GNAS Methylation-Specific MLPA

Turnaround Time

3 weeks

CPT Code(s)

81479

Cost

$600

Genes

• Pseudohypoparathyroidism (PHP)
• Pseudopseudohypoparathyroidism (PPHP)
• Osteoma Cutis (OC)
• Progressive Osseous Heteroplasia (POH)

Pseudohypoparathyroidism (PHP) is caused by inactivation of GNAS. There are multiple PHP phenotypes resulting from GNAS inactivation, including PHP -Ia, -Ib, and –Ic. Additional phenotypes include pseudopseudohypoparathyroidism (PPHP), progressive osseous heteroplasia (POH), and osteoma cutis (OC). These disorders are associated with varying degrees of hormone resistance and skeletal abnormalities including short stature, subcutaneous ossifications, and brachydactyly. GNAS is an imprinted locus on chromosome 20q13.32 with a primary mechanism for GNAS inactivation resulting from methylation defects. Epigenetic inactivation of GNAS can occur through paternal uniparental disomy, isolated epimutations, and alterations in regulatory elements of GNAS or STX16. Disorders of GNAS inactivation are examples of disorders involving imprinted genes. Imprinted genes are only expressed from either the maternally or paternally derived member of a homologous chromosome pair. Disorders of GNAS inactivation are caused by the lack of expression of GNAS. PHP -Ia, -Ib, and –Ic are caused by inactivation of the maternal GNAS complex locus while PPHP, POH, and OC are caused by inactivation of the paternal GNAS complex locus.

Methylation testing and copy number status is useful to confirm the diagnosis. Further testing may be necessary to determine the etiology of the disorder. GNAS methylation is appropriate when a pathogenic variant is not identified on sequence analysis.

A methylation sensitive multiplex ligation-dependent probe amplification (MLPA) assay is used to determine methylation status at 20q13.32 as well as any copy number variants in this region.

10-60% of probands with a disorder of GNAS Inactivation have a diagnostic change detectable by methylation analysis. This increases to almost 100% for probands with PHP-Ib.

The preferred sample type is 3-5 ml of peripheral blood collected in an EDTA (purple top) tube. Extracted DNA is also accepted for this test.

The specimen should be kept at room temperature and delivered via overnight shipping. If shipment is delayed by one or two days, the specimen should be refrigerated and shipped at room temperature. Do not freeze the specimen. Samples collected on Friday can be safely designated for Monday delivery.