Molecular characterization of two galactosemia mutations and one polymorphism: implications for structure-function analysis of human galactose-1-phosphate uridyltransferase

Clinical and molecular characteristics and time of diagnosis of patients with classical galactosemia in an unscreened population in Turkey

Classical galactosemia is an autosomal recessive inborn error of metabolism caused by biallelic pathogenic variants in the GALT gene. With the benefit of early diagnosis by newborn screening, the acute presentation of galactosemia can be prevented. In this study, we describe the clinical phenotypes, time of diagnosis and GALT genotypes of 76 galactosemia patients from Turkey, where the disease is not yet included in the newborn screening program. The median age at first symptom was 10 days (range 5-20), while the median age at diagnosis was 30 days (range 17-53). Nearly half of the patients (36 patients, 47.4%) were diagnosed later than age 1 month. Fifty-eight individuals were found to have 18 different pathogenic variants in their 116 mutant alleles. In our sample, Q188R variant has the highest frequency with 53%, the other half of the allele frequency of the patients showed 17 different genotypes. Despite presenting with typical clinical manifestations, classical galactosemia patients are diagnosed late in Turkey. Due to the geographical location of our country, different pathogenic GALT variants may be seen in Turkish patients. In the present study, a clear genotype-phenotype correlation could not be established in patients.

Cryptic residual GALT activity is a potential modifier of scholastic outcome in school age children with classic galactosemia

Classic galactosemia is a potentially lethal disorder that results from profound deficiency of galactose-1-phosphate uridylyltransferase (GALT), the second enzyme in the Leloir pathway of galactose metabolism. Although early diagnosis and rigorous dietary restriction of galactose prevent or resolve the potentially lethal acute symptoms, patients are at markedly increased risk of long-term complications including significant cognitive, speech, and behavioral difficulties, among other problems. The mechanisms that underlie these long-term complications remain unclear, as do the factors that modify their severity. Here we explored the scholastic and behavioral outcomes experienced by a cohort of 54 school age children with classic galactosemia. Data collected included survey responses from parents and teachers, school records including standardized test scores, and GALT genotype data used to estimate predicted residual GALT activity based on a yeast expression system. As expected, many but not all of the children in our study demonstrated speech, scholastic, and behavioral difficulties. Perhaps most striking, we found that predicted cryptic residual GALT activity, often below the threshold of detection of clinical assays, appeared to modify scholastic outcome. These data raise the intriguing possibility that cryptic GALT activity might also influence the severity of other long-term complications in classic galactosemia.

Misfolding of galactose 1-phosphate uridylyltransferase can result in type I galactosemia

Type I galactosemia is a genetic disorder that is caused by the impairment of galactose-1-phosphate uridylyltransferase (GALT; EC 2.7.7.12). Although a large number of mutations have been detected through genetic screening of the human GALT (hGALT) locus, for many it is not known how they cause their effects. The majority of these mutations are missense, with predicted substitutions scattered throughout the enzyme structure and thus causing impairment by other means rather than direct alterations to the active site. To clarify the fundamental, molecular basis of hGALT impairment we studied five disease-associated variants p.D28Y, p.L74P, p.F171S, p.F194L and p.R333G using both a yeast model and purified, recombinant proteins. In a yeast expression system there was a correlation between lysate activity and the ability to rescue growth in the presence of galactose, except for p.R333G. Kinetic analysis of the purified proteins quantified each variant's level of enzymatic impairment and demonstrated that this was largely due to altered substrate binding. Increased surface hydrophobicity, altered thermal stability and changes in proteolytic sensitivity were also detected. Our results demonstrate that hGALT requires a level of flexibility to function optimally and that altered folding is the underlying reason of impairment in all the variants tested here. This indicates that misfolding is a common, molecular basis of hGALT deficiency and suggests the potential of pharmacological chaperones and proteostasis regulators as novel therapeutic approaches for type I galactosemia.

Mutation spectrum in the French cohort of galactosemic patients and structural simulation of 27 novel missense variations

BACKGROUND

Classic galactosemia refers to galactose-1-phosphate uridyltransferase (GALT) deficiency and is characterized by long-term complications of unknown mechanism and high allelic heterogeneity of GALT gene.

AIM

To report molecular characterization of GALT variations in 210 French families, to analyze the structural effects of novel missense variations and to assess informativity of structural data in predicting outcome.

