GNAS-Related Disorders via the GNAS Gene

The GNAS locus encodes multiple gene products including the alpha-subunit of the stimulatory guanine nucleotide-binding protein (Gsα). Germline genetic defects at the GNAS complex imprinted locus can cause five different autosomal dominant disorders depending on variant type and parental origin of the defective allele: pseudohypoparathyroidism type-Ia (PHP-Ia), pseudohypoparathyroidism type-Ic (PHP-Ic), pseudo-pseudohypoparathyroidism (PPHP), progressive osseous heteroplasia (POH), and pseudohypoparathyroidism type-Ib (PHP-Ib) (Turan et al. 2015; Lemos et al. 2015; Linglart et al. 2013).

Pseudohypoparathyroidism refers to a group of heterogeneous disorders characterized by end-organ resistance to parathyroid hormone (PTH). In addition to PTH, patients with PHP-Ia or PHP-Ic also have resistance to other hormones including thyroid-stimulating hormone (TSH) and gonadotropins. Both PHP-Ia and PHP-Ic have a constellation of clinical features called Albright hereditary osteodystrophy (AHO), which includes obesity, round facies, short stature, subcutaneous ossifications, brachydactyly, and intellectual disability. Both PHP-Ia and PHP-Ic are caused by maternally inherited inactivating Gsα-encoding GNAS variants and show blunted urinary cAMP and phosphate excretion in response to exogenous PTH administration. The difference between these two forms of PHP is that erythrocyte Gsα activity is normal in PHP-Ic, but reduced in PHP-Ia.

In addition to PHP-Ia and PHP-Ic, inactivating Gsα-encoding GNAS variants can cause AHO features without hormone resistance, which is termed as pseudo-pseudohypoparathyroidism (PPHP). In contrast to PHP-Ia and PHP-Ic, PPHP is caused by paternally inherited inactivating Gsα-encoding GNAS variants. PPHP has been commonly found in the same family with PHP-Ia, but they are never present in the same sibship due to different parental origin of the familial inactivating variant. Patients with PPHP have a normal urinary cAMP and phosphate excretion in response to exogenous PTH administration but erythrocyte Gsα activity is reduced.

Progressive osseous heteroplasia (POH) is also caused by paternally inherited inactivating Gsα-encoding GNAS variants and is characterized by dermal ossification beginning in infancy and progressive ossification into skeletal muscle and deep connective tissues in childhood. In most cases, AHO features or hormone resistance are absent. Like PPHP, patients with POH have a normal urinary cAMP and phosphate excretion in response to exogenous PTH administration but erythrocyte Gsα activity is reduced.

Pseudohypoparathyroidism type-Ib (PHP-Ib) is the type of PHP that PTH resistance manifests only in renal proximal tubules with hypocalcemia, hyperphosphatemia, and increased serum PTH, but typically without AHO features. Patients with PHP-Ib have a decreased urinary cAMP and phosphate excretion in response to exogenous PTH administration but erythrocyte Gsα activity is normal. Instead of inactivating Gsα-encoding GNAS variants, PHP-Ib is caused by maternally inherited methylation defects at the GNAS locus due to large deletions altering the differentially methylated regions (DMRs) upstream of the gene (the STX16 gene locus, the vicinity of NESP55 exon 1, or exons 3-4 of the antisense transcript).

The GNAS locus encodes multiple gene products including the alpha-subunit of the stimulatory guanine nucleotide-binding protein (Gsα), extra-large Gsα (XLαs), neuroendocrine secretory protein 55 (NESP55), GNAS-A/B and GNAS-AS1 (Turan et al. 2015). Except for the GNAS-AS1 transcript being derived from the antisense strand, other four transcripts share the same coding regions through exons 2 to 13 but have their own unique first exons.

Gsα is a ubiquitous signaling protein critical in pathways of hormones, neurotransmitters, and paracrine/autocrine factors. Documented genetic defects throughout Gsα-encoding GNAS exons (13 coding exons) include missense and various truncating variants. Large deletions and duplications have also been reported at the region of Gsα-encoding exons (Human Gene Mutation Database).

As described in the Clinical Features section, germline genetic defects at the GNAS locus cause five different autosomal dominant disorders depending on variant type and parental origin of the defective allele (Turan et al. 2015; Lemos et al. 2015; Linglart et al. 2013). For inactivating Gsα-encoding GNAS variants, however, no clear genotype-phenotype correlation has been found. Most of the pathogenic variants are private in individual families and de novo changes are common (Elli et al. 2013; Thiele et al. 2015).

To date, well-established pathogenic variants beyond the region of Gsα-encoding exons (upstream of Gsα-encoding GNAS exon 1) are only those large deletions that alter the differentially methylated regions (DMRs). The correlation of small variants (single nucleotide substitutions and small indels) in the vicinity of these reported deletions (the STX16 gene locus, the vicinity of NESP55 exon 1, or exons 3-4 of the antisense transcript) has been rarely reported and is still inconclusive although two small deletions (33 and 40bp in size) of uncertain significance in intron 1 of the NESP55 transcript have been reported in families with PHP-1b (Rezwan et al. 2015).

In a study of 78 unrelated patients with clinical diagnosis of pseudohypoparathyroidism type-Ia (PHP-Ia) or pseudo-pseudohypoparathyroidism (PPHP), sequencing analysis of Gsα-encoding GNAS exons found a heterozygous inactivating pathogenic variant in 49 patients (~63%) (Elli et al. 2013). In another study of 88 unrelated patients with variable clinical features of Albright hereditary osteodystrophy (AHO), sequencing analysis of Gsα-encoding GNAS exons found a heterozygous inactivating pathogenic variant in 64 patients (~73%; 26 familial and 40 de novo) (Thiele et al. 2015).

Sequencing analysis of Gsα-encoding GNAS exons found a heterozygous inactivating pathogenic variant in 13 (~72%) of 18 probands with progressive osseous heteroplasia (POH) (Shore et al. 2002).

In a study of 56 unrelated families (72 affected individuals in total) with clinical suspicion of PHP-Ia, pseudohypoparathyroidism type-Ib (PHP-Ib) or PPHP, sequencing analysis of Gsα-encoding GNAS exons found a heterozygous inactivating pathogenic variant in 25 families (~45%) (Fernández-Rebollo et al. 2013). In patients with no inactivating variants, six families (~11%) had the 3.2-kb microdeletion in STX16.

Of the 31 unrelated patients with PHP-Ib, 18 (58%) carried a deletion within STX16 (Linglart et al. 2005).

Garin et al. reported large deletions of various sizes affecting the GNAS locus including multi-exon deletions within the region of Gsα-encoding exons in 7 (6.25%) of 112 probands with PHP-Ia or PPHP (Garin et al. 2015).

This test is performed using Next-Generation sequencing with additional Sanger sequencing as necessary.

This test provides full coverage of all coding exons of the GNAS gene, plus ~10 bases of flanking noncoding DNA. We define full coverage as >20X NGS reads or Sanger sequencing.

This test can be used for detection of mosaic variants in GNAS for McCune-Albright syndrome, but please also note that detection of low level mosaicism is limited (<10% mosaicism may not be detected).