Congenital Disorder of Glycosylation IIh: COG8 Gene Sequencing

Congenital disorders of glycosylation (CDG) are a group of autosomal recessive genetic disorders caused by the alteration in synthesis and structure of protein and lipid glycosylation. In the past decade, over 30 genetic diseases have been identified that alter glycan synthesis, structure and ultimately the function of nearly all organ systems.

CDG type I (CDGI) disorders result from impaired synthesis of the incomplete lipid linked oligosaccharide (LLO) and/or its attachment to the growing polypeptide chain. CDG-Ia is the most common form reported, due to phosphomannomutase deficiency, an enzyme that converts mannose-6-phosphate to mannose-1-phosphate. CDG-Ib (phosphomannose isomerase, MPI deficiency) is the only known treatable form, by giving mannose orally. CDG type II (CDGII) includes defects in processing of N-glycans.

Phenotypes of this disorder are extremely variable. Manifestations range from severe developmental delay and hypotonia with multiple organ system involvement beginning in infancy, to hypoglycemia and protein-losing enteropathy with normal development. Most subtypes have been described in only a few individuals, however, thus understanding of the phenotypes is limited.

The current diagnostic test for CDG is analysis of serum transferrin glycoforms, also called "transferrin isoforms analysis", or "carbohydrate-deficient transferrin analysis." If positive, this testing can be followed by DNA testing to identify mutations in the gene involved.

Two infants affected with CDG IIh had severe psychomotor delay, hypotonia, seizures, esotropia, failure to thrive, and progressive microcephaly.

Mutations causing CDG IIh have been found in the COG8 (16q22.1) gene.

For patients with suspected CDG IIh, sequence analysis is recommended as the first step in mutation identification. For patients in whom mutations are not identified by full gene sequencing, deletion/duplication analysis is appropriate.

References:

• Freeze HH. Congenital disorders of glycosylation: CDG-I, CDG-II, and beyond. Curr Mol Med 2007; 7:389-396.
• GeneTests: Congenital Disorders of Glycosylation Overview
• Jaeken J, Matthijs G. Congenital disorders of glycosylation: A rapidly expanding disease family. Annu Rev Genomics Hum Genet 2007; 8:261-278.

This test is indicated for:

• Confirmation of a clinical/biochemical diagnosis of CDG IIh
• Carrier testing in adults with a family history of CDG IIh

PCR amplification of 5 exons contained in the COG8 gene is performed on the patient's genomic DNA. Direct sequencing of amplification products is performed in both forward and reverse directions, using automated fluorescence dideoxy sequencing methods. The patient's gene sequences are then compared to a normal reference sequence. Sequence variations are classified as mutations, benign variants unrelated to disease, or variations of unknown clinical significance. Variants of unknown clinical significance may require further studies of the patient and/or family members. This assay does not interrogate the promoter region, deep intronic regions, or other regulatory elements, and does not detect large deletions.

• Deletion/duplication analysis of the COG8 gene by CGH array is available for those individuals in whom sequence analysis is negative.
• Analysis of other CDG genes is also available.
• Biochemical carbohydrate deficient transferrin analysis for CDGs is also available.
• Custom diagnostic mutation analysis (KM) is available to family members if mutations are identified by targeted mutation testing or sequencing analysis.
• Prenatal testing is available to adult couples who are confirmed carriers of mutations. Please contact the laboratory genetic counselor to discuss appropriate testing prior to collecting a prenatal specimen.

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