METHODS

Sequencing of exons and intron-exon junctions of GALT gene was completed in unsolved cases by analysis of a long range PCR product. Structural consequences of novel missense variations were predicted using a homology model of GALT protein and a semi-automated analysis which integrates simulation of variations, structural analyses and two web servers dedicated to identify mutation-induced change of protein stability.

RESULTS

Forty four novel variations were identified, among them 27 nucleotide substitutions. In silico modeling of these missense variations showed that 12 variations are predicted to impair subunit interactions and/or active site conformation and that 23 variations modify H-bond or salt-bridge networks. Twenty variations decrease the global stability of the protein. Five variations had apparently no structural effect.

CONCLUSION

Our results expand the mutation spectrum in GALT gene and the list of GALT variations analyzed at the structural level, providing new data to assess the pathophysiology of galactosemia.

N- and O-linked glycosylation of total plasma glycoproteins in galactosemia

Classic galactosemia is a potentially lethal metabolic disorder that results from profound impairment of the enzyme galactose-1-phosphate uridylyltransferase (GALT); despite decades of research, the underlying mechanism of pathophysiology remains unclear. Previous studies of plasma and tissue samples from patients with classic galactosemia have revealed defects of protein and lipid glycosylation, however, the underlying bases for these defects and their clinical significance, if any, has remained unclear. As a step toward addressing these questions we characterized both the N- and O-linked glycomes of plasma proteins from neonates, infants, children, and adults with galactosemia using mass spectrometry and asked (1) whether similar or disparate defects exist for N-linked and O-linked modifications, (2) what factors correlate with the severity of these defects in different patients, and perhaps most important, (3) whether there is any apparent relationship between chronic glycosylation defects and long-term outcome in patients. We found that some but not all of the galactosemic neonates tested exhibited abnormal N- and O-linked glycosylation of plasma proteins. The types of abnormalities seen were similar between N- and O-linked moieties, but the extent of the defects varied between patients. Age, gender, GALT genotype, and predicted residual GALT activity all failed to explain the extent of the glycosylation defect in the samples studied. Dietary galactose restriction markedly normalized both the N- and O-linked glycosylation patterns for all infants tested; however, any remaining glycosylation defects evident in the plasma of older children or adults on galactose-restricted diets showed no correlation with clinical outcome. These data cannot rule out the possibility that subtle or localized glycosylation defects, not detectable by our methods or not reflected in plasma, may contribute to acute or long-term outcome severity.

Structural and molecular biology of type I galactosemia: disease-associated mutations

Type I galactosemia results from reduced galactose 1-phosphate uridylyltransferase (GALT) activity. Signs of disease include damage to the eyes, brain, liver, and ovaries. However, the exact nature and severity of the pathology depends on the mutation(s) in the patient's genes and his/her environment. Considerable enzymological and structural knowledge has been accumulated and this provides a basis to explain, at a biochemical level, impairment in the enzyme in the more than 230 disease-associated variants, which have been described. The most common variant, Q188R, occurs close to the active site and the dimer interface. The substitution probably disrupts both UDP-sugar binding and homodimer stability. Other alterations, for example K285N, occur close to the surface of the enzyme and most likely affect the folding and stability of the enzyme. There are a number of unanswered questions in the field, which require resolution. These include the possibility that the main enzymes of galactose metabolism form a supramolecular complex and the need for a high resolution crystal structure of human GALT.

Analysis of galactosemia-linked mutations of GALT enzyme using a computational biology approach

We describe the prediction of the structural and functional effects of mutations on the enzyme galactose-1-phosphate uridyltransferase related to the genetic disease galactosemia, using a fully computational approach. One hundred and seven single-point mutants were simulated starting from the structural model of the enzyme obtained by homology modeling methods. Several bioinformatics programs were then applied to each resulting mutant protein to analyze the effect of the mutations. The mutations have a direct effect on the active site, or on the dimer assembly and stability, or on the monomer stability. We describe how mutations may exert their effect at a molecular level by altering H-bonds, salt bridges, secondary structure or surface features. The alteration of protein stability, at level of monomer and/or dimer, is the main effect observed. We found an agreement between our results and the functional experimental data available in literature for some mutants. The data and analyses for all the mutants are fully available in the web-accessible database hosted at http://bioinformatica.isa.cnr.it/GALT.

