Characterization of two missense mutations in human galactose-1-phosphate uridyltransferase: Different molecular mechanisms for galactosemia

We report the molecular characterization of two novel galactosemia mutations that exhibit different molecular phenotypes. Both are of the missense type with low or no residual enzyme activity. The R148W mutation results in an unstable protein, although messenger RNA is still produced. In contrast, the L195P mutation produces stable but inactive immunoreactive protein. The R148W mutation alters an amino acid that is not evolutionarily conserved, while the L195P mutation affects a well-conserved residue nine amino acids down-stream from the putative active site nucleophile. These mutations provide evidence that different mechanisms can result in galactosemia: destabilizing mutations in any given area of the protein and missense mutations in conserved domains of the enzyme resulting in low or no activity. These two mutant alleles represent the fifth and sixth galactosemia mutations and confirm the hypothesis that galactosemia results from a multiplicity of mutations at the molecular level.

Genetic basis of galactosemia

Classic galactosemia is an inborn error of galactose metabolism and results from deficiency of the ubiquitously expressed enzyme galactose-1-phosphate uridyltransferase (GALT). Nine missense mutations, three splicing mutations, three GALT protein polymorphisms, and one silent nucleotide substitution have been identified to date. Most of the disease-causing mutations are rare among patients. The most common mutation, Q188R, has a frequency of only one-fourth in the patient population examined. Three classes of disease-causing mutations have been reported: CRM+ missense mutations (the most common class), CRM- missense mutations, and splicing mutations. Thus, galactosemia is heterogeneous at the molecular level, which is noteworthy in light of the well-documented clinical variability observed in this disorder. It has also been shown that eight of nine galactosemia missense mutations occur in evolutionarily well-conserved domains, suggesting that they affect functionally and/or structurally important residues. In contrast, all protein polymorphisms alter variable amino acids which presumably are not important for the enzyme's function.

Molecular characterization of two galactosemia mutations: correlation of mutations with highly conserved domains in galactose-1-phosphate uridyl transferase

Galactosemia is an autosomal recessive disorder of human galactose metabolism caused by deficiency of the enzyme galactose-1-phosphate uridyl transferase (GALT). The molecular basis of this disorder is at present not well understood. We report here two missense mutations which result in low or undetectable enzymatic activity. First, we identified at nucleotide 591 a transition which substitutes glutamine 188 by arginine. The mutated glutamine is not only highly conserved in evolution (conserved also in Escherichia coli and Saccharomyces cerevisiae), but is also two amino acid residues downstream from the active site histidine-proline-histidine triad and results in about 10% of normal enzymatic activity. The arginine 188 mutation is the most common galactosemia mutation characterized to date. It accounts for one-fourth of the galactosemia alleles studied. Second, we report the substitution of arginine 333 by tryptophan, caused by a transition at nucleotide 1025. The area surrounding this missense mutation is the most highly conserved domain in the homologous enzymes from E. coli, yeast, and humans, and this mutation results in undetectable enzymatic activity, suggesting that this is a severe mutation. This second mutation appears to be rare, since it was found only in the patient we sequenced. Our data provide further evidence for the heterogeneity of galactosemia at the molecular level, heterogeneity which might be related to the variable clinical outcome observed in this disorder.

Modulation of rat tissue galactose-1-phosphate uridyltransferase by uridine and uridine triphosphate

Uridine-containing sugar nucleotides, uridine diphosphate (UDP)-glucose and UDPgalactose are important intermediates in galactose metabolism, and tissue UDPgalactose may be a salient factor in the etiology of the long-term clinical manifestations of patients with galactose-1-phosphate uridyltransferase deficient galactosemia. Because uridine and uridine nucleotides such as uridine triphosphate (UTP) are known inhibitors of rat hepatic transferase, we have examined the effects of these compounds on the activity of the enzyme in homogenates of rat brain and ovary which are target organs of galactose toxicity in classical galactosemia. In addition, the concerted effect of uridine and UTP together on hepatic transferase has been assessed. These investigations have been prompted by considerations that uridine administration may have a therapeutic role in the long-term treatment of classical galactosemia. Both uridine and UTP have been found to be potent inhibitors of brain and ovarian transferase activity. Brain enzyme activity is more sensitive to these compounds than is that of the ovary. They are competitive inhibitors of UDPglucose in both newborn and adult brain enzyme preparations with a ki of 0.15 to 0.20 mM. Uridine and UTP at low concentrations were found to have an additive effect on rat hepatic transferase activity, which is especially significant in that uridine administration is known to increase hepatic UTP concentration. These findings suggest judicious caution should be used in giving uridine to patients with genetically limited transferase activity because the possibility exists of inhibiting small amounts of residual enzyme in the tissues of affected subjects.

