The GALT gene provides instructions for making an enzyme called galactose-1-phosphate uridylyltransferase. This enzyme enables the body to process a simple sugar called galactose, which is present in small amounts in many foods. Galactose is primarily part of a larger sugar called lactose, which is found in all dairy products and many baby formulas.GHR
More than 300 mutations in the GALT gene have been identified in people with the classic form of galactosemia, a condition that causes life-threatening signs and symptoms beginning shortly after birth. Most of these mutations severely reduce or eliminate the activity of galactose-1-phosphate uridylyltransferase. A shortage of this enzyme prevents cells from processing galactose obtained from the diet. As a result, galactose-1-phosphate and related compounds can build up to toxic levels in the body. The accumulation of these substances damages tissues and organs, leading to the serious medical problems associated with classic galactosemia.GHR
The most common GALT mutation in white Europeans and North Americans replaces the amino acid glutamine with the amino acid arginine at position 188 in the enzyme (written as Gln188Arg or Q188R; rs75391579). Another mutation occurs almost exclusively in people of African descent. This genetic change substitutes the amino acid leucine for the amino acid serine at position 135 (written as Ser135Leu or S135L; rs111033690).GHR
A particular GALT mutation called the Duarte variant results in a form of galactosemia with less serious complications than the classic type. This mutation replaces the amino acid asparagine with the amino acid aspartic acid at protein position 314 (written as Asn314Asp or N314D; rs2070074). The Duarte variant reduces but does not eliminate the activity of galactose-1-phosphate uridylyltransferase. The signs and symptoms associated with this variant tend to be milder because the enzyme retains 5 percent to 20 percent of its normal activity.GHR
A fairly well known deletion potentially resulting in galactosemia consists of a ~5kb+ chunk out of the GALT gene on ch 9, predominantly seen in Ashkenazi Jews.[PMID 17079880] Seen in perhaps up to 10% of carriers, it does wipe out several SNPs so it should be possible to predict a deletion based on seeing either (a) a significant run of "absent" calls for chromosomally adjacent SNPs, assuming homozygosity for deleted areas on both maternal and paternal chromosomes, or (b) seeing a statistically unlikely run of homozygosity for a string of chromosomally adjacent SNPs on an autosome, assuming the deletion is only occurring on one chromosome.
Technically, the 5kb deletion allele is complex, in that it involves a 3163-nt deletion of the GALT promoter and 5' gene region along with a 2295-bp deletion of a portion of towards the 3' end of the gene; only segments of exon 8 and intron 8 are retained [PMID 17079880].