Uniparental disomy in steroid 5alpha-reductase 2 deficiency

Molecular Characterization of Two Known SRD5A2 Gene Variants in Mexican Patients With Disorder of Sexual Development

Background: The 5α-reductase type 2 deficiency (5α-RD2) is a specific form of disorder of sexual development (DSD). Pathogenic variants in the SRD5A2 gene, which encodes this enzyme, are responsible for 46,XY DSD.

Objective: The objective of the study was to investigate the genetic etiology of 46,XY DSD in two Mexican families with affected children.

Materials and methods: The SRD5A2 gene of the parents and affected children was screened in both families via polymerase chain reaction amplification and DNA direct sequencing. The role of genetic variants in enzymatic activity was tested by site-directed mutagenesis.

Results: Subject 1 presented two variants: p.Glu197Asp and p.Pro212Arg. Subject 2 was homozygous for the variant p.Glu197Asp. The two variants were reported in early studies. The directed mutagenesis study showed that the p.Glu197Asp and p.Pro212Arg variants lead to a total loss of enzymatic activity and, consequently, abnormal genitalia development in the patients.

Conclusion: These results suggest that p.Glu197Asp and p.Pro212Arg are pathogenic variants that lead to the phenotypic expression of DSD. 5α-RD2 is of extreme importance not only because of its frequency (it is rare) but also because of its significance in understanding the mechanism of androgen action, the process of sexual differentiation, and the factors that influence normal sexual behavior.

Exome sequencing identified compound heterozygous mutations in the SRD5A2 gene in a case of 46,XY ambiguous genitalia

The disorders of sexual development (DSD) represent an array of phenotypes with ambiguous genitalia. The present case had microphallus with fused and bifid scrotum and was initially assigned androgen insensitivity syndrome; however, sequencing of the complete coding region of the androgen receptor gene failed to identify a causative mutation. We undertook whole exome sequencing for identification of the pathogenic mutation. The most promising pathogenic variants were genotyped using Sanger sequencing to confirm the genotypes. We found compound heterozygous mutations, c.169G>T and c.586G>A in the SRD5A2 gene in this case, resulting in a nonsense (p.Glu57Ter) and a nonsynonymous substitution (p.Gly196Ser), respectively. While the nonsense mutation would result in a truncated protein, p.Gly196Ser substitution has been previously reported to be pathogenic. The mutations were confirmed by Sanger sequencing. Sequencing of 96 normal male individuals did not show the above mutations, suggesting their pathogenic nature. In conclusion, we identified compound heterozygous pathogenic mutations, c.169G>T (p.Glu57Ter) and c.586G>A (p.Gly196Ser), in the SRD5A2 gene in a case of ambiguous genitalia. p.Glu57Ter is a novel mutation, which in compound heterozygote combination with Gly196Ser causes 5a reductase deficiency.

Integrative and Analytical Review of the 5-Alpha-Reductase Type 2 Deficiency Worldwide

Introduction

The conversion of testosterone into dihydrotestosterone is catalyzed by the 5α-reductase type 2 enzyme which plays a crucial role in the external genitalia virilization. It is encoded by the SRD5A2 gene. Allelic variants in this gene cause a 46,XY DSD with no genotype-phenotype relationship. It was firstly reported in the early 70s from isolated clusters. Since then, several cases have been reported. Putting together, it will expand the knowledge on the molecular bases of androgen milieu.

Methods

We searched for SRD5A2 allelic variants (AV) in the literature (PubMed, Embase, MEDLINE) and websites (ensembl, HGMD, ClinVar). Only cases with AV in both alleles, either in homozygous or compound heterozygous were included. The included cases were analyzed according to ethnicity, exon, domain, aminoacid (aa) conservation, age at diagnosis, sex assignment, gender reassignment, external genitalia virilization and functional studies. External genitalia virilization was scored using Sinnecker scale. Conservation analysis was carried out using the CONSURF platform. For categorical variables, we used X2 test and Cramer's V. Continuous variables were analyzed by t test or ANOVA. Concordance was estimated by Kappa.

