Unit Code 8333:
Galactose-1-Phosphate Uridyltransferase (GALT), Blood
Useful For
Diagnosis of GALT deficiency, the most common cause of
galactosemia
Confirmation of abnormal state newborn screening results
Clinical Information
Three clinically important inborn errors of galactose metabolism
that result in galactosemia have been described. They are
transmitted by autosomal recessive inheritance. The genetic
disturbance is expressed as a deficiency of galactokinase (GALK),
galactose-1-phosphate uridyltransferase (GALT), or UDP
galactose-4-epimerase (GALE), the enzymes catalyzing the
reactions in converting galactose to glucose:
Kinase Transferase Epimerase
Gal ATP -----> Gal-1-P UDP-Glu ---------> UDP-Gal --------> UDP-Glu Glu-1-P
The transferase (GALT) deficiency, referred to as classical
galactosemia or GG genotype, is the most commonly occurring
of the 3 disorders. Typically, individuals with classic galactosemia
are diagnosed via newborn screening and a lactose-restricted
diet is initiated during the neonatal period. In the unfortunate
circumstance where diagnosis and dietary treatment is delayed,
infants usually present with gram-negative sepsis, inanition
(weakness and weight loss due to severe lack of food), failure
to thrive, vomiting, life-threatening liver disease, cataracts, and
ultimately, mental retardation if galactose ingestion continues.
Treatment with lactose-restricted diet can typically reverse the
acute symptoms. Mild growth retardation, cognitive impairment,
and intellectual deficit have been variably described as
complications of treated galactosemia in some patients.
Ovarian dysfunction is an almost unavoidable
consequence of classic galactosemia, even with adherence to a
strict diet. Osteoporosis secondary to decreased calcium intake
can also occur.
A patient with a GG genotype must be on a galactose-free diet.
Additionally, patients with a DG genotype (Duarte variant/G
compound heterozygotes) must be monitored closely, and in
some cases, dietary treatment may be recommended.
Galactokinase deficiency is the second most common cause
of galactosemia and results in a milder variant of galactosemia
than that which results from GALT deficiency. Galactokinase
deficiency is very rare and usually is expressed by occurrence
of juvenile cataracts in the absence of mental retardation (which
occurs in transferase deficiency).
Epimerase deficiency is an extremely rare cause of
galactosemia.
Galactosuria may indicate galactosemia.
Reference Values
> or = 18.5 U/g of hemoglobin
Interpretation
The laboratory provides an interpretation of the results.
Galactosemia occurs in patients whose enzymes levels
are extremely low.
For kinase deficiency see #8628 "Galactokinase, Blood."
If galactosuria is unexplainable by either a kinase or transferase
defect, a red cell specimen can be sent to the appropriate
research laboratory on request.
See "Galactosemia Testing Algorithm" in Special Instructions
for additional information.
Cautions
This assay is not useful for monitoring dietary compliance by
galactosemics, see #80337 "Galactose-1-Phosphate (Gal-1-P),
Erythrocytes."
The presence of galactose in blood and urine is dependent upon
dietary consumption, and this test is valid only after the patient has
consumed either galactose or lactose in their normal diet.
Special Instructions and Forms
Clinical Reference
1. Holton JB, Walter JH, Tyfield LA: Galactosemia. In The Metabolic
and Molecular Basis of Inherited Disease. Vol. 1. 8th edition. Edited
by CR Scriver, AL Beadut. New York, McGraw-Hill Book Company,
2001, pp 1553-1587
2. Walter JH, Collins JE, Leonard JV: Recommendations for the
management of galactosemia. Arch Dis Child 1999 Jan;80(1):93-96
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