Drosophila melanogaster Models of Galactosemia

The galactosemias are a family of autosomal recessive genetic disorders resulting from impaired function of the Leloir pathway of galactose metabolism. Type I, or classic galactosemia, results from profound deficiency of galactose-1-phosphate uridylyltransferase, the second enzyme in the Leloir pathway. Type II galactosemia results from profound deficiency of galactokinase, the first enzyme in the Leloir pathway. Type III galactosemia results from partial deficiency of UDP galactose 4'-epimerase, the third enzyme in the Leloir pathway. Although at least classic galactosemia has been recognized clinically for more than 100 years, and detectable by newborn screening for more than 50 years, all three galactosemias remain poorly understood. Early detection and dietary restriction of galactose prevent neonatal lethality, but many affected infants grow to experience a broad range of developmental and other disabilities. To date, there is no intervention known that prevents or reverses these long-term complications. Drosophila melanogaster provides a genetically and biochemically facile model for these conditions, enabling studies that address mechanism and open the door for novel approaches to intervention.

Analysis of UDP-galactose 4'-epimerase mutations associated with the intermediate form of type III galactosaemia

Type III galactosaemia is a hereditary disease caused by reduced activity in the Leloir pathway enzyme, UDP-galactose 4'-epimerase (GALE). Traditionally, the condition has been divided into two forms-a mild, or peripheral, form and a severe, or generalized, form. Recently it has become apparent that there are disease states which are intermediate between these two extremes. Three mutations associated with this intermediate form (S81R, T150M and P293L) were analysed for their kinetic and structural properties in vitro and their effects on galactose-sensitivity of Saccharomyces cerevisiae cells that were deleted for the yeast GALE homologue Gal10p. All three mutations result in impairment of the kinetic parameters (principally the turnover number, k (cat)) compared with the wild-type enzyme. However, the degree of impairment was mild compared with that seen with the mutation (V94M) associated with the generalized form of epimerase deficiency galactosaemia. None of the three mutations tested affected the ability of the protein to dimerize in solution or its susceptibility to limited proteolysis in vitro. Finally, in the yeast model, each of the mutated patient alleles was able to complement the galactose-sensitivity of gal10Delta cells as fully as was the wild-type human allele. Furthermore, there was no difference from control in metabolite profile following galactose exposure for any of these strains. Thus we conclude that the subtle biochemical and metabolic abnormalities detected in patients expressing these GALE alleles likely reflect, at least in part, the reduced enzymatic activity of the encoded GALE proteins.

A yeast model reveals biochemical severity associated with each of three variant alleles of galactose-1P uridylyltransferase segregating in a single family

Classic galactosaemia is a potentially lethal inborn error of metabolism that results from profound impairment of galactose-1P uridylyltransferase (GALT). Like many autosomal recessive disorders, classic galactosaemia demonstrates marked allelic heterogeneity; many if not most patients are compound heterozygotes. Owing in part to the fact that most GALT mutations are never observed in patients in the homozygous state, in part to concerns of possible allelic interaction, and in part to the broad range of GALT activity levels associated with the affected, carrier, and control states, definition of the specific functional consequence of individual variant GALT alleles from studies of clinical samples alone can be a challenging task. To overcome this problem we previously developed and applied a null-background yeast system to enable functional analyses of human GALT alleles expressed individually or in defined pairs. We report here the application of this system to characterize three distinct variant alleles of GALT identified within a single family. Of these alleles, one carried a missense mutation (K285N) that has previously been reported and characterized, one carried a nonsense mutation (R204X) that has previously been reported but not characterized, and the third carried a missense substitution (T268N) that was novel. Our studies reported here reconfirm the profound nature of the K285N mutation, demonstrate that the R204X mutation severely compromises both expression and function of human GALT, and finally implicate T268N as one of a very small number of naturally occurring rare but neutral missense polymorphisms in human GALT.