The clinical and molecular spectrum of galactosemia in patients from the Cape Town region of South Africa

BACKGROUND

The objective of this study was to document the clinical, laboratory and genetic features of galactosemia in patients from the Cape Town metropolitan region.

METHODS

Diagnoses were based on thin layer chromatography for galactosuria/galactosemia and assays of erythrocyte galactose-1-phosphate uridyltransferase (GALT) and galactokinase activities. Patients were screened for the common S135L and Q188R transferase gene mutations, using PCR-based assays. Screening for the S135L mutation in black newborns was used to estimate the carrier rate for galactosemia in black South Africans.

RESULTS

A positive diagnosis of galactosemia was made in 17 patients between the years 1980 to 2001. All had very low or absent galactose-1-phosphate uridyltransferase (GALT) activity, and normal galactokinase levels. The mean age at diagnosis was 5.1 months (range 4 days to 6.5 months). A review of 9 patients showed that hepatomegaly (9/9), and splenomegaly, failure to thrive, developmental delay, bilateral cataracts (6/9) were the most frequent features at diagnosis. Six had conjugated hyperbilirubinemia. Four experienced invasive E. coli infection before diagnosis. Ten patients were submitted to DNA analysis. All 4 black patients and 2 of mixed extraction were homozygous for the S135L allele, while all 3 white patients were homozygous for the Q188R allele. The remaining patient of mixed extraction was heterozygous for the Q188R allele. The estimated carrier frequency of the S135L mutation in 725 healthy black newborns was 1/60.

CONCLUSIONS

In the absence of newborn screening the delay in diagnosis is most often unacceptably long. Also, carrier frequency data predict a galactosemia incidence of approximately 1/14 400 for black newborns in the Cape Metropole, which is much higher than the current detection rate. It is thus likely that many patients go undetected.

Relationship between genotype, activity, and galactose sensitivity in yeast expressing patient alleles of human galactose-1-phosphate uridylyltransferase

Impairment of the human enzyme galactose-1-phosphate uridylyltransferase (GALT) results in the potentially lethal disorder galactosemia; the biochemical basis of pathophysiology in galactosemia remains unknown. We have applied a yeast expression system for human GALT to test the hypothesis that genotype will correlate with GALT activity measured in vitro and with metabolite levels and galactose sensitivity measured in vivo. In particular, we have determined the relative degree of functional impairment associated with each of 16 patient-derived hGALT alleles; activities ranged from null to essentially normal. Next, we utilized strains expressing these alleles to demonstrate a clear inverse relationship between GALT activity and galactose sensitivity. Finally, we monitored accumulation of galactose-1-P, UDP-gal, and UDP-glc in yeast expressing a subset of these alleles. As reported for humans, yeast deficient in GALT, but not their wild type counterparts, demonstrated elevated levels of galactose 1-phosphate and diminished UDP-gal upon exposure to galactose. These results present the first clear evidence in a genetically and biochemically amenable model system of a relationship between GALT genotype, enzyme activity, sensitivity to galactose, and aberrant metabolite accumulation. As such, these data lay a foundation for future studies into the underlying mechanism(s) of galactose sensitivity in yeast and perhaps other eukaryotes, including humans.

Covalent heterogeneity of the human enzyme galactose-1-phosphate uridylyltransferase

Galactose-1-phosphate uridylyltransferase (GALT) acts by a double displacement mechanism, catalyzing the second step in the Leloir pathway of galactose metabolism. Impairment of this enzyme results in the potentially lethal disorder, galactosemia. Although the microheterogeneity of native human GALT has long been recognized, the biochemical basis for this heterogeneity has remained obscure. We have explored the possibility of covalent GALT heterogeneity using denaturing two-dimensional gel electrophoresis and Western blot analysis to fractionate and visualize hemolysate hGALT, as well as the human enzyme expressed in yeast. In both contexts, two predominant GALT species were observed. To define the contribution of uridylylated enzyme intermediate to the two-spot pattern, we exploited the null allele, H186G-hGALT. The Escherichia coli counterpart of this mutant protein (H166G-eGALT) has previously been demonstrated to fold properly, although it cannot form covalent intermediate. Analysis of the H186G-hGALT protein demonstrated a single predominant species, implicating covalent intermediate as the basis for the second spot in the wild-type pattern. In contrast, three naturally occurring mutations, N314D, Q188R, and S135L-hGALT, all demonstrated the two-spot pattern. Together, these data suggest that uridylylated hGALT comprises a significant fraction of the total GALT enzyme pool in normal human cells and that three of the most common patient mutations do not disrupt this distribution.

