided into groups of Null, A, B, and C, from severe to mild. The correlation between different genotype groups and clinical typing was observed.

Results

The study analyzed a total of 67 patients. Among them, 44 (65.67%) were males and 23 (34.33%) were females, with a male-to-female ratio of approximately 1.9:1. A total of 27 pathogenic variants were identified in the 67 patients, of which micro-conversion accounted for 61.9%, new variants of CYP21A2 accounted for 8.2%; deletion accounted for 22.4% (CYP21A2 single deletion and chimeric TNXA/TNXB accounted for 12.7%, chimeric CYP21A1P/CYP21A2 accounted for 9.7%); and duplication accounted for 3.0% (CYP21A2 Gene Duplication). I2G was the most common variant (26.9%). Targeted capture LRS and MLPA combined with Long-PCR detection of CYP21A2 mutations showed 30 detection results with differences. The overall genotype-phenotype correlation was 82.1%. The positive predictive rate of the Null group for salt wasting (SW) type was 84.6%, the A group for SW type was 88.9%, the group B for simple virilization (SV) type was 82.4%, and the group C for SV type was 62.5%. The correlation coefficient rs between the severity of the phenotype and the genotype group was 0.682 (P < 0.05).

Conclusion

Targeted capture combined with LRS is an integrated approach for detecting CYP21A2 mutations, allowing precise determination of connected sites for multiple deletions/insertions and cis/trans configurations without analyzing parental genomic samples. The overall genotype-phenotype correlation for 21-OHD is generally strong, with higher associations observed between genotype and phenotype for group Null, A, and B mutations, and larger genotype-phenotype variation in group C mutations. Targeted capture with LRS sequencing offers a new method for genetic diagnosis in 21-OHD patients.

Therapeutic difficulties in a patient with Ehlers-Danlos syndrome and numerous symptomatic premature ventricular contractions-case report and literature review

A 28-year-old female patient diagnosed with Ehlers-Danlos syndrome type III (hypermobile EDS, hEDS) was admitted to the cardiology clinic due to a 3-year history of symptomatic ventricular arrhythmia in the form of multiple premature ventricular contractions (PVCs). Attempts at antiarrhythmic treatment with beta-blockers, propafenone, and verapamil were unsuccessful. Due to the diagnosis of hEDS and the high risk of vascular complications related to the ablation procedure, invasive treatment was abandoned, and it was decided to implement flecainide. After the flecainide treatment initiation, a spectacular improvement in the number of ventricular arrhythmias was observed, along with the disappearance of the complaints previously reported by the patient. To the best of our knowledge, this is the first described case of spectacular flecainide antiarrhythmic effect in a patient with numerous PVCs also diagnosed with EDS. Flecainide treatment in the EDS group could be a successful alternative to ablation, which can lead to serious vascular and even life-threatening complications, especially after the failure of propafenone and beta-blockers treatment.

Molecular basis and genetic testing strategies for diagnosing 21-hydroxylase deficiency, including CAH-X syndrome

Congenital adrenal hyperplasia (CAH) is a group of autosomally recessive disorders that result from impaired synthesis of glucocorticoid and mineralocorticoid. Most cases (~95%) are caused by mutations in the CYP21A2 gene, which encodes steroid 21-hydroxylase. CAH patients manifest a wide phenotypic spectrum according to their degree of residual enzyme activity. CYP21A2 and its pseudogene (CYP21A1P) are located 30 kb apart in the 6q21.3 region and share approximately 98% of their sequences in the coding region. Both genes are aligned in tandem with the C4, SKT19, and TNX genes, forming 2 segments of the RCCX modules that are arranged as STK19-C4A-CYP21A1P-TNXA-STK19B-C4B-CYP21A2-TNXB. The high sequence homology between the active gene and pseudogene leads to frequent microconversions and large rearrangements through intergenic recombination. The TNXB gene encodes an extracellular matrix glycoprotein, tenascin-X (TNX), and defects in TNXB cause Ehlers-Danlos syndrome. Deletions affecting both CYP21A2 and TNXB result in a contiguous gene deletion syndrome known as CAH-X syndrome. Because of the high homology between CYP21A2 and CYP21A1P, genetic testing for CAH should include an evaluation of copy number variations, as well as Sanger sequencing. Although it poses challenges for genetic testing, a large number of mutations and their associated phenotypes have been identified, which has helped to establish genotype-phenotype correlations. The genotype is helpful for guiding early treatment, predicting the clinical phenotype and prognosis, and providing genetic counseling. In particular, it can help ensure proper management of the potential complications of CAH-X syndrome, such as musculoskeletal and cardiac defects. This review focuses on the molecular pathophysiology and genetic diagnosis of 21-hydroxylase deficiency and highlights genetic testing strategies for CAH-X syndrome.

