Galactosemia

BACKGROUND:

The most common cause of galactosemia is a deficiency of the enzyme galactose-1-phosphate uridyl transferase. This is referred to as classical galactosemia or galactosemia type I. This enzyme catalyzes the conversion of galactose-1-phosphate and UDP-glucose to UDP-galactose and glucose-1-phosphate. The lack of tranferase enzyme results in an elevation of blood galactose, high tissue levels of galactose-1-phosphate, and sometimes increased urine galactose excretion.

INCIDENCE:

The population frequency determined through U.S. newborn screening is approximately 1:40,000. A number of mutant alleles are known, including the Duarte variant, which causes a reduction in erythrocyte enzyme activity.

INDICATIONS FOR TESTING*:

Tranferase deficient infants are healthy until they are fed milk containing lactose. The most common symptom is failure to thrive, but vomiting and diarrhea, hepatomegaly and jaundice are common by the end of the first week of life.

*New Jersey routinely tests all newborns within the State for galactosemia.

PROCEDURE:

Galactose-1-phosphate uridyl tranferase is measured spectrophotometrically by using the UDPG consumption method. Variants of galactosemia can be identified using starch gel electrophoresis. Galactose-1-phosphate can be measured spectrophotometrically to monitor dietary management of galactosemic patients and low activity variants such as Duarte-galactosemia heterozygotes.

SAMPLE REQUIREMENTS:

At least 5cc of blood in a green top (sodium heparin) vacutainer tube is required for enzyme analysis and electrophoresis. An additional 3 cc in a green top tube would be required for galactose-1 phosphate levels.

INTERPRETATION:

GALT activity (in units per gram Hgb) is generally as follows:

Normal: >19
Los Angeles-Normal: >27
Duarte-Normal: 14-19
Galactosemia-Normal(carrier): 7-14
Duarte homozygote: 5-9
Duarte-Galactosemia heterozygote: 2-6
Galactosemia homozygote: 0-1

Other mutant alleles are seen on occasion.

In starch gel electrophoresis, normal and galactosemia carrier patients show 1 band, while Duarte-Normal patients have 3 distinct bands. Los Angeles-Normal patients have 3 strong bands, while Duarte-Galactosemia patients have 3 faint bands. Galactosemic patients have no bands on electrophoresis.

Normal individuals would have no galactose-1-phosphate in their blood. Patients with galactosemia who exhibit good dietary control have galactose-1-phosphate levels of 3-4mg%.

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