Functional consequence of substitutions at residue 171 in human galactose-1-phosphate uridylyltransferase

Impairment of the human enzyme galactose-1-phosphate uridylyltransferase (hGALT) results in the potentially lethal disorder classic galactosemia. Although a variety of naturally occurring mutations have been identified in patient alleles, few have been well characterized. We have explored the functional significance of a common patient mutation, F171S, using a strategy of conservative substitution at the defined residue followed by expression of the wild-type and, alternatively, substituted proteins in a null-background strain of yeast. As expected from patient studies, the F171S-hGALT protein demonstrated <0.1% wild-type levels of activity, although two of three conservatively substituted moieties, F171L- and F171Y-hGALT, demonstrated approximately 10% and approximately 4% activity, respectively. The third protein, F171W, demonstrated severely reduced abundance, precluding further study. Detailed kinetic analyses of purified wild-type, F171L- and F171Y-hGALT enzymes, coupled with homology modeling of these proteins, enabled us to suggest that the effects of these substitutions resulted largely from altering the position of a catalytically important residue, Gln-188, and secondarily, by altering the subunit interface and perturbing hexose binding to the uridylylated enzyme. These results not only provide insight into the functional impact of a single common patient allele and offer a paradigm for similar studies of other clinically or biochemically important residues, but they further help to elucidate activity of the wild-type human GALT enzyme.

Classical galactosemia and mutations at the galactose-1-phosphate uridyl transferase (GALT) gene

Classical galactosemia is caused by a deficiency in activity of the enzyme galactose-1-phosphate uridyl transferase (GALT), which, in turn, is caused by mutations at the GALT gene. The disorder exhibits considerable allelic heterogeneity and, at the end of 1998, more than 150 different base changes were recorded in 24 different populations and ethnic groups in 15 countries worldwide. The mutations most frequently cited are Q188R, K285N, S135L, and N314D. Q188R is the most common mutation in European populations or in those predominantly of European descent. Overall, it accounts for 60-70% of mutant chromosomes, but there are significant differences in its relative frequency in individual populations. Individuals homoallelic for Q188R tend to have a severe phenotype and this is in keeping with the virtually complete loss of enzyme activity observed in in vitro expression systems. Globally, K285N is rarer, but in many European populations it can be found on 25-40% of mutant chromosomes. It is invariably associated with a severe phenotype. S135L is found almost exclusively in African Americans. In vitro expression results are discrepant, but some individuals carrying S135L appear to exhibit GALT activity in some tissues. Duarte 1 (or Los Angeles) and Duarte 2 (or Duarte) variants carry the same amino acid substitution, N314D, even though D1 is associated with increased erythrocyte GALT activity and D2 with reduced activity. N314D is in linkage disequilibrium with other base changes that differ on the D1 and D2 alleles. N314D does not impair GALT activity in in vitro expression systems. However, there are differences in the abundance of GALT protein in lymphoblastoid cells lines from D2 and D1 individuals. It is unclear whether the specific molecular changes that distinguish the D1 and D2 alleles account for the different activities. The considerable genetic heterogeneity documented to date undoubtedly contributes to the phenotypic heterogeneity that is observed in galactosemia. The additional effects of nonallelic variation and other constitutional factors on phenotypic variability remain to be elucidated.

The molecular basis of transferase galactosaemia in South African negroids

Transferase galactosaemia is an autosomal recessively inherited disorder caused by a deficiency of galactose-1-phosphate uridyltransferase (GALT). Manifestations include jaundice, vomiting, cataracts, mental retardation, speech abnormalities and poor growth. The GALT gene has been mapped and sequenced. The S135L mutation accounts for approximately 48% of galactosaemia alleles in African Americans and has been found to account for about 91% of galactosaemia alleles in negroid South African patients which suggested that the mutation had an African origin. We have calculated the S135L allele frequency (+/- 1SE) in a sample of healthy unrelated negroid South Africans to be 0.0067 (+/- 0.0024). The S135L mutation was also detected in negroid populations from other regions of Africa confirming its African origin.