Salt-wasting congenital adrenal hyperplasia phenotype as a result of the TNXA/TNXB chimera 1 (CAH-X CH-1) and the pathogenic IVS2-13A/C > G in CYP21A2 gene

BACKGROUND

Genetic diversity of mutations in the CYP21A2 gene is the main cause of the monogenic congenital adrenal hyperplasia (CAH) disorder. On chromosome 6p21.3, the CYP21A2 gene is partially overlapped by the TNXB gene, the two residing in tandem with their highly homologous corresponding pseudogenes (CYP21A1P and TNXA), which leads to recurrent homologous recombination.

METHODS AND RESULTS

In the present study, the genetic status of an ethnic Greek-Cypriot family, with a female neonate that was originally classified as male and manifested the salt-wasting (SW) form, is presented. Genetic defects in the CYP21A2 and TNXB genes were investigated by Sanger sequencing multiplex ligation-dependent probe amplification (MLPA) and a real-time PCR assay. The neonate carried in compound heterozygosity the TNXA/TNXB chimeric gene complex (termed CAH-X CH-1) that results in a contiguous CYP21A2 and TNXB deletion and in her second allele the pathogenic IVS2-13A/C > G (c.655A/C > G) in CYP21A2.

CONCLUSIONS

The classic SW-CAH due to 21-hydroxylase (21-OH) deficiency may result from various complex etiological mechanisms and, as such, can involve the formation of monoallelic TNXA/TNXB chimeras found in trans with other CYP21A2 pathogenic variants. This is a rare case of CAH due to 21-hydroxylase deficiency, which elucidates the role of the complex RCCX CNV structure in the development of the disease. Identification of the correct CAH genotypes for a given phenotype is of considerable value in assisting clinicians in prenatal diagnosis, appropriate treatment, and genetic counseling.

Pseudogene TNXA Variants May Interfere with the Genetic Testing of CAH-X

CAH-X is a hypermobility-type Ehlers-Danlos syndrome connective tissue dysplasia affecting approximately 15% of patients with 21-hydroxylase deficiency (21-OHD) congenital adrenal hyperplasia (CAH) due to contiguous deletion of CYP21A2 and TNXB genes. The two most common genetic causes of CAH-X are CYP21A1P-TNXA/TNXB chimeras with pseudogene TNXA substitution for TNXB exons 35-44 (CAH-X CH-1) and TNXB exons 40-44 (CAH-X CH-2). A total of 45 subjects (40 families) from a cohort of 278 subjects (135 families of 21-OHD and 11 families of other conditions) were found to have excessive TNXB exon 40 copy number as measured by digital PCR. Here, we report that 42 subjects (37 families) had at least one copy of a TNXA variant allele carrying a TNXB exon 40 sequence, whose overall allele frequency was 10.3% (48/467). Most of the TNXA variant alleles were in cis with either a normal (22/48) or an In2G (12/48) CYP21A2 allele. There is potential interference with CAH-X molecular genetic testing based on copy number assessment, such as with digital PCR and multiplex ligation-dependent probe amplification, since this TNXA variant allele might mask a real copy number loss in TNXB exon 40. This interference most likely happens amongst genotypes of CAH-X CH-2 with an in trans normal or In2G CYP21A2 allele.

Long-Term Outcomes of Congenital Adrenal Hyperplasia

A plethora of negative long-term outcomes have been associated with congenital adrenal hyperplasia (CAH). The causes are multiple and involve supra-physiological gluco- and mineralocorticoid replacement, excess adrenal androgens both intrauterine and postnatal, elevated steroid precursor and adrenocorticotropic hormone levels, living with a congenital condition as well as the proximity of the cytochrome P450 family 21 subfamily A member 2 (CYP21A2) gene to other genes. This review aims to discuss the different long-term outcomes of CAH.

Prevalence of CAH-X Syndrome in Italian Patients with Congenital Adrenal Hyperplasia (CAH) Due to 21-Hydroxylase Deficiency

