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.
Timal et al. (2012) identified a hemizygous de novo pathogenic variant in the ALG13 gene (Xq23) in an individual with CDG-Is. His features included epilepsy, hepatomegaly, recurrent infections, swelling, microcephaly, and bilateral optic nerve atrophy. The same pathogenic variant, also de novo, was identified by the Epi4K Consortium in an unrelated female with seizures and severe intellectual disability.
- Epi4K Consortium. (2013), Nature, 501:217-221.
- Timal et al. (2012), Hum Mol Genet, 21:4151-4161.
- OMIM #300776: ALG13 gene
- OMIM #300884: CDG-Is
- Confirmation of a clinical diagnosis of CDG-Is in an individual in whom sequence analysis was negative.
- Carrier testing in adults with a family history of CDG-Is in whom sequence analysis was negative.
Please note that a "backbone" of probes across the entire genome are included on the array for analytical and quality control purposes. Rarely, off-target copy number variants causative of disease may be identified that may or may not be related to the patient's phenotype. Only known pathogenic off-target copy number variants will be reported. Off-target copy number variants of unknown clinical significance will not be reported.
Type: Whole Blood
Specimen Requirements:In EDTA (purple top) or ACD (yellow top) tube: Infants (<2 years): 2-3 ml Children (>2 years): 3-5 ml Older Children & Adults: 5-10 ml
Specimen Collection and Shipping: Refrigerate until time of shipment. Ship sample within 5 days of collection at room temperature with overnight delivery.
Specimen Requirements:OrageneTM Saliva Collection kit (available through EGL) used according to manufacturer instructions.
Specimen Collection and Shipping: Please do not refrigerate or freeze saliva sample. Please store and ship at room temperature.
Sequence analysis is required before deletion/duplication analysis by targeted CGH array. If sequencing is performed outside of Emory Genetics Laboratory, please submit a copy of the sequencing report with the test requisition.
- Sequence analysis of the ALG13 gene is available and is required before deletion/duplication analysis.
- 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.