The molecular biology of galactosemia

Classic galactosemia is an autosomal recessive disorder caused by the deficiency of galactose 1-phosphate uridyltransferase (GALT). Although the potentially lethal, neonatal hepatotoxic syndrome is prevented by newborn screening and galactose restriction, long-term outcome for older patients with galactosemia remains problematic. After the cloning and sequencing of the GALT gene, more than 130 mutations in the GALT gene have been associated with GALT deficiency; this review relates them to function and clinical outcome. Two common mutations, Q188R and K285N, account for more than 70% of G alleles in the white population and are associated with classic galactosemia and impaired GALT function. In the black population, S135L accounts for 62% of the alleles causing galactosemia and is associated with good outcomes. A large 5 kb deletion in the GALT gene is found in Ashkenazim Jews. The Duarte galactosemia variant is caused by N314D. Homozygosity for N314D reduces GALT activity to 50%. When either E203K or a 1721C-->T transition (Los Angeles variant) are present in cis with N314D, GALT activity reverts to normal. In this review, we discuss the structural biology of these mutations as they affect both the GALT enzyme and patient outcome.

Molecular and biochemical basis of galactosemia

Galactosemia is a clinically heterogeneous autosomal recessive inborn error of metabolism caused by deficiency of galactose-1-phosphate uridylyltransferase (GALT). Despite the numerous point mutations identified in the GALT gene, the prevalence of these mutations in different ethnic groups has not been studied. Reports on genotype/phenotype correlation are not consistent due to the small sample sizes studied and the lack of a sensitive enzyme assay. We applied multiplex PCR/ASO dot blot analysis to screen 293 galactosemic patients for 17 known point mutations in exons 5, 6, and 10. Our data demonstrate that only 7 of these mutations were detected in our patients, accounting for 65% of the GALT mutant alleles. Although Q188R is the most common mutation in Caucasian and Hispanic patients, the S135L mutation is most common in African-Americans. Another mutation, F171S, was observed only among African-American patients. An improved, sensitive, and accurate method was used to measure GALT activity in patient's red blood cells. The results indicated that patients homozygous for Q188R have no enzyme activity while those homozygous for S135L had residual enzyme activity. Interestingly, both Q188R/S135L and S135L/F171S compound heterozygotes demonstrated zero enzyme activity. Overall, 85% of Q188R compound heterozygotes also did not have any enzyme activity, whereas the remaining Q188R and the majority of S135L compound heterozygotes expressed variable amounts of GALT activity. We speculate that heterodimeric subunit interaction plays an important role in determining the overall enzymatic activity. Various genotypes thus result in biochemical and clinical heterogeneity among the patients.

Biochemical characterization of the S135L allele of galactose-1-phosphate uridylyltransferase associated with galactosaemia

Impairment of the human enzyme galactose-1-phosphate uridylyltransferase (GALT) results in the potentially lethal disorder galactosaemia. The S135L mutation, which accounts for almost 50% of the GALT alleles in galactosaemia patients of African-American descent, has been associated with activities ranging from null to wild-type by different investigators examining cell lysates representing different tissues or model systems. Because of the crude nature of the lysates examined, however, and the absence of quantitative measures concerning GALT abundance in most of those lysates, the available data do not distinguish between differences in GALT enzyme expression/abundance, specific activity, or kinetic constants in these different tissues or systems. In an effort to overcome this uncertainty and investigate the biochemical impact of the S135L substitution on human GALT function under defined conditions, we have overexpressed both wild-type and S135L-mutant GALT sequences in a null-background yeast expression system, and purified both proteins to near homogeneity. Abundance of the wild-type and mutant proteins in crude yeast lysates differed by approximately 2-fold. Kinetic studies of the purified proteins, however, demonstrated that although K(m) values differed by < 2-fold, specific activities differed by 10-fold. Temperature-activity profiles revealed no significant differences, and coprecipitation studies demonstrated that S135L-hGALT subunits remained competent to self-associate in vivo. We conclude that the S135L substitution causes either steric or electrochemical changes sufficiently close to the active site in human GALT to result in partial impairment of the transferase reaction.