Molecular basis of galactosemia: mutations and polymorphisms in the gene encoding human galactose-1-phosphate uridylyltransferase

We describe the molecular characterization of two mutations responsible for galactosemia, an inherited disorder of galatose metabolism that causes jaundice, cataracts, and mental retardation in humans. The coding region of galactose-1-phosphate uridylyltransferase (GALT; UDPglucose:alpha-D-galactose-1-phosphate uridylyltransferase, EC 2.7.7.12) was amplified by the polymerase chain reaction from total cDNA of a classic galactosemic individual and was characterized by direct sequencing of the products. Two missense mutations were identified: (i) replacement of valine-44 by methionine and (ii) replacement of methionine-142 by lysine. These mutations led to a drastic reduction in GALT activity when individual mutant cDNAs were overexpressed in a mammalian cell system, although full-length protein is synthesized in this assay. The two galactosemia mutations account for 3 of the 15 galactosemia alleles analyzed. These results suggest that galactosemia is caused by a variety of mutations, which might be responsible for the observed clinical heterogeneity of this disorder. We also present the molecular characterization of two GALT polymorphisms: (i) replacement of leucine-62 by methionine and (ii) replacement of asparagine-314 by aspartate. It appears that galactosemia mutations tend to occur in regions that are highly conserved throughout evolution while the polymorphisms change variable residues.

Aldose reductase messenger RNA in the lens epithelium in vivo: effects of diabetes mellitus and galactosaemia

1. Quantitative in situ hybridization histochemistry was used to examine the regulation of aldose reductase messenger RNA in the rat lens after the induction of diabetes mellitus or after feeding a 50% (w/w) galactose diet. 2. Although increased staining for aldose reductase in the lens epithelium has previously been observed by immunohistochemistry after 3 weeks of diabetes or after 7 days of galactose feeding, we have not been able to detect any increase in the amount of aldose reductase messenger RNA in these cells as compared with controls (113 +/- 7%, 105 +/- 9%, 100 +/- 7%, respectively) at these time points (P greater than 0.05). 3. After 15 days of galactose feeding, however, there was a significant increase of 140% (+/- 12%) in the amount of aldose reductase messenger RNA in the lens epithelial cells as compared with controls (P = less than 0.001). 4. These results demonstrate that increased availability of galactose, a high-affinity substrate for the enzyme, leads to increased aldose reductase messenger RNA, which suggests a role for aldose reductase in sugar metabolism in the lens.

Preliminary results of UDP galactose pyrophosphorylase in the erythrocytes of healthy persons and in patients with galactosaemia

Direct chromatographic isolation of UDP galactose and galactose-1-phosphate was used for determination of the activity of UDP galactose pyrophosphorylase in erythrocytes. The activity was determined by measuring the amount of UDP galactose produced from galactose-1-phosphate and uridine triphosphate. In homozygotes with galactosaemia the activity of the enzyme was nearly ten times lower than in controls, so this difference was highly significant statistically (p less than or equal to 0.001) and the respective values were 0.0051 +/- 0.0003 and 0.0418 +/- 0.0038 mumol of UDP galactose formed during 1 hour by 1 ml of erythrocytes (300 mg of haemoglobin). In heterozygotes with galactosaemia the activity of the enzyme had intermediate values between those in homozygotes and healthy controls.

Culture of galactosaemic fibroblasts in the presence of galactose: effect of inosine

Fibroblasts from three galactosaemics had no galactose-1-phosphate uridyltransferase (GALT) activity. These fibroblasts cells were cultured in different media supplemented with dialysed fetal calf serum. Galactosaemic and control cell strains stopped growing in hexose-free medium. In glucose-free medium containing galactose, galatosaemic cells, in contrast to control cells, stopped growing after two days and died. In the same medium supplemented with inosine, they exhibited the same growth pattern as the control cell strains although in the presence of high concentrations of galactose-1-phosphate (Gal-1-P). These findings indicated that the glucose-free medium containing galactose supplemented with dialysed fetal calf serum and inosine, as a ribose donor, was appropriate for further in vitro investigations of galactose metabolism in galactosaemic cells.

Regulation of galactose metabolism: implications for therapy

In view of evidence that dietary therapy of galactose-1-phosphate uridyltransferase deficiency has failed to prevent complications of the disorder, there is a need for new strategies in treatment. The enhancement of residual enzyme activity in tissues of galactosaemic patients should provide such an approach. This possibility is derived from knowledge of the regulation of transferase activity in normal animal tissues. The pertinent observations summarized herein are: (1) that hepatic transferase activity is modulated by various cellular metabolites, uridine nucleotides being of particular significance; (2) that transferase activity in the young rat liver is subject to developmental programming with a several-fold increase after birth; (3) that transferase activity in pregnant rat liver is significantly increased which may be related to hormonal effects of progesterone; and (4) that pharmacological doses of folic acid may increase transferase activity. The basis of such regulation can give insight into sufficient augmentation of the residual activity to increase galactose utilization and thereby better the long-term outcome.