A PCR-based method for detecting known mutations in the human UDP galactose-4'-epimerase gene associated with epimerase-deficiency galactosemia

Epimerase-deficiency galactosemia results from impairment of the human enzyme UDP galactose-4'-epimerase (GALE). We report a rapid, internally controlled PCR-based method for detection of nine naturally occurring point mutations in human GALE associated with epimerase deficiency. These mutations were derived from patients whose clinical presentations ranged from mild to severe; all but one of these mutations have been reported previously. The tests described here work well on both cDNA and genomic samples and require no specialized equipment beyond a thermal cycler and an agarose gel electrophoresis system. Finally, although these tests in no way replace the need for biochemical diagnosis in epimerase-deficiency galactosemia, they do provide the possibility of additional molecular information to support a biochemical diagnosis and facilitate the possibility of more accurate carrier testing, should that option be desired.

The brefeldin A resistance protein Bfr1p is a component of polyribosome-associated mRNP complexes in yeast

The yeast gene BFR1 was originally isolated from a genetic screen for high-copy suppressors of brefeldin A-induced lethality in Saccharomyces cerevisiae. While this result suggested a possible role for the encoded protein, Bfr1p, in the secretory pathway, subsequent data have not fully supported this conclusion. Alternatively, Bfr1p has also been found by yeast two-hybrid analysis to interact with Bbp1p, a component of the spindle pole body. Finally, we have reported that Bfr1p associates with cytoplasmic mRNP complexes containing Scp160p, raising the possibility that Bfr1p may function in mRNA metabolism. Here, we have explored this possibility further. We report that Bfr1p associates with yeast polyribosomes and mRNP complexes even in the absence of Scp160p, and that its interaction with Scp160p-containing mRNP complexes is RNA-dependent. Furthermore, we have determined by fluorescence microscopy and subcellular fractionation that Bfr1p and Scp160p demonstrate similar cytoplasmic localization with enrichment around the nuclear envelope/endoplasmic reticulum. Finally, we report that loss of Bfr1p disrupts the interaction of Scp160p with polyribosomes, thereby demonstrating that the relationship between these two proteins is functional as well as physical. Considered together, these data raise the intriguing possibility that Bfr1p may provide a link between mRNA metabolism, the chromosomal segregation machinery and perhaps secretion in yeast.

Molecular basis for severe epimerase deficiency galactosemia. X-ray structure of the human V94m-substituted UDP-galactose 4-epimerase

Galactosemia is an inherited disorder characterized by an inability to metabolize galactose. Although classical galactosemia results from impairment of the second enzyme of the Leloir pathway, namely galactose-1-phosphate uridylyltransferase, alternate forms of the disorder can occur due to either galactokinase or UDP-galactose 4-epimerase deficiencies. One of the more severe cases of epimerase deficiency galactosemia arises from an amino acid substitution at position 94. It has been previously demonstrated that the V94M protein is impaired relative to the wild-type enzyme predominantly at the level of V(max) rather than K(m). To address the molecular consequences the mutation imparts on the three-dimensional architecture of the enzyme, we have solved the structures of the V94M-substituted human epimerase complexed with NADH and UDP-glucose, UDP-galactose, UDP-GlcNAc, or UDP-GalNAc. In the wild-type enzyme, the hydrophobic side chain of Val(94) packs near the aromatic group of the catalytic Tyr(157) and serves as a molecular "fence" to limit the rotation of the glycosyl portions of the UDP-sugar substrates within the active site. The net effect of the V94M substitution is an opening up of the Ala(93) to Glu(96) surface loop, which allows free rotation of the sugars into nonproductive binding modes.

