Test ID: GCT    
Galactosemia Reflex, Blood

Diagnosis, carrier detection, and determination of genotype of galactose-1-phosphate uridyltransferase deficiency, the most common cause of galactosemia

Differentiating Duarte variant galactosemia from classic galactosemia

Confirming results of newborn screening programs

Preferred test to evaluate for possible diagnosis of galactosemia, routine carrier screening, and follow-up of abnormal newborn screening results. Comprehensive reflex test begins with quantitative galactose-1-phosphate uridyltransferase (GALT) enzyme analysis (GALT/8333 Galactose-1-Phosphate Uridyltransferase [GALT], Blood). If quantitative GALT enzyme value is consistent with a diagnosis of or carrier status for galactosemia, DNA analysis of the GALT gene (GAL6/84366 Galactosemia Gene Analysis [6-Mutation Panel]) is performed to detect the 4 most frequently encountered classic galactosemia alleles (Q188R, S135L, K285N, and L195P) in addition to the N314D (Duarte) and L218L (Los Angeles) variants.

Galactosemia is an autosomal recessive disorder that results from a deficiency of 1 of the 3 enzymes catalyzing the conversion of galactose to glucose: galactose-1-phosphate uridyltransferase (GALT), galactokinase (GALK), and uridine diphosphate galactose-4-epimerase (GALE). GALT deficiency is the most common cause of galactosemia and is often referred to as classic galactosemia. The complete or near-complete deficiency of GALT enzyme is life-threatening if left untreated. Complications in the neonatal period include failure to thrive, liver failure, sepsis, and death; even with survival, long-term intellectual disability can occur. Galactosemia is treated by a galactose-restricted diet, which allows for rapid recovery from the acute symptoms and a generally good prognosis. Despite adequate treatment from an early age, individuals with galactosemia remain at increased risk for developmental delays, speech problems, and abnormalities of motor function. Females with galactosemia are at increased risk for premature ovarian failure. Based upon reports by newborn screening programs, the frequency of classic galactosemia in the United States is approximately 1 in 30,000, although literature reports range from 1 in 10,000 to 1 in 60,000 live births.

Galactose-1-phosphate (Gal-1-P) accumulates in the erythrocytes of patients with galactosemia. The quantitative measurement of Gal-1-P is useful for monitoring compliance with dietary therapy. Gal-1-P is thought to be the causative factor for development of liver disease in these patients and, because of this, patients should maintain low levels and be monitored on a regular basis.

Duarte-variant galactosemia (compound heterozygosity for the Duarte mutation, N314D, and a classic mutation) is generally associated with higher levels of enzyme activity (5%-20%) than classic galactosemia (<5%); however, this may be indistinguishable by newborn screening assays. Typically, individuals with Duarte-variant galactosemia have a milder phenotype, but are also often treated with a low galactose diet during infancy. The LA variant, which consists of N314D and a second mutation, L218L, is associated with higher levels of GALT enzyme activity than the Duarte-variant allele.

Newborn screening, which identifies potentially affected individuals by measuring total galactose (galactose and Gal-1-P) and/or determining the activity of the GALT enzyme, varies from state to state. The diagnosis of galactosemia is established by follow-up quantitative measurement of GALT enzyme activity. If enzyme levels are indicative of carrier or affected status, molecular testing for common GALT mutations may be performed. If 1 or both disease-causing mutations are not detected by targeted mutation analysis and biochemical testing has confirmed the diagnosis of galactosemia, sequencing of the GALT gene is available to identify private mutations.

Several disease-causing mutations are common in patients with classic galactosemia (G/G genotype). The most frequently observed is the Q188R classic mutation. This mutation accounts for 60% to 70% of classical galactosemia alleles. The S135L mutation is the most frequently observed mutation in African Americans and accounts for approximately 50% of the mutant alleles in this population. The K285N mutation is common in those of eastern European descent and accounts for 25% to 40% of the alleles in this population. The L195P mutation is observed in 5% to 7% of classical galactosemia. The Duarte mutation (N314D) is observed in 5% of the general US population.

For more information regarding diagnostic strategy, refer to Galactosemia: Current Testing Strategy and Aids for Test Selection, Mayo Medical Laboratories Communique 2005 May;30(5).

See Galactosemia Testing Algorithm in Special Instructions for additional information.

> or =18.5 U/g of hemoglobin

The laboratory provides an interpretation of the results, including galactose-1-phosphate uridyltransferase enzyme activity and genotype, if necessary. This interpretation provides an overview of the results and their significance, a correlation to available clinical information, elements of differential diagnosis, and recommendations for additional testing.

Any specimen where enzyme activity is <18.5 U/g hemoglobin will be analyzed for the presence of the 4 mutations associated with classic galactosemia, as well as the 2 variants (Duarte and LA). See Galactosemia Reflex Algorithm in Special Instructions for testing algorithm and additional information.

This test detects 4 of the most frequently encountered classic galactosemia alleles (Q188R, S135L, K285N, and L195P), as well as the N314D Duarte and L218L LA variants. A high proportion (20%) of patients with classic galactosemia have a private mutation. Since our assay does not investigate for the presence of private mutations, when GG, DG, or NG genotype is predicted by enzymatic studies and the current panel does not identify a mutation, molecular sequencing may be indicated.

This assay is not useful for monitoring dietary compliance by galactosemics, see GAL1P/80337 Galactose-1-Phosphate (Gal-1-P), Erythrocytes.

This assay will not detect all of the mutations that cause galactosemia. Therefore, the absence of a detectable mutation(s) does not rule out the possibility that an individual is a carrier of or affected with this disease.

Test results should be interpreted in the context of clinical findings, family history, and other laboratory data. Errors in our interpretation of results may occur if information given is inaccurate or incomplete.

Many disorders may present with symptoms similar to those associated with galactosemia. Therefore, biochemical testing is performed to establish the diagnosis of galactosemia prior to DNA analysis.

1. Elsas LJ 2nd, Lai K: The molecular biology of galactosemia. Genet Med 1998 Nov-Dec;1(1):40-48

2. Novelli G, Reichardt JK: Molecular basis of disorders of human galactose metabolism: past, present, and future. Mol Genet Metab 2000 Sep-Oct;71(1-2):62-65

3. Walter JH, Collins JE, Leonard JV: Recommendations for the management of galactosemia. Arch Dis Child 1999 Jan;80(1):93-96

• Informed Consent for Genetic Testing
• Galactosemia Testing Algorithm