Aim: Determine whether variations in the paternally expressed GNAS splice variant XL-alpha S affect normal fetal growth?

Background: The term pseudohypoparathyroidism (PHP) refers to several rare, yet related human disorders. PHP type Ia (PHP-Ia) is characterized by an abnormal regulation of calcium and phosphate homeostasis because of resistance toward parathyroid hormone (PTH) in the proximal renal tubules; affected patients furthermore have developmental abnormalities referred to as Albright Hereditary Osteodystrophy (AHO) and often resistance towards other hormones such as TSH and GHRH that mediate their actions through Gs-alpha-coupled receptors. PHP type Ib (PHP-Ib) is characterized by hypocalcemia and hyperphosphatemia due PTH-resistance, typically without evidence for AHO. PHP-Ia and PHP-Ib are both imprinting disorder that are caused by maternal mutations within or up-stream of the GNAS locus on chromosome 20q13.3. This complex locus encodes the stimulatory G protein (Gs-alpha; GNAS exons 1-13) as well as several splice variants thereof, including XL-alpha S (GNAS exons 2-13, plus an XL-specific first exon), the extra-large form of Gs-alpha that is expressed only from the paternal, non-methylated GNAS allele.

Hypothesis: Are variations in XL-alpha S associated with normal fetal growth?

Very recently, we discovered through a collaboration with colleagues in France that paternal GNAS mutations are associated with intrauterine growth retardation (IUGR) (Richard et al., JCEM, 2013). Interestingly, mutations in GNAS exon 1, i.e. the exon specific for Gs-alpha, do not seem to lead to severe IUGR. This suggests that XL-alpha S, which uses the XL-specific first exon, contributes primarily to growth retardation. Conversely, biallelic XL-alpha S expression, as observed in patients with patUPD20q, is associated with enhanced fetal and probably also post-natal growth.

Our observations, which are surprising, led us to conclude that normal XL-alpha S expression from the paternal GNAS allele is essential for the prevention of IUGR and that XL-alpha S expression from both parental alleles results in much enhanced growth rates.

To investigate these findings in rare human disorders further and to determine whether the paternal GNAS haplotype is associated with fetal growth, we now propose to re-analyze some of the recently published GWAS data (Horikoshi et al 2013), namely the data from ALSPAC mother-child pairs.

The investigative plan would be as follows:

1) Define the region of interest at the GNAS locus on chromosome 20q13.3 and select SNPs from the maternal and fetal GWAS data within this region.

2) Deduce the paternally inherited allele, where possible, across the GNAS locus for each of these SNPs.

2) Use these deduced paternal allele data to determine whether inheritance of paternal (but not maternal) alleles is associated with impaired growth.

If this provides further evidence for a role of XL-alpha S in fetal development, we would then seek replication of the associations in further studies, e.g. in the EGG Consortium. If the association holds up, we could search in IUGR patients for paternal mutations in exon XL or its regulatory regions, i.e. in the exon specific for XL-alpha S; the identification of such XL-specific mutations would obviously be very exciting, as not much is known about the role of XL in normal human biology. In fact, our recent birth weight data for patients with either paternally or maternally inherited GNAS mutations have provided the first hints regarding its role in humans.