Scientific background:

Summary: The gene product is involved cGMP signal transduction. Imprinting influences which isofoms are transcribed, spliced, and translated. Deficiency causes pseudohypoparathyroidism, Albright osteodystrophy and pituitary tumor.

Gene: The gene is located on chromosome 10 (20q13.2), spans approximately 74kb, and consists of 13 exons. Four splice variants are known. The differ predominantly in their first exon.
   The protein product of the splice variant with the most upstream first exon is also known as NESP55. Exons 2-13 are also transcribed but translation is terminated by a stop codon in exon 1. The function of NESP55 remains to be elucidated.
   The protein product of the splice variant with the most downstream transcription start is called Gs, a component of G-coupled receptors. Exons 2-13 express all essential components of normal protein function.
   A third splice variant with an exceptional large exon 1 is named XLAS. As exons 2-13 are identically expressed its function seems to be similar to Gs.
   Still an other exon 1, dubbed 1A, with a different pattern of imprinting is postulated. The understanding of gene function is further convoluted by the discovery of a NESP55 complementary transcription product.
   Genomic imprinting ensures different gene products from the maternal and paternal alleles expressed in renal tubules. Genomic imprinting is an epigenetic phenomenon associated with methylation of the promoter (at cytidines within CpG dinucleotides) that inactivates distinct splice variants. Normally the splice variants XLAS and A1 are inactivated on the maternal allele whereas NESP55 is inactivated on the paternal allele.

Interpretation: Mutations that affect the coding region of the Gs splice variant result in Albright hereditary osteodystrophy (AOH) characterized by skeletal abnormalities and endocronological dysfunctions. If inherited from father AOH is associated with pseudopseudohypoparathyroidism (PPHP) whereas maternal transmission results in several endocrinological dysfunctions including thyreotropin resistance and pseudohypoparathyroidism (PHP1A).
   If both alleles exhibit paternal imprinting (lack of methylation) of the promotor of the splice variant A1, then in the distal tubule insufficient Gs will be transcribed and a PTH resistence of the kidney ensues (PHP1B).

Methodology:

	clinical test	Method		Genomic sequencing of the entire coding region
		Turn-around time		25 working days
		Effort		medium
		Specimen		DNA
		Quality assessment		Internal quality control only
	All known and new missense, nonsense and splice mutations can be detected.
	clinical test	Method		Methylation test
		Turn-around time		25 working days
		Effort		little
		Specimen		DNA
		Quality assessment		Internal quality control only
	clinical test	Method		Multiplex Ligation-Dependent Probe Amplification
		Turn-around time		25 working days
		Effort		little
		Specimen		DNA
		Quality assessment		Internal quality control only
	clinical test	Method		Carrier testing
		Turn-around time		5 working days
		Effort		little
		Specimen		DNA
		Quality assessment		Internal quality control only
	The test is only specific about the mutation already known in this kindred.

Literature: 

Jan de Beur S et al. (2003) Discordance between genetic and epigenetic defects in pseudohypoparathyroidism type 1b revealed by inconsistent loss of maternal imprinting at GNAS1.
Freson K et al. (2002) Pseudohypoparathyroidism type Ib with disturbed imprinting in the GNAS1 cluster and Gsalpha deficiency in platelets.
Hayward BE et al. (2001) Imprinting of the G(s)alpha gene GNAS1 in the pathogenesis of acromegaly.
Liu J et al. (2000) A GNAS1 imprinting defect in pseudohypoparathyroidism type IB.
Hayward BE et al. (1998) Bidirectional imprinting of a single gene: GNAS1 encodes maternally, paternally, and biallelically derived proteins.
Hayward BE et al. (1998) The human GNAS1 gene is imprinted and encodes distinct paternally and biallelically expressed G proteins.