Results

We identified 434 cases of 5ARD2 deficiencies from 44 countries. Most came from Turkey (23%), China (17%), Italy (9%), and Brazil (7%). Sixty-nine percent were assigned as female. There were 70% of homozygous allelic variants and 30% compound heterozygous. Most were missense variants (76%). However, small indels (11%), splicing (5%) and large deletions (4%) were all reported. They were distributed along with all exons with exon 1 (33%) and exon 4 (25%) predominance. Allelic variants in the exon 4 (NADPH-binding domain) resulted in lower virilization (p<0.0001). The codons 55, 65, 196, 235 and 246 are hotspots making up 25% of all allelic variants. Most of them (76%) were located at conserved aa. However, allelic variants at non-conserved aa were more frequently indels (28% vs 6%; p<0.01). The overall rate of gender change from female to male ranged from 16% to 70%. The lowest rate of gender change from female to male occurred in Turkey and the highest in Brazil. External genitalia virilization was similar between those who changed and those who kept their assigned gender. The gender change rate was significantly different across the countries (V=0.44; p<0.001) even with similar virilization scores.

Conclusion

5ARD2 deficiency has a worldwide distribution. Allelic variants at the NADPH-ligand region cause lower virilization. Genitalia virilization influenced sex assignment but not gender change which was influenced by cultural aspects across the countries. Molecular diagnosis influenced on sex assignment, favoring male sex assignment in newborns with 5α-reductase type 2 deficiency.

Primary congenital glaucoma due to paternal uniparental isodisomy of chromosome 2 and CYP1B1 deletion

Background

CYP1B1 variants and deletions are the most common cause of primary congenital glaucoma (PCG).

Methods

We investigated an individual with PCG from the Australian and New Zealand Registry of Advanced Glaucoma. We performed sequencing of the CYP1B1 gene, followed by Multiplex Ligation‐dependent Probe Amplification and SNP array.

Results

We identified a homozygous deletion of the CYP1B1 gene by Multiplex Ligation‐dependent Probe Amplification and confirmed that the father was heterozygous for a CYP1B1 deletion but the mother had normal gene copy number. SNP array identified paternal uniparental isodisomy of the entire chromosome 2.

Conclusions

This study is the first report of a homozygous CYP1B1 whole gene deletion due to paternal uniparental isodisomy of chromosome 2 as a cause of PCG. These results illustrate the importance of genetic testing in providing appropriate genetic counseling regarding the risks of recurrence.

5α-Reductase-2 Deficiency: Clinical Findings, Endocrine Pitfalls, and Genetic Features in a Large Italian Cohort

Clinical records (n = 24) with an established diagnosis of 5α-reductase-2 deficiency were reviewed. A previous misdiagnosis was present in about 70% (period from first observation to definitive diagnosis: 9.1 ± 10.8 years), and in 8 children gonadal removal was performed before certain diagnosis. Initial sex assignment was female in 16/24 (67%) and male in 8/24 (33%) cases. After diagnosis, sex re-assignment was performed in 5 babies (4 girls to male sex; 1 boy to female sex). Baseline testosterone/DHT ratio was diagnostic in 6/12 subjects (first months of life n = 4; puberty n = 2), while post-hCG testosterone/DHT ratio was diagnostic in all tested individuals (choosing both the cut-off value 15 or 10). Eighteen different mutations in the steroid-5α-reductase-2 (SRD5A2) gene were identified, 5 of which have never been reported. In conclusion, a time lag exists before the diagnosis of 5α-reductase-2 deficiency is established; sex assignment and gonadal removal may be performed before certain diagnosis. Sex re-assignment is usually female to male, but the contrary may occur. A large variability in clinical phenotypes and genetic mutations was present in this cohort. Accurate endocrine evaluation is recommended in babies possibly affected by 5α-reductase-2 deficiency, since the use of appropriate cut-off values of testosterone/DHT ratio after hCG stimulation may permit to select individuals for SRD5A2 gene analysis. A genotype-phenotype correlation was not found in this study.