Black children deficient in galactose 1-phosphate uridyltransferase: correlation of activity and immunoreactive protein in erythrocytes and leukocytes

A recent study found a high prevalence of a missense mutation (S135L) in the gene for galactose 1-phosphate uridyltransferase (GALT) in black children with galactosemia (J Pediatr 1996; 128:89-95). In the present study, GALT activity and GALT protein content were measured in erythrocytes and leukocytes of eight black and seven white galactosemic (GALT-deficient) children, for correlation with the presence of the S135L and Q188R (highly prevalent in white galactosemic children) missense mutations. The S135L mutation was found in 9 of 16 alleles of black children but not in white children; the Q188R mutation was found in 10 of 14 alleles examined in white galactosemic children and in 4 of 16 alleles in black galactosemic children. The GALT activity was near zero in the erythrocytes of white and black galactosemic children (0.26 +/- 0.28 vs 0.33 +/- 0.25 mumol/hr per gram of hemoglobin, respectively; p = 0.61) (normal 17 to 26 mumol/hr per gram), and no correlation of erythrocyte activity with genotype was observed. The GALT activity was higher in the leukocytes of black galactosemic children compared with white children (5 +/- 6 vs 1 +/- 2 mumol/hr per gram, respectively) (normal 172 to 374 mumol/hr per gram), but the difference was not statistically significant (p = 0.11). Analysis by genotype revealed that the two S135L homozygotes had much more leukocyte activity (9 and 17 mumol/hr per gram) than Q188R homozygotes or than all non-S135L allelic genotypes. Compound heterozygotes (S135L/G) had intermediate activity. The GALT protein was not detectable by Western blot in the erythrocytes of either white or black galactosemic children, as determined by antibodies specific for both C- and N-terminal sequences. The GALT protein was undetectable in the leukocytes of white galactosemic children, but leukocytes from black galactosemic children with the S135L mutation contained reduced but readily detectable GALT protein. Erythrocyte galactose 1-phosphate levels were significantly lower in galactosemic children with an S135L mutant allele (1.1 +/- 0.2 gm/dl) compared with children who had other mutations (3.1 +/- 0.9 mg/dl; p = 0.0001). The correlation of protein content data with activity levels in the blood cells suggests that the S135L missense mutation affects the stability of GALT protein to produce a deficiency state.

Three missense mutations in the galactose-1-phosphate uridyltransferase gene of three families with mild galactosaemia

Classical galactosaemia caused by deficiency of galactose-1-phosphate uridyltransferase (GALT) is characterized by acute symptoms of hepatocellular dysfunction, sepsis, cataracts and failure to thrive. Galactose limitation reverses these complications immediately, however, most of these children have a long-term complication of verbal dyspraxia mental retardation and ovarian failure. The GALT gene was cloned and several mutations including the common Q188R have been reported. In this study the coding region of GALT was amplified by polymerase chain reaction from genomic DNA of classical galactosaemic individuals and characterized by direct sequencing of the products. Three missense mutations were identified in three patients with a mild galactosaemic variant: (1) replacement of threonine-138 by methionine (T138M); (2) replacement of arginine by tryptophan (R259W); and (3) replacement of threonine by alanine (T350A). All three galactosaemic individuals, one girl and two boys, have varying degrees of residual GALT activity in RBC and their galactose-1-phosphate levels decreased much faster than in other galactosaemic patients. These missense mutations occur in regions that are not highly conserved domains.

CONCLUSION

The study of the molecular basis related to the phenotype variation may indeed help to prognosticate the outcome of patients with classical galactosaemia.

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.

Characterization of two stop codon mutations in the galactose-1-phosphate uridyltransferase gene of three male galactosemic patients with severe clinical manifestation

Classical galactosemia, which is caused by deficiency of galactose-1-phosphate uridyltransferase, is characterized by acute problems of hepatocellular dysfunction, sepsis, cataracts and failure to thrive. Galactose limitation reverses these symptoms immediately; however, the long-term complications, such as mental retardation and ovarian failures are major problems in most of these patients. In order to investigate the molecular basis for phenotype variation in galactosemia, we have screened the most common mutation in the GALT gene, Q188R. We have further examined those patients who are heterozygous for Q188R or negative for this mutation by SSCP analysis and direct sequencing. In three male patients, we have identified, for the first time, two stop-codon mutations in the GALT gene, G212X (exon 7) and E340X (exon 10). Two patients of 8 and 28 years of age, respectively, who are compound heterozygotes for Q188R and G212X, have severe mental retardation and their general clinical condition is more severe than that of patients with missense mutations. The third patient, who is 8 years of age and who is homozygous for E340X, the N314D polymorphism and a silent substitution L218L, presents with a relatively normal physical and mental condition to date.