21-hydroxylase deficiency (21OHD), the most common form of congenital adrenal hyperplasia (CAH), is associated with pathogenic variants in CYP21A2 gene. The clinical form of the disease ranges from classic or severe to non-classic (NC) or mild late onset. The CYP21A2 gene is located on the long arm of chromosome 6, within the RCCX region, one of the most complex loci in the human genome. The 3′untranslated sequence of CYP21A2 exon 10 overlap the last exon of TNXB gene (these genes lie on the opposite strands of DNA and have the opposite transcriptional direction) that encodes an extracellular matrix glycoprotein tenascin-X (TNX). A recombination event between TNXB and its pseudogene TNXA causes a 30 kb deletion producing a chimeric TNXA/TNXB gene (CAH-X chimera) where both CYP21A2 and TNXB genes are impaired. This genetic condition characterizes a subset of patients with 21OHD who display the hypermobility phenotype of Ehlers–Danlos syndrome (hEDS) (CAH-X Syndrome). The aim of this study was to assess the prevalence of CAH-X syndrome in an Italian cohort of patients with 21OHD. At this purpose, 196 probands were recruited. Multiplex ligation-dependent probe amplification (MLPA) and Sanger sequencing were used to identify the CAH-X genotype. Twenty-one individuals showed the heterozygous continuous deletion involving the CYP21A2 and part of the TNXB gene. EDS-related clinical manifestations were identified in most patients carrying the CAH-X chimera. A CAH-X prevalence of 10.7% was estimated in our population.

CAH-X Syndrome: Genetic and Clinical Profile

The term CAH-X was coined to describe a subset of patients with 21-hydroxylase deficiency displaying a phenotype compatible with the hypermobility type of Ehlers Danlos syndrome. The genetic defect is due to the monoallelic presence of a CYP21A2 deletion extending into the gene encoding tenascin X (TNXB), a connective tissue extracellular matrix protein. The result is a chimeric TNXA/TNXB gene causing tenascin-X haploinsufficiency. The prevalence of CAH-X was estimated to be around 14-15% in large cohorts of patients with 21-hydroxylase deficiency. However, population studies are still scarce and the clinical picture of the syndrome has yet to be fully defined. In this review, we discuss the current knowledge regarding the genetic and clinical profile of the CAH-X syndrome.

Congenital Adrenal Hyperplasia and Ehlers-Danlos Syndrome

Congenital adrenal hyperplasia (CAH) secondary to 21-hydroxylase deficiency is an autosomal recessive disorder. The 21-hydroxylase enzyme P450c21 is encoded by the CYP21A2 gene located on chromosome 6p21.33 within the HLA major histocompatibility complex. This locus also contains the CYP21A1P, a non-functional pseudogene, that is highly homologous to the CYP21A2 gene. Other duplicated genes are C4A and C4B, that encode two isoforms of complement factor C4, the RP1 gene that encodes a serine/threonine protein kinase, and the TNXB gene that, encodes the extracellular matrix glycoprotein tenascin-X (TNX). TNX plays a role in collagen deposition by dermal fibroblasts and is expressed in the dermis of the skin and the connective tissue of the heart and skeletal muscle. During meiosis, misalignment may occur producing large gene deletions or gene conversion events resulting in chimeric genes. Chimeric recombination may occur between TNXB and TNXA. Three TNXA/TNXB chimeras have been described that differ in the junction site (CH1 to CH3) and result in a contiguous CYP21A2 and TNXB gene deletion, causing CAH-X syndrome. TNXB deficiency is associated with Ehlers Danlos syndrome (EDS). EDS comprises a clinically and genetically heterogeneous group of connective tissue disorders. As molecular analysis of the TNXB gene is challenging, the TNX-deficient type EDS is probably underdiagnosed. In this minireview, we will address the different strategies of molecular analysis of the TNXB-gene, as well as copy number variations and genetic status of TNXB in different cohorts. Furthermore, clinical features of EDS and clinical recommendations for long-term follow-up are discussed.

Genes and Pseudogenes: Complexity of the RCCX Locus and Disease

Copy Number Variations (CNVs) account for a large proportion of human genome and are a primary contributor to human phenotypic variation, in addition to being the molecular basis of a wide spectrum of disease. Multiallelic CNVs represent a considerable fraction of large CNVs and are strictly related to segmental duplications according to their prevalent duplicate alleles. RCCX CNV is a complex, multiallelic and tandem CNV located in the major histocompatibility complex (MHC) class III region. RCCX structure is typically defined by the copy number of a DNA segment containing a series of genes - the serine/threonine kinase 19 (STK19), the complement 4 (C4), the steroid 21-hydroxylase (CYP21), and the tenascin-X (TNX) - lie close to each other. In the Caucasian population, the most common RCCX haplotype (69%) consists of two segments containing the genes STK19-C4A-CYP21A1P-TNXA-STK19B-C4B-CYP21A2-TNXB, with a telomere-to-centromere orientation. Nonallelic homologous recombination (NAHR) plays a key role into the RCCX genetic diversity: unequal crossover facilitates large structural rearrangements and copy number changes, whereas gene conversion mediates relatively short sequence transfers. The results of these events increased the RCCX genetic diversity and are responsible of specific human diseases. This review provides an overview on RCCX complexity pointing out the molecular bases of Congenital Adrenal Hyperplasia (CAH) due to CYP21A2 deficiency, CAH-X Syndrome and disorders related to CNV of complement component C4.