46,XY disorder of sexual development resulting from a novel monoallelic mutation (p.Ser31Phe) in the steroid 5α-reductase type-2 (SRD5A2) gene

Inactivating mutations of the 5α-steroid reductase type-2 (SRD5A2) gene result in a broad spectrum of masculinization defects, ranging from a male phenotype with hypospadias to a female phenotype with Wolffian structures. Molecular studies of the SRD5A2 revealed a new heterozygous gene variant within the coding region that results in phenotypic expression. A c.92C>T transition changing serine to phenylalanine at codon 31 of exon 1 (p.Ser31Phe) was identified in a patient with 46,XY disorder of sexual development who displayed glandular hypospadias with micropenis and bilateral cryptorchidism. The restoration of the p.Ser31Phe mutation by site-directed mutagenesis and transient expression assays using cultured HEK-293 cells showed that this novel substitution does not abolish but does deregulate the catalytic efficiency of the enzyme. Thus, the maximum velocity (V_max) value was higher for the mutant enzyme (22.5 ± 6.9 nmol DHT mg protein⁻¹ h⁻¹) than for the wild-type enzyme (9.8 ± 2.0 nmol DHT mg protein⁻¹ h⁻¹). Increased in vitro activity of the p.Ser31Phe mutant suggested an activating effect. This case provides evidence that heterozygous missense mutations in SRD5A2 may induce the abnormal development of male external genitalia.

Practical approach to steroid 5alpha-reductase type 2 deficiency

The aim of this article is to review the literature on steroid 5alpha-reductase type 2 deficiency (5α-RD2) to provide clinicians with information to guide their management of patients with this disorder. The 5alpha-reductase type 2 is encoded by the 5alpha-reductase type 2 gene (SRD5A2) on chromosome 2 and is predominantly expressed in external genital tissues and the prostate. Mutations of the SRD5A2 gene leads to an uncommon autosomal recessive disorder affecting sexual differentiation in individuals with 46,XY karyotype; their phenotype can range from almost normal female structures to a distinct male phenotype with ambiguous genitalia at birth. These phenotypes result from impaired conversion of testosterone to dihydrotestosterone due to mutations in the SRD5A2 gene. Patients exhibit virilization at puberty without breast development, which is often accompanied by gender identity change from female to male. More than 40 mutations have been reported in all five exons of the SRD5A2 gene. Phenotype-genotype correlations for 5α-RD2 have not been well established. The newborn phenotypes of male pseudohermaphrodites with 5α-RD2, partial androgen insensitivity syndrome (PAIS), or 17β-hydroxysteroid dehydrogenase type 3 (17β-HSD3) enzyme deficiency may be indistinguishable. We conclude that steroid 5α-RD2 should be included in the differential diagnosis of newborns with 46,XY DSD. It is important that the diagnosis be made in infancy by biochemical and molecular studies before gender assignment or any surgical intervention because these patients should be considered males at birth.

Inherited surfactant deficiency caused by uniparental disomy of rare mutations in the surfactant protein-B and ATP binding cassette, subfamily a, member 3 genes

OBJECTIVE

To characterize inheritance of homozygous, rare, recessive loss-of-function mutations in surfactant protein-B (SFTPB) or ATP binding cassette, subfamily A, member 3 (ABCA3) genes in newborns with lethal respiratory failure.

STUDY DESIGN

We resequenced genes from parents whose infants were homozygous for mutations in SFTPB or ABCA3. For infants with only 1 heterozygous parent, we performed microsatellite analysis for chromosomes 2 (SFTPB) and 16 (ABCA3).

RESULTS

We identified 1 infant homozygous for the g.1549C > GAA mutation (121ins2) in SFTPB for whom only the mother was heterozygous and 3 infants homozygous for mutations in ABCA3 (p.K914R, p.P147L, and c.806_7insGCT) for whom only the fathers were heterozygous. For the SP-B-deficient infant, microsatellite markers confirmed maternal heterodisomy with segmental isodisomy. Microsatellite analysis confirmed paternal isodisomy for the 3 ABCA3-deficient infants. Two ABCA3-deficient infants underwent lung transplantation at 3 and 5 months of age, respectively, and 2 infants died. None exhibited any nonpulmonary phenotype.

CONCLUSIONS

Uniparental disomy should be suspected in infants with rare homozygous mutations in SFTPB or ABCA3. Confirmation of parental carrier status is important to provide recurrence risk and to monitor expression of other phenotypes that may emerge through reduction to homozygosity of recessive alleles.