Evaluation of reduced activity galactose-1-phosphate uridyl transferase by combined radioisotopic assay and high-resolution isoelectric focusing

Fifty-nine patients referred for testing of galactose-1-phosphate uridyl transferase (GALT) activity were evaluated by a combination of radioisotopic assay and an improved high-resolution isoelectric focusing (IEF) method for determination of individual genotypes. The majority of evaluations (43 of 59) were undertaken because of an abnormal fluorometric GALT screening assay conducted as part of a state newborn galactosemia screening program or as a diagnostic test for an infant with one or more clinical signs of galactosemia. Only 12 of the 43 patients with an abnormal fluorometric screening test were found to have less than 3% GALT activity by quantitative assay, no GALT-staining by IEF, and a clinical history consistent with classical galactosemia. The majority of the remainder (24 of 43) were found to be heterozygotes for galactosemia, the Duarte variant, or both. IEF was found to be a fast and accurate technique for assignment of GALT genotypes in conjunction with quantitative enzymatic assay.

[Ciliary body changes associate with aldose reductase in galactosemic rats]

Morphological changes of the ciliary body epithelium in rats fed a 50% galactose diet for 10 months with or without aldose reductase inhibitor (ARI) were investigated by both light microscopic observation and measurement of cell size. Aldose reductase was immunohistochemically observed to be present in non-pigmented epithelial cells of ciliary body and structural change (hypertrophy) was observed only in non-pigmented epithelial cells of galactosemic rats. In contrast, no hypertrophic changes were seen in rats fed with ARI. These results strongly suggest that aldose reductase is associated with hypertrophic change of non-pigmented epithelial cells of the ciliary body.

Galactose-1-phosphate uridyl transferase in density-fractionated erythrocytes. Studies of normal and mutant enzymes

Galactose-1-phosphate uridyl transferase (GALT), the deficient enzyme in classical galactosemia, was studied by Percoll-gradient age-fractionation of erythrocytes. For normal GALT, a rapid and substantial decrease in GALT activity and loss of most of two isozymes was found to occur in the reticulocyte fractions. The loss of activity was then followed by relative stabilization of both GALT-specific activity and microheterogeneity in mature and aging erythrocytes. When applied to the study of mutant GALT from galactosemic patients, the Percoll-gradient fractionation method permitted detection in the reticulocyte-enriched fractions of up to 5% of normal GALT-specific activity and an isoelectric focusing pattern essentially the same as that of normal GALT. Percoll-gradient fractionation of erythrocytes offers a simple and direct method to study characteristics of GALT activity and microheterogeneity in normal and galactosemic human erythrocytes.

Gonadal failure in young women and galactose-1-phosphate uridyl transferase activity

The present study allows the conclusion that low Gal-1-P transferase activity resulting from GtD/gt heterozygosity is not an major cause of early ovarian failure. However, larger groups of patients need to be examined to fully exclude an association between these rare conditions.

Idiopathic presenile cataract formation and galactosaemia

Five hundred patients undergoing cataract surgery were prospectively examined, and 46 Caucasian patients were found to have strictly idiopathic cataracts severe enough to warrant surgery on or before age 55. In a masked fashion we determined the activity of galactokinase (GK) and galactose-1-phosphate uridyl transferase (GPUT) in these patients as well as on 53 age matched controls. With respect to GK no cataract patient had an enzyme level of less than 2 standard deviations below the control mean. However, 3 of 45 (6.7%) patients in the cataract group had a GPUT level less than 2 standard deviations below the mean for controls, and were presumably heterozygotes for this enzyme. In comparison with the expected population rate of 0.8% this is highly significant (p = 0.006). Abnormalities in galactose pathway enzymes may therefore predispose to development of presenile cataracts. In affected people there is a possibility of treating these patients clinically by dietary restriction of dairy products or by using aldose reductase inhibitors to prevent or reverse cataract formation.

[Double heterozygosity (transferase-/epimerase-defect) and galactosemia cataract]

The mother of a boy who suffered from classical galactosaemia (galactose-1-phosphate uridyl transferase deficiency) has unilateral cataracta. In addition the boy had a decreased activity of the UDP-galactose-4-epimerase. The latter defect could also be demonstrated in the erythrocytes from the mother and the grandmother. In contrast to the finding of cataracta in the mother the grandmother with the same type of double heterozygosity was ophthalmologically normal. The implication of partial maternal disorders of galactose metabolism will be discussed in view of their possible role for the origin of cataracta.

