Familial Pseudohypoparathyroidism Type IB Associated with an SVA Retrotransposon Insertion in the GNAS Locus

(Epi)genetic and clinical characteristics in 84 patients with pseudohypoparathyroidism type 1B

OBJECTIVE

Pseudohypoparathyroidism type 1B (PHP1B) caused by methylation defects of differentially methylated regions (DMRs) on the GNAS locus can be categorized into groups according to etiologies and methylation defect patterns of the DMRs. The aim of this study was to clarify the clinical characteristics of each group.

DESIGN

Comprehensive molecular analyses consisting of methylation, copy number, and microsatellite analyses.

METHODS

Eighty-four patients with PHP1B were included in this study. We classified them into 5 groups, namely, autosomal dominant inheritance-PHP1B (Group 1, G1), sporadic-PHP1B (G2), and atypical-PHP1B (G3-G5), based on the methylation defect patterns in 4 DMRs on the GNAS locus and etiologies and evaluated the clinical findings in each group and compared them among the groups.

RESULTS

G2 had the youngest age and the highest serum intact parathyroid hormone levels among the 5 groups at the time of diagnosis. The most common symptoms at the time of diagnosis were tetany in G1, and seizures or loss of consciousness in G2. Albright's hereditary osteodystrophy and PHP-suggestive features were most frequently observed in the G2 proband. Nine patients had neurodevelopmental disorders (NDs) consisting of mild to borderline intellectual disability and/or developmental delay. There were no significant correlations between the average methylation ratios of 7 CpG sites in the GNAS-A/B:TSS-DMR and hormonal and biochemical findings.

CONCLUSION

This study revealed the differences in some clinical characteristics, particularly clinical features, and ages at the time of diagnosis between G2 and other groups and detailed NDs observed in some patients with PHP1B.

SINE-VNTR-Alu retrotransposon insertion as a novel mutational event underlying Glanzmann thrombasthenia

BACKGROUND

Glanzmann thrombasthenia (GT) is an autosomal recessive platelet aggregation disorder caused by mutations in ITGA2B or ITGB3.

OBJECTIVES

We aimed to assess the phenotype and investigate the genetic etiology of a GT pedigree.

METHODS

A patient with bleeding manifestations and mild mental retardation was enrolled. Complete blood count, coagulation, and platelet aggregation tests were performed. Causal mutations were identified via whole exome and genome sequencing and subsequently confirmed through polymerase chain reaction and Sanger sequencing. The transcription of ITGB3 was characterized using RNA sequencing and reverse transcription polymerase chain reaction. The αⅡb and β3 biosynthesis was investigated via whole blood flow cytometry and in vitro studies.

RESULTS

GT was diagnosed in a patient with defective platelet aggregation. Novel compound heterozygous ITGB3 variants were identified, with a maternal nonsense mutation (c.2222G>A, p.Trp741∗) and a paternal SINE-VNTR-Alu (SVA) retrotransposon insertion. The 5' truncated SVA element was inserted in a sense orientation in intron 11 of ITGB3, resulting in aberrant splicing of ITGB3 and significantly reducing β3 protein content. Meanwhile, both the expression and transportation of β3 were damaged by the ITGB3 c.2222G>A. Almost no αⅡb and β3 expressions were detected on the patient's platelets surface.

CONCLUSION

Novel compound heterozygous ITGB3 mutations were identified in the GT pedigree, resulting in defects of αⅡbβ3 biosynthesis. This is the first report of SVA retrotransposon insertion in the genetic pathogenesis of GT. Our study highlights the importance of combining multiple high-throughput sequencing technologies for the molecular diagnosis of genetic disorders.

The long-range interaction between two GNAS imprinting control regions delineates pseudohypoparathyroidism type 1B pathogenesis

Genetic defects of GNAS, the imprinted gene encoding the stimulatory G protein α-subunit, are responsible for multiple diseases. Abnormal GNAS imprinting causes pseudohypoparathyroidism type 1B (PHP1B), a prototype of mammalian end-organ hormone resistance. Hypomethylation at the maternally methylated GNAS A/B region is the only shared defect in patients with PHP1B. In autosomal dominant (AD) PHP1B kindreds, A/B hypomethylation is associated with maternal microdeletions at either the GNAS NESP55 differentially methylated region or the STX16 gene located approximately 170 kb upstream. Functional evidence is meager regarding the causality of these microdeletions. Moreover, the mechanisms linking A/B methylation and the putative imprinting control regions (ICRs) NESP-ICR and STX16-ICR remain unknown. Here, we generated a human embryonic stem cell model of AD-PHP1B by introducing ICR deletions using CRISPR/Cas9. With this model, we showed that the NESP-ICR is required for methylation and transcriptional silencing of A/B on the maternal allele. We also found that the SXT16-ICR is a long-range enhancer of NESP55 transcription, which originates from the maternal NESP-ICR. Furthermore, we demonstrated that the STX16-ICR is an embryonic stage-specific enhancer enabled by the direct binding of pluripotency factors. Our findings uncover an essential GNAS imprinting control mechanism and advance the molecular understanding of PHP1B pathogenesis.