Three-dimensional structure of galactose-1-phosphate uridylyltransferase from Escherichia coli at 1.8 A resolution

Galactose-1-phosphate uridylyltransferase catalyzes the reversible transfer of the uridine 5'-monophosphoryl moiety of UDP-glucose to the phosphate group of galactose 1-phosphate to form UDP-galactose. This enzyme participates in the Leloir pathway of galactose metabolism, and its absence is the primary cause of the potentially lethal disease galactosemia. The three-dimensional structure of the dimeric enzyme from Escherichia coli complexed with uridine 5'-diphosphate is reported here. The structure was solved by multiple isomorphous replacement and electron density modification techniques and has been refined to 1.8 A resolution. Enzyme subunits consist of a single domain with the topology of a "half-barrel". The barrel staves are formed by nine strands of antiparallel beta-sheet. The barrel axis is approximately parallel to the local dyad that relates each subunit. Two amphipathic helices fill the half-barrel sequestering its hydrophobic interior. An iron atom resides on the outside of the barrel, centered in the subunit interface. Intrasubunit coordination to iron resembles a distorted square pyramid formed by the equatorial ligation of two histidines and a bidentate carboxylate group and a single axial histidine. The subunit interface is stabilized by this coordination and is further characterized by the formation of two intermolecular "mini-sheets" distinct from the strands of the half-barrel. Loops that connect the mini-sheet strands contribute to the formation of the active site, which resides on the external surface of the barrel rim. Loops of the barrel strands are tethered together by a structural zinc atom that orients the local fold in a manner essential for catalysis. In one of the latter loops, S gamma of a cysteine is modified by beta-mercaptoethanol, which prevents the alpha-phosphorus of the nucleotide from access to the nucleophile His166. This conformation does not appear to perturb the interactions to the uracil and ribose moieties as mediated through the side chains of Leu54, Ohe75, Asn77, Asp78, Phe79, and Val108. Several of the latter residues have been implicated in human galactosemia. The present structure explains the deleterious effects of many of those mutations.

Mutations in the galactose-1-phosphate uridyltransferase gene of two families with mild galactosaemia variants

Classical galactosaemia, deficiency of galactose-1-phosphate uridyltransferase (GALT), is characterized by acute symptoms of hepatomegaly, jaundice, sepsis, cataracts and growth retardation. Treatment with dietary galactose restriction corrects these complications immediately; however, most of these children develop long-term complications of verbal dyspraxia, mental retardation and ovarian failure. Our previous molecular study showed that the most common mutation of the GALT gene is a missense mutation of Q188R (replacement of glutamine-188 by arginine) in approximately 60-65% of the German galactosaemic population. The coding region of GALT was amplified by the polymerase chain reaction from genomic DNA of classical galactosaemic individuals, who are negative or heterozygous for Q188R, and was further characterized by direct sequencing. Three new disease-causing mutations, two missense and a stop codon mutation, were identified in three patients from two families with mild galactosaemic variants: firstly R67C, replacement of arginine-67 by cysteine and W316X, the stop codon at tryptophan-316 in one male; secondly A330V, replacement of alanine-330 by valine in two female siblings. In the first family the patient was also heterozygous for the polymorphism N314D and in the second family both girls were compound heterozygotes for Q188R and A330V. All three galactosaemic individuals have a considerable amount of the residual GALT activity in RBC and the galactose-1-phosphate (GALP) level decreased much faster on treatment than that of other galactosaemic patients with missense mutations such as Q188R. The clinical and biochemical data of these patients were much more favourable in comparison with those of two female galactosaemic individuals, one homozygous for L195P and the other compound heterozygous for Q188R and L195P. These three missense mutations (R67C, L195P and A330V) also occur in highly conserved regions. These observations suggest that the phenotypic variation in galactosaemic individuals may be due to different molecular aetiologies.