Transient neonatal galactosaemia

A 4 week old infant who failed to thrive was found to have galactose in his urine. Plasma galactose concentration was grossly raised (4.48 mmol/l; reference range less than 0.24 mmol/l) but red cell transferase and epimerase activities were normal. He improved when dietary lactose was excluded. Clinical and biochemical tolerance to galactose was evident by 7 months of age.

Adenosine triphosphatase in nerves and ganglia of rats with streptozotocin-induced diabetes or galactosaemia; effects of aldose reductase inhibition

This study measured the ouabain-sensitive and ouabain-resistant adenosine triphosphatase activity in homogenates of the sciatic nerves and of pooled fourth and fifth lumbar dorsal root ganglia from rats fed 20% galactose or made diabetic with streptozotocin for either 4 or 8 weeks. Diabetes caused reductions in both fractions of sciatic nerve adenosine triphosphatase activity. After 8 weeks the ouabain-sensitive fraction was 54% of control (p less than 0.05) and the ouabain-resistant fraction was 57% of control (p less than 0.05). Galactose feeding more than doubled the ouabain-sensitive adenosine triphosphatase activity in the sciatic nerve (225% of control after 4 weeks, 215% of control after 8 weeks of galactose feeding, both p less than 0.01) and produced a progressive increase in the ouabain-resistant fraction (119% of control at 4 weeks (p less than 0.05) and 176% of control at 8 weeks (p less than 0.01)). In a group of rats fed galactose for 5 days, sciatic nerve ouabain-sensitive adenosine triphosphatase activity was 165% of control. Treatment with the aldose-reductase inhibitors tolrestat, ponalrestat or sorbinil prevented accumulation of polyol and depletion of myo-inositol in the sciatic nerves, indicating effective inhibition of aldose reductase. These drugs prevented completely the effect of galactose on the sciatic nerve adenosine triphosphatase activity, but had no significant effect on the reduction in adenosine triphosphatase activity in the sciatic nerves of diabetic rats. In the dorsal root ganglia galactose feeding had no measurable effect on the adenosine triphosphatase activity. Diabetes caused a modest numerical reduction in the ouabain-sensitive activity only.(ABSTRACT TRUNCATED AT 250 WORDS)

Galactosaemia

This paper reviews galactosaemia and describes the experience of the Willink Biochemical Genetics Unit in the management of classical galactosaemia. Galactokinase and UDPgalactose-4-epimerase deficiency are dealt with briefly. The former disorder is readily treated with a galactose free diet and if this is started early in life, the only complication, cataracts, is avoided. Epimerase deficiency is a relatively 'new' disorder and little is known about the eventual outcome of affected patients. Early observations suggest that the prognosis is likely to be poor even in those patients diagnosed and treated soon after birth. Classical galactosaemia leads to a number of long term complications. The prognosis for normal mental development in affected patients is poor. Unfortunately this does not appear to be reversible by early diagnosis and treatment and whilst every effort should be made to establish an early diagnosis our experience suggests that there is no difference in the ultimate mental development between those who are detected within the first two weeks of life and those before the age of six weeks. In addition female homozygotes are at a very high risk of developing ovarian failure at an early age.

Opposite effects of diabetes and galactosaemia on adenosine triphosphatase activity in rat nervous tissue

This study measured the ouabain-sensitive adenosine triphosphatase activity in sciatic nerve, lumbar dorsal root ganglia and superior cervical ganglia from control rats, rats with 8 weeks streptozotocin-induced diabetes and rats fed a diet containing 20% galactose for 8 weeks. Whilst the sciatic nerves of the diabetic rats showed a 42% reduction in ouabain-sensitive adenosine triphosphatase activity, the galactose-fed rats showed an increase of 124% (p less than 0.01 and p less than 0.005, respectively, compared to controls). There was also a reduction (by 30% compared to controls; p less than 0.05) in the ouabain-sensitive adenosine triphosphatase activity of the dorsal root ganglia from the diabetic rats, but their superior cervical ganglia did not show a significant fall. The ganglia of the galactosaemic rats showed no change in ouabain-sensitive adenosine triphosphatase activity compared to controls. These changes coexisted with increases in appropriate polyol pathway metabolites in all tissues of both diabetic and galactosaemic rats. There were also depletions of myo-inositol in the sciatic nerves and dorsal root ganglia of diabetic and galactosaemic rats, but their superior cervical ganglia contained levels of myo-inositol which were similar to those of controls. The nerves of the galactosaemic rats showed increased water content; the nerves of the diabetic rats did not. The data argue against a simple relationship between myo-inositol depletion and impaired Na/K adenosine triphosphatase activity in association with exaggerated polyol pathway flux in peripheral nervous tissue.