A molecular approach to galactosemia

Classical galactosemia (G/G) is caused by the lack of galactose-1-phosphate uridyltransferase (GALT) activity. A more common clinical variant, Duarte/Classical (D/G) produces partial enzymatic impairment. Although neonatal death due to G/G galactosemia has been largely eliminated by population-based screening and intervention, long-term outcome in some is associated with impaired growth, ovarian failure, dyspraxic speech and neurologic deficits. At least 32 variants in the nucleotide sequence of the GALT gene have been identified and 9 have transferred impaired GALT activity to transformed cells in transfection experiments. We here define the prevalence and biochemical phenotype of two mutations. An A to G transition in exon 6 of the GALT gene converts a predicted glutamine at codon 188 to an arginine (Q188R), and introduces a new HpaII cut site into the gene which enables population screening by polymerase chain reaction. An A to G transition in exon 10 in the GALT gene produces a codon change converting an asparagine to aspartic acid at codon 314 (N314D) and adds an AVA II cut site. We screened a large population for the Q188R and N314D sequence changes to investigate the prevalence of Q188R in G/G galactosemia, the effect of homozygosity for Q188R on outcome, and the prevalence and biochemical phenotype of the N314D sequence change. We found that the Q188R mutation has a prevalence of 62% in a predominately Caucasian population of 107 patients with G/G galactosemia. Homozygosity for Q188R was associated with a poor clinical outcome in a subgroup of these patients. The N314D mutation is associated with the Duarte biochemical phenotype with extraordinary concordance.

Duarte allele impairs biostability of galactose-1-phosphate uridyltransferase in human lymphoblasts

The Duarte allele (D) is a missense mutation (N314D) that produces a characteristic isoform and partial impairment of galactose-1-phosphate uridyltransferase (GALT) in human erythrocytes, fibroblasts, and transformed lymphoblasts. The position of this amino acid is distant, however, from presumptive catalytic site(s) as deduced from a three-dimensional model of crystallized Escherichia coli galT protein. To evaluate the mechanism(s) involved in the partial impairment of enzymatic activity, we compared the activity, abundance, biological stability, and mRNA of GALT in human lymphoblastoid cell lines cultured from individuals homozygous for wild-type (WT/WT) and Duarte alleles (N314D/N314D). No other nucleotide differences were present in their GALT genes. The apparent Vmax was reduced in N314D/N314D cells to 31 +/- 3.6 compared to WT/WT of 54 +/- 6.5 nmole UDP-galactose formed/g cell protein/hour. Both genotypes had similar apparent KMs for UDP-glucose of 0.142 +/- 0.057 mM and 0.133 +/- 0.056 mM. This reduced Vmax was associated with a reduced abundance of the 86kD GALT dimer as determined by Western blots and densitometry. Using RNase protection assays, this reduced GALT protein in the N314D/N314D cell lines was not associated with reduced abundance of GALT mRNA. Using cycloheximide (3-[2-(3,5-Dimethyl-2-oxocyclohexyl)-2-hydroxyethyl]glutarimide) inhibition of de novo protein synthesis, GALT enzyme activity, and its dimeric protein had a biological T1/2 of approximately 24 hours in N314D/N314D cell lines as compared to 50 hours for WT/WT lymphoblasts. Upon exposure to 50 degrees C for 15 minutes, N314D/ N314D lymphoblasts retained 45% of GALT activity, whereas controls retained 77% activity. Reduced activity and thermal sensitivity caused by the N314D mutation reverted to control values when a lysine was substituted for a glutamic acid at amino acid 203 in cis (E203K). In summary, N314D/N314D lymphoblasts have reduced GALT enzyme capacity, dimeric protein abundance, biological, and thermal stability. We conclude that the substitution of aspartate for asparagine at amino acid 314 in the human GALT protein reduces the biostability of the active enzyme in human lymphoblasts.

Molecular basis for Duarte and Los Angeles variant galactosemia

Human orythrocytes that are homozygous for the Duarte enzyme variant of galactosemia (D/D) have a characteristic isoform on isoelectric focusing and 50% reduction in galactose-1-phosphate uridyltransferase (GALT) enzyme activity. The Duarte biochemical phenotype has a molecular genotype of N314D/N314D. The characteristic Duarte isoform is also associated with a variant called the "Los Angeles (LA) phenotype," which has increased GALT enzyme activity. We evaluated GALT enzyme activity and screened the GALT genes of 145 patients with one or more N314D-containing alleles. We found seven with the LA biochemical phenotype, and all had a 1721C-->T transition in exon 7 in cis with the N314D missense mutation. The 1721C-->T transition is a neutral polymorphism for leucine at amino acid 218 (L218L). In pedigree analyses, this 1721C-->T transition segregated with the LA phenotype of increased GALT activity in three different biochemical phenotypes (LA/N, LA/G, and LA/D). To determine the mechanism for increased activity of the LA variant, we compared GALT mRNA, protein abundance, and enzyme thermal stability in lymphoblast cell lines of D and LA phenotypes with comparable genotypes. GALT protein abundance was increased in LA compared to D alleles, but mRNA was similar among all genotypes. When LA/D and D/D GALT biochemical phenotypes were compared to N/N GALT phenotypes, both had 50%, as compared to 21%, reduction in GALT activity in the wild type (N/N) after exposure at identical initial enzyme activity to 50 degrees C for 15 min. We conclude that the codon change N314D in cis with the base-pair transition 1721C-->T produces the LA variant of galactosemia and that this nucleotide change increases GALT activity by increasing GALT protein abundance without increasing transcription or decreasing thermal lability. A favorable codon bias for the mutated codon with consequently increased translation rates is postulated as the mechanism.