Molecular characterization of galactosemia (type 1) mutations in Japanese

We characterized two novel mutations of the galactose-1-phosphate uridyltransferase (GALT) gene in two Japanese patients with GALT deficiency and identified N314D and R333W mutations, previously found in Caucasians. One novel missense mutation was an G-to-A transition in exon 8, resulting in the substitution of arginine by histidine at the codon 231 (R231H). GALT activity of the R231H mutant construct was reduced to 15% of normal controls in a COS cell expression system. The other was a splicing mutation, an A-to-G transition at the 38th nucleotide in exon 3 (318A-->G), resulting in a 38-bp deletion in the GALT cDNA by activating a cryptic splice acceptor site. In seven Japanese families (14 alleles for classic form and one allele for Duarte variant) with GALT deficiency, the R231H and 318A-->G mutations were found only on both alleles of the proband. The N314D and R333W mutations were found on one allele each. The Q188R was prevalent in the United States but not in Japanese patients. The N314D mutation was associated with the Duarte variant in Japanese persons, as well as in the United States. We speculate that classic galactosemia mutations appear to differ between Japanese and Caucasian patients. Our limited data set on galactosemia mutations in Japanese suggests that the N314D GALT mutation encoding the Duarte variant arose before Asian and Caucasian people diverged and that classic galactosemia mutations arose and/or accumulated after the divergence of Asian and Caucasian populations.

Regulatory effects of galactose on galactose-1-phosphate uridyltransferase activity on human hepatoblastoma HepG2 cells

Galactose-1-phosphate uridyltransferase (GALT) deficiency results in galactosemia in man. We have studied the regulation of the GALT gene expression on the HepG2 cell line by growing the cells in glucose or galactose medium. No difference of Km values was observed in glucose or galactose media but the Vmax value with galactose was 50% higher than that with glucose. Also in galactose medium, an increased GALT specific activity was detected suggesting the production of more enzyme proteins. Yet, slot dot quantification of GALT mRNA revealed a decreased amount of these transcripts in cells cultured with galactose or inosine while Northern blot analysis revealed the normal 1.4 kb transcript in all culture media used. Finally, IEF gel analysis displayed different isozymic patterns for the GALT enzyme in cells grown in glucose, galactose or inosine media. With glucose-free media, the major band of GALT corresponds to that found in human liver. Altogether, these results suggest that the control of GALT gene expression in HepG2 cells is located at the post-transcriptional level and correlated to the growth rate of the cell.

Biochemical and molecular studies of 132 patients with galactosemia

We evaluated 132 galactosemia patients for the Q188R (glutamine-188 to arginine) mutation in the human galactose-1-phosphate uridyltransferase (GALT) gene and for GALT activity in their hemolysates by a sensitive radioisotopic method. In those without any detectable GALT activity (GG), the Q188R mutation constituted 67% of the alleles. In patients with detectable GALT activity (GV), only 16% of the alleles were accounted for by Q188R. In all patients who were homozygous for the Q188R mutation, no erythrocyte GALT activity could be demonstrated. There was an extensive variation in the amount of detectable GALT activity ranging from 0.1% to 5% of the normal values among the GV patients. There was a difference in the frequency of Q188R mutation in the GALT alleles among patients belonging to different racial and ethnic groups. In Caucasian and Hispanic patients, the frequency was not far different (64% and 58%, respectively). On the other hand, only 12% of the GALT alleles with Q188R were found in African-American patients.

A yeast expression system for human galactose-1-phosphate uridylyltransferase

Galactose-1-phosphate uridylyltransferase (GALT) (UTP: alpha-D-hexose-1-phosphate uridylyltransferase, EC 2.7.7.10) is an essential enzyme of the Leloir pathway of galactose metabolism. Mutations in human GALT are associated with the potentially lethal disorder galactosemia, which affects 1 in 30,000-60,000 live-born infants. Although a number of base substitutions have been identified in the GALT alleles of galactosemia patients, the detailed biochemical impact of these mutations on GALT enzymatic activity remains obscure. Similarly, little is known about the sequence/structure/function relationships for wild-type human GALT. As a first step toward addressing these questions, we have developed a yeast-based expression system for the human enzyme. The wild-type human GALT coding sequence has been introduced into a strain of Saccharomyces cerevisiae that carries a disruption of the GALT-encoding GAL7 gene and, therefore, expresses no endogenous GALT. Transformants were tested for restoration of GALT activity both indirectly, by cell growth on galactose, and directly, by analysis of enzyme activity in cell extracts. The results of both tests were striking; wild-type human GALT functioned in yeast almost as well as the endogenous enzyme. In contrast, cells transformed with either human or yeast GALT sequences engineered to carry a common human GALT mutation, Q188R (changing Gln188 to Arg), exhibited essentially no detectable GALT activity and failed to grow on galactose. Lymphoblasts from patients homozygous for the Q188R mutation similarly exhibited essentially no detectable GALT activity in parallel assays. The results reported here establish the utility of the yeast-based expression system for human GALT and set the stage for more detailed studies of this important enzyme and its role in galactosemia.

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.