Human UDP-galactose 4-epimerase. Accommodation of UDP-N-acetylglucosamine within the active site

UDP-galactose 4-epimerase catalyzes the interconversion of UDP-galactose and UDP-glucose during normal galactose metabolism. One of the key structural features in the proposed reaction mechanism for the enzyme is the rotation of a 4'-ketopyranose intermediate within the active site pocket. Recently, the three-dimensional structure of the human enzyme with bound NADH and UDP-glucose was determined. Unlike that observed for the protein isolated from Escherichia coli, the human enzyme can also turn over UDP-GlcNAc to UDP-GalNAc and vice versa. Here we describe the three-dimensional structure of human epimerase complexed with NADH and UDP-GlcNAc. To accommodate the additional N-acetyl group at the C2 position of the sugar, the side chain of Asn-207 rotates toward the interior of the protein and interacts with Glu-199. Strikingly, in the human enzyme, the structural equivalent of Tyr-299 in the E. coli protein is replaced with a cysteine residue (Cys-307) and the active site volume for the human protein is calculated to be approximately 15% larger than that observed for the bacterial epimerase. This combination of a larger active site cavity and amino acid residue replacement most likely accounts for the inability of the E. coli enzyme to interconvert UDP-GlcNAc and UDP-GalNAc.

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.

Studies of the V94M-substituted human UDPgalactose-4-epimerase enzyme associated with generalized epimerase-deficiency galactosaemia

Impairment of the human enzyme UDPgalactose 4-epimerase (hGALE) results in epimerase-deficiency galactosaemia, an inborn error of metabolism with variable biochemical presentation and clinical outcomes reported to range from benign to severe. Molecular studies of the hGALE loci from patients with epimerase deficiency reveal significant allelic heterogeneity, raising the possibility that variable genotypes may constitute at least one factor contributing to the biochemical and clinical heterogeneity observed. Previously we have identified a single substitution mutation, V94M, present in the homozygous state in all patients genotyped with the severe, generalized form of epimerase-deficiency galactosaemia. We report here further studies of the V94M-hGALE enzyme, overexpressed and purified from a null-background yeast expression system. Our results demonstrate that the mutant protein is impaired relative to the wild-type enzyme predominantly at the level of Vmax rather than of Km. Studies using UDP-N-acetylgalactosamine as a competitor of UDPgalactose further demonstrate that the Km values for these two substrates vary by less than a factor of 3 for both the wild-type and mutant proteins. Finally, we have explored the impact of the V94M substitution on susceptibility of yeast expressing human GALE to galactose toxicity, including changes in the levels of galactose 1-phosphate (gal-1-P) accumulated in these cells at different times following exposure to galactose. We have observed an inverse correlation between the level of GALE activity expressed in a given culture and the degree of galactose toxicity observed. We have further observed an inverse correlation between the level of GALE activity expressed in a culture and the concentration of gal-1-P accumulated in the cells. These data support the hypothesis that elevated levels of gal-1-P may underlie the observed toxicity. They further raise the intriguing possibility that yeast may provide a valuable model not only for assessing the impact of given patient mutations on hGALE function, but also for exploring the metabolic imbalance resulting from impaired activity of GALE in living cells.

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.

Subcellular localization of galactose-1-phosphate uridylyltransferase in the yeast Saccharomyces cerevisiae

The enzyme galactose-1-phosphate uridylyltransferase (GALT) catalyzes the second step of the Leloir pathway of galactose metabolism, following galactokinase (GALK) and preceding UDP-galactose-4-epimerase (GALE). Impairment of GALT in humans results in the potentially lethal disorder classic galactosemia. Standard lysis protocols of bacteria, yeast, or mammalian cells release all three Leloir enzymes in the soluble fraction, leading to the historical assumption that all three function as free cytosolic enzymes. We have tested this assumption with regard to GALT in vivo using the yeast Saccharomyces cerevisiae, by linking a GFP-tag onto the amino terminus of Gal7p, the endogenous yeast GALT. We find clear evidence of localization of the fusion protein to discrete spots in the cytoplasm of the majority of cells expressing all three Leloir enzymes, although GFP alone appears freely cytosolic. In contrast, yeast expressing GFP-Gal7p but lacking Gal1p (GALK), Gal10p (GALE), or both do not demonstrate spots in the majority of cells, implicating a role, either direct or indirect, for these other Leloir proteins in the Gal7p localization process. Preliminary truncation experiments reveal that amino acids 1-134 of Gal7p are sufficient to drive localization of the fusion protein, while amino acids 1-66 are not. Finally, GFP-tagged human GALT expressed in yeast also localizes to spots, demonstrating that at least some of the intrinsic determinants of localization have been conserved. These observations raise the intriguing possibility that GALT may function in a sequestered rather than a freely diffusible state, and that this subcellular organization may have been conserved through evolution.

