Pseudohypoparathyroidism Type 1b: First Case Report in Chinese and Literature Review
Objective: Pseudohypoparathyroidism type 1b is a rare genetic endocrine disease. We would like to increase awareness of this condition and highlight what clinical geneticists can contribute in the patient management. Method: Case report and literature review. Case presentation: A 10-year-old boy was referred for suspected pseudohypoparathyroidism with parathyroid hormone resistance on biochemical investigation. Physical examination showed no features of Albright hereditary osteodystrophy and normal intelligence. There was no family history of endocrine or developmental problem. Based on above, the diagnosis of pseudohypoparathyroidism type 1b was suspected. It was subsequently confirmed by epigenetic change over GNAS gene using methylation genetic study. Conclusion: Pseudohypoparathyroidism 1b is a rare endocrine disease caused by epigenetic change in GNAS gene. All patients with parathyroid hormone resistance should be referred to the clinical geneticist for proper assessment, genetic testing, and genetic counselling.
Keyword : Chinese; Epigenetic; Pseudohypoparathyroidism type 1b
Pseudohypoparathyroidism (PHP) is a clinically and genetically heterogeneous disease characterised by parathyroid hormome resistance (PTH). It is an uncommon disorder and can be further divided into several subtypes based on the presence of hormonal resistance pattern, molecular abnormalities and Albright hereditary osteodystrophy (AHO). Pseudohypoparathyroidism type 1b (PHP-1b) (MIM#603233) is a rare subtype of PHP that is caused by loss of imprinting at the GNAS locus at chromosome 20q13.32 region. Methylation study targeting the GNAS gene will show specific pattern including hypermethylation at the NESP55 and hypomethylation at the NESPAS, GNAS XL and GNAS A/B differential methylation regions. Affected individuals mainly have renal resistance to PTH with some patients also having mild resistance to thyroid stimulating hormone. Typically, PHP-1b patients do not have features of AHO, though obesity, short stature and subtle bony abnormalities have been reported.1 Up to now, no more than 100 cases of PHP-1b were reported in the literature. Here we reported the first case in Chinese population that was confirmed by molecular testing.
The proband was a 10-year-old boy. He was the first child of a non-consanginuous Chinese couple. His perinatal history was uneventful, and he enjoyed good past health with normal development. There was no family history of endocrine problems or developmental delay. He initially presented with absence seizure at the age of 6. Investigations at that time showed hypocalcaemia, hyperphosphataemia and elevated PTH level. The initial biochemistry results were summarised in Table 1.
Based on the initial workup, the diagnosis of PHP was made. Further investigations after correction of hypocalcaemia, including brain imaging and electroencephalogram were normal. Skeletal survey showed no rachitic changes. He was put on vitamin D and calcium supplement and was regularly followed up in the endocrine clinic. Repeated ultrasound kidney showed no nephrocalcinosis. He was subsequently referred to Clinical Genetic Service for genetic investigation and counselling at the age of 10.
Physical examination at the genetic clinic showed normal growth parameters with both body weight and height at the 25th percentile. He had no cataract or craniofacial dysmorphism. There was no features of AHO like round face, brachydactyly and shortened metacarpal/metatarsal bones. Examination of parents was also normal. In view of the normal intelligence and lack of AHO features, PHP type 1b was suspected. Molecular testing using methylation specific-multiplex ligation dependent probe amplification (MS-MLPA) targeting the GNAS gene (SALSA MLPA kit ME031-A1 MRC-Holland) was performed. The results showed hypermethylation at the NESP55 and hypomethylation at the NESPAS, GNAS XL and GNAS A/B differential methylation regions (Figures 1a and 1b) compared with normal controls. There was no copy number change of the GNAS and STX16 genes. Parental genetic testing were normal. Paternal uniparental disomy of chromosome 20 was excluded by single nucleotide polymorphism (SNP)-array study (Agilent SurePrint G3 Human CGH+SNP Microarray 4x180K) (Figure 1c). The overall picture substantiated the diagnosis of sporadic PHP-1b due to imprinting defect.