A prevalent mutation for galactosemia among black Americans

OBJECTIVE

To define the mutation causing galactosemia in patients of black American origin who have no galactose-1-phosphate uridyltransferase (GALT) activity in erythrocytes but good clinical outcome.

METHODS

We discovered a mutation caused by a C-->T transition at base-pair 1158 of the GALT gene that results in a serine-to-leucine substitution at codon 135 (S135L). We developed a method with which to screen populations for its prevalence. We compared galactose-1-phosphate uridyltransferase among erythrocytes, leukocytes, and transformed lymphoblasts, as well as total body oxidation of D-(13C)-galactose to 13CO2 among three genotypes for GALT (S135L/S135L, Q188R/Q188R, and Normal/Normal).

RESULTS

We found a 48% prevalence of the S135L mutation among 17 black American patients with classic galactosemia and a 1% prevalence in a population of 50 black Americans without galactosemia. The S135L mutation was not found in 84 white patients with G/G galactosemia nor in 87 white control subjects without galactosemia. We found normal whole body oxidation of D-(13C)-galactose by the patient homozygous for S135L and various degrees of enzyme impairment among different tissues.

CONCLUSIONS

The S135L mutation in the GALT gene is a prevalent cause of galactosemia among black patients. Because GALT activity varies in different tissues of patients homozygous for S135L, they may have a better clinical outcome than patients who are homozygous for Q188R when both are treated from infancy.

Galactosemia: a strategy to identify new biochemical phenotypes and molecular genotypes

We describe a stratagem for identifying new mutations in the galactose-1-phosphate uridyl transferase (GALT) gene. GALT enzyme activity and isoforms were defined in erythrocytes from probands and their first-degree relatives. If the biochemical phenotypes segregated in an autosomal recessive pattern, we screened for common mutations by using multiplex PCR and restriction endonuclease digestions. If common mutant alleles were not present, the 11 exons of the GALT gene were amplified by PCR, and variations from the normal nucleotide sequences were identified by SSCP. The suspected region(s) was then analyzed by direct DNA sequencing. We identified 86 mutant GALT alleles that reduced erythrocyte GALT activity. Seventy-five of these GALT genomes had abnormal SSCP patterns, of which 41 were sequenced, yielding 12 new and 21 previously reported, rare mutations. Among the novel group of 12 new mutations, an unusual biochemical phenotype was found in a family whose newborn proband has classical galactosemia. He had inherited two mutations in cis (N314D-E203K) from his father, whose GALT activity was near normal, and an additional GALT mutation in the splice-acceptor site of intron C (IVSC) from his mother. The substitution of a positively charged E203K mutation created a unique isoform-banding pattern. An asymptomatic sister's GALT genes carries three mutations (E203K-N314D/N314D) with eight distinct isoform bands. Surprisingly, her erythrocytes have normal GALT activity. We conclude that the synergism of pedigree, biochemical, SSCP, and direct GALT gene analyses is an efficient protocol for identifying new mutations and speculate that E203K and N314D codon changes produce intraallelic complementation when in cis.

A common mutation associated with the Duarte galactosemia allele

The human cDNA and gene for galactose-1-phosphate uridyl transferase (GALT) have been cloned and sequenced. A prevalent mutation (Q188R) is known to cause classic galactosemia (G/G). G/G galactosemia has an incidence of 1/38,886 in 1,396,766 Georgia live-born infants, but a more common variant of galactosemia, Duarte, has an unknown incidence. The proposed Duarte biochemical phenotypes of GALT are as follows: D/N, D/D, and D/G, which have approximately 75%, 50%, and 25% of normal GALT activity respectively. In addition, the D allele has isoforms of its enzyme that have more acidic pI than normal. Here we systematically determine (a) the prevalence of an A-to-G transition at base pair 2744 of exon 10 in the GALT gene, transition that produces a codon change converting asparagine to aspartic acid at position 314 (N314D), and (b) the association of this mutation with the Duarte biochemical phenotype. The 2744G nucleotide change adds an AvaII (SinI) cut site, which was identified in PCR-amplified DNA. In 111 biochemically unphenotyped controls with no history of galactosemia, 13 N314D alleles were identified (prevalence 5.9%). In a prospective study, 40 D alleles were biochemically phenotyped, and 40 N314D alleles were found. By contrast, in 36 individuals known not to have the Duarte biochemical phenotype, no N314D alleles were found. We conclude that the N314D mutation is a common allele that probably causes the Duarte GALT biochemical phenotype and occurs in a predominantly Caucasian, nongalactosemic population, with a prevalence of 5.9%.