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.

Crystallographic evidence for Tyr 157 functioning as the active site base in human UDP-galactose 4-epimerase

UDP-galactose 4-epimerase catalyzes the interconversion of UDP-glucose and UDP-galactose during normal galactose metabolism. In humans, deficiencies in this enzyme lead to the complex disorder referred to as epimerase-deficiency galactosemia. Here, we describe the high-resolution X-ray crystallographic structures of human epimerase in the resting state (i.e., with bound NAD(+)) and in a ternary complex with bound NADH and UDP-glucose. Those amino acid side chains responsible for anchoring the NAD(+) to the protein include Asp 33, Asn 37, Asp 66, Tyr 157, and Lys 161. The glucosyl group of the substrate is bound to the protein via the side-chain carboxamide groups of Asn 187 and Asn 207. Additionally, O(gamma) of Ser 132 and O(eta) of Tyr 157 lie within 2.4 and 3.1 A, respectively, of the 4'-hydroxyl group of the sugar. Comparison of the polypeptide chains for the resting enzyme and for the protein with bound NADH and UDP-glucose demonstrates that the major conformational changes which occur upon substrate binding are limited primarily to the regions defined by Glu 199 to Asp 240 and Gly 274 to Tyr 308. Additionally, this investigation reveals for the first time that a conserved tyrosine, namely Tyr 157, is in the proper position to interact directly with the 4'-hydroxyl group of the sugar substrate and to thus serve as the active-site base. A low barrier hydrogen bond between the 4'-hydroxyl group of the sugar and O(gamma) of Ser 132 facilitates proton transfer from the sugar 4'-hydroxyl group to O(eta) of Tyr 157.

Scp160p, a multiple KH-domain protein, is a component of mRNP complexes in yeast

Scp160p is a 160 kDa protein in the yeast Saccharomyces cerevisiae that contains 14 repeats of the hnRNP K-homology (KH) domain, and demonstrates significant sequence homology to a family of proteins collectively known as vigilins. As a first step towards defining the function of Scp160p, we have characterized the subcellular distribution and in vivo interactions of this protein. Using sucrose gradient fractionation studies we have demonstrated that Scp160p in cytoplasmic lysates is predominantly associated with polyribosomes. Furthermore, we have found that Scp160p is released from polyribosomes by EDTA in the form of a large complex of> or =1300 kDa that is sensitive both to RNase and NaCl. Using affinity-chromatography to isolate these complexes, we have identified two protein components other than Scp160p: poly(A) binding protein, Pab1p, and Bfr1p. The presence of Pab1p confirms these complexes to be mRNPs. The presence of Bfr1p is intriguing because the null phenotype for this gene is essentially the same as that reported for scp160 -null cells: increased cell size and aberrant DNA content. These results demonstrate that Scp160p associates with polyribosome-bound mRNP complexes in vivo, implicating a role for this protein in one or more levels of mRNA metabolism in yeast.