With the coexistence of hypocalcaemia, hyperphosphataemia, elevated PTH levels without vitamin D and renal abnormalities, the diagnosis of PHP was easily made. However, in difficult cases, the diagnosis could only be confirmed by PTH infusion test or subcutaneous challenge before the molecular era. With more understanding of the underlying genetic and epigenetic mechanisms of PHP together with the highly sensitive and specific GNAS assay, GNAS molecular testing should be considered the first line investigation for all suspected PHP cases. The detection rates for typical PHP-1a and PHP-1b by GNAS mutation/epimutation screening are approximately 70% and 80-90%, respectively.2 Apart from confirmation of the clinical diagnosis that serves to guide patient management, knowledge of the underlying genetic or epigentic defect also facilitates genetic counselling in terms of risk assessment and prenatal diagnosis.
In contrast to PHP-1a characterised by presence of AHO features, developmental delay and/or other hormones resistance which is caused by heterozygous maternally derived mutations of the GNAS gene, PHP-1b is caused by loss of imprinting at the GNAS complex locus. The GNAS locus is located at chromosome 20q13.32 region and consists of at least 4 differential methylated regions (DMRs) (Figure 1a). By parent-specific methylation of different promoters, different transcripts are produced from the GNAS locus, namely Gsa (a subunit of the stimulatory G protein), XLas (Gas extra-large variant), NESP55 (neuroendocrine protein 55), A/B (untranslated exon A/B) and AS (antisense transcript).3-6 The NESP55 is maternal expressed while XLas, A/B and AS are paternally expressed.3,6 All PHP-1b patients have loss of methylation at the A/B DMR. There are 2 subtypes of PHP-1b with different genetic mechanisms. One subtype, accounting for 10-15% of PHP-1b cases, is associated with a maternally derived 3 kb microdeletion involving the STX16 gene, and is inherited in autosomal dominant fashion.7 The other subtype is associated with more extensive loss of imprinting at the GNAS locus that affects at least one additional DMR (hypermethylation at NESP and hypomethylation at AS and/or XL region ) without microdeletion of the STX16 or AS gene. It is sporadic and accounts for 80-85% of the cases.7 In order to study the epigenetic defects in PHP-1b, methylation study with techniques like methylation specific-multiplex ligation dependent probe amplification (MS-MLPA), methylation-specific polymerase chain reaction or pyrosequencing can be used. Despite recent progress in the understanding of the epigenetic defects associated with PHP-1b, the exact pathogenesis is yet to be elucidated.
The clinical manifestations of autosomal dominant and sporadic forms of PHP-1b are similar. Therefore, distinguishing between them by appropriate molecular testing is important for genetic counselling. In the literature, about 2 to 20% of PHP-1b cases were reported to be associated with paternal uniparental disomy (UPD) chromosome 20, thus UPD20 should be excluded in all case of PHP-1b, either by microsatellites analysis or SNP-array study. If UPD20 is excluded, the recurrence risk to subsequent siblings and offspring is negligible, while for the autosomal dominant form the recurrence risk is 50%.
In summary, we have reported the first case of sporadic PHP-1b due to imprinting defect in Chinese. In the presence of biochemical features of PHP like hypocalcaemia, hyperphosphataemia, elevated PTH, normal vitamin D and renal function, but lack of intellectual disability or skeletal features of AHO, the diagnosis of PHP-1b should be the considered. Epigenetic/epimutation study rather than sequencing of the coding region of GNAS gene should be performed as the initial genetic investigation (Figure 1d). All cases of PHP should be managed by specialists from multiple disciplines including clinical geneticist, so as to provide the best medical management and genetic counselling.
Conflict of Interest
The authors declare that they have no conflict of interest.
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