Maternal uniparental heterodisomy with partial isodisomy of a chromosome 2 carrying a splice acceptor site mutation (IVS9-2A>T) in ALS2 causes infantile-onset ascending spastic paralysis (IAHSP)

Infantile-onset ascending spastic paralysis (OMIM #607225) is a rare autosomal recessive early onset motor neuron disease caused by mutations in the gene ALS2. We report on a splice acceptor site mutation in intron 9 of ALS2 (IVS9-2A>T) in a German patient from nonconsanguineous parents. The mutation results in skipping of exon 10. This causes a frame-shift in exon 11 and a premature stop codon. Analysis of the parental ALS2 gene revealed heterozygosity for the mutation in the mother but not in the father. Therefore, we studied polymorphic markers scattered along chromosome 2 in both parents and the patient and found maternal uniparental disomy in the patient. While homozygosity was observed at several loci of chromosome 2 including ALS2, other loci were heterozygous, i.e., both maternal alleles were present. The findings can be explained by at least four recombination events during maternal meiosis followed by a meiosis I error and postzygotic trisomy rescue or gamete complementation.

Donnai-Barrow syndrome (DBS/FOAR) in a child with a homozygous LRP2 mutation due to complete chromosome 2 paternal isodisomy

Donnai-Barrow syndrome [Faciooculoacousticorenal (FOAR) syndrome; DBS/FOAR] is a rare autosomal recessive disorder resulting from mutations in the LRP2 gene located on chromosome 2q31.1. We report a unique DBS/FOAR patient homozygous for a 4-bp LRP2 deletion secondary to paternal uniparental isodisomy for chromosome 2. The propositus inherited the mutation from his heterozygous carrier father, whereas the mother carried only wild-type LRP2 alleles. This is the first case of DBS/FOAR resulting from uniparental disomy (UPD) and the fourth published case of any paternal UPD 2 ascertained through unmasking of an autosomal recessive disorder. The absence of clinical symptoms above and beyond the classical phenotype in this and the other disorders suggests that paternal chromosome 2 is unlikely to contain imprinted genes notably affecting either growth or development. This report highlights the importance of parental genotyping in order to give accurate genetic counseling for autosomal recessive disorders.

Novel compound heterozygous mutations in the SRD5A2 gene from 46,XY infants with ambiguous external genitalia

Dihydrotestosterone is crucial for normal development of external genitalia and prostate in the male embryo. Autosomal recessive mutations in the 5 alpha-reductase type 2 (SRD5A2) gene disrupt the synthesis of dihydrotestosterone in the urogenital tract and give rise to genetic males with undervirilized external genitalia that may be female-like or ambiguous. In this study, three unrelated 46,XY children (0.5, 3, and 8 years old) who presented severe undermasculinization at birth were examined for genetic abnormalities in the SRD5A2 gene. Coding sequence abnormalities were ascertained by exon-specific polymerase chain reaction (PCR), single-stranded conformational polymorphism (SSCP), and sequencing analysis. Functional properties of the mutant alleles were investigated by means of site-directed mutagenesis assays. DNA molecular studies showed that all three patients were compound heterozygotes for SRD5A2 mutations. Patient 1 had a point mutation 547G --> A in exon 3 (G183S) and a novel dinucleotidic mutation 634,635CC --> TG in exon 4 (P212X). This double change results in premature termination signal (TGA) at codon 212, which predicts the expression of a truncated 211-amino acid protein. Patient 2 was the carrier of mutations G115D in exon 3 and S210F in exon 4. Patient 3 had two substitution mutations in exon 1, including a novel G --> C transversion at nucleotide 169 (E57Q) and a G --> A transition at nucleotide 254 (G85D). In transitory transfection assays, the recombinant cDNAs harboring mutations E57Q and G85D showed residual 5 alpha-reductase activity, whereas those with mutations G115D, S210F, and P212X were devoid of activity. In contrast, the G183S substitution affected the catalytic activity of the enzyme by decreasing its affinity for testosterone substrate. We describe six different mutations of the SRD5A2 gene detected in three children with genital ambiguity. These genotypes are consistent with the clinical phenotype of steroid 5 alpha-reductase 2 deficiency. Our data suggest that the combined gene variants (E57Q/G85D, G115D/S210F, and G183S/P212X) result in subfunctional or nonfunctional enzymes, causing masculinization defects in these patients. This further underscores that exon 4 of SRD5A2 may be a site prone to inactivating mutations.