Identification and characterization of a mutation, in the human UDP-galactose-4-epimerase gene, associated with generalized epimerase-deficiency galactosemia

Epimerase-deficiency galactosemia results from impairment of the human enzyme UDP-galactose-4-epimerase (hGALE). We and others have identified substitution mutations in the hGALE alleles of patients with the clinically mild, peripheral form of epimerase deficiency. We report here the first identification of an hGALE mutation in a patient with the clinically severe, generalized form of epimerase deficiency. The mutation, V94M, was found on both GALE alleles of this patient. This same mutation also was found in the homozygous state in two additional patients with generalized epimerase deficiency. The specific activity of the V94M-hGALE protein expressed in yeast was severely reduced with regard to UDP-galactose and partially reduced with regard to UDP-N-acetylgalactosamine. In contrast, two GALE-variant proteins associated with peripheral epimerase deficiency, L313M-hGALE and D103G-hGALE, demonstrated near-normal levels of activity with regard to both substrates, but a third allele, G90E-hGALE, demonstrated little, if any, detectable activity, despite near-normal abundance. G90E originally was identified in a heterozygous patient whose other allele remains uncharacterized. Thermal lability and protease-sensitivity studies demonstrated compromised stability in all of the partially active mutant enzymes.

Characterization of two mutations associated with epimerase-deficiency galactosemia, by use of a yeast expression system for human UDP-galactose-4-epimerase

UDP-galactose-4-epimerase (GALE) is a highly conserved enzyme that catalyzes the interconversion of UDP-galactose and UDP-glucose. Impairment of this enzyme in humans results in one of two clinically distinct forms of epimerase-deficiency galactosemia-one benign, the other severe. The molecular and biochemical distinction between these disorders remains unknown. To enable structural and functional studies of both wild-type and patient-derived alleles of human GALE (hGALE), we have developed and applied a null-background yeast expression system for the human enzyme. We have demonstrated that wild-type hGALE sequences phenotypically complement a yeast gal10 deletion, and we have biochemically characterized the wild-type human enzyme isolated from these cells. Furthermore, we have expressed and characterized two mutant alleles, L183P-hGALE and N34S-hGALE, both derived from a patient with no detectable GALE activity in red blood cells but with approximately 14% activity in cultured lymphoblasts. Analyses of crude extracts of yeast expressing L183P-hGALE demonstrated 4% wild-type activity and 6% wild-type abundance. Extracts of yeast expressing N34S-hGALE demonstrated approximately 70% wild-type activity and normal abundance. However, yeast coexpressing both L183P-hGALE and N34S-hGALE exhibited only approximately 7% wild-type levels of activity, thereby confirming the functional impact of both substitutions and raising the intriguing possibility that some form of dominant-negative interaction may exist between the mutant alleles found in this patient. The results reported here establish the utility of the yeast-based hGALE-expression system and set the stage for more-detailed studies of this important enzyme and its role in epimerase-deficiency galactosemia.

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.

Genetic approaches to biochemical questions: insights into the functional requirements of proline 185 in the active site of human galactose-1-phosphate uridylyltransferase

Saturating random mutagenesis at a given position within a polypeptide sequence can provide powerful insights into the functional requirements of the position. By coupling this genetic methodology with expression of human proteins in yeast, we and others have begun to ask pointed and important questions about the structure-function relationships of proteins associated with human genetic disease.

The Q188R mutation in human galactose-1-phosphate uridylyltransferase acts as a partial dominant negative

A longstanding goal in the fields of molecular genetics and biochemistry has been to explain how naturally occurring mutations associated with human metabolic disease impair activity of the enzymes involved. This goal is particularly complex for enzymes composed of multiple subunits, because single mutations may exert both intra- and intersubunit effects on holoenzyme structure and function. We have previously applied a yeast coexpression system for human galactose-1-phosphate uridylyltransferase, a dimeric enzyme associated with galactosemia, to investigate the impact of naturally occurring mutations on subunit association and holoenzyme function (). Here we describe the purification and characterization of two heterodimers, R333W/wild type (WT) and Q188R/WT, revealing that although the first exhibits approximately 50% wild-type activity, the second exhibits only approximately 15% wild-type activity. Neither heterodimer varied significantly from the wild type with regard to apparent Km for either substrate, although Q188R/WT but not R333W/WT heterodimers demonstrated significantly increased thermal sensitivity relative to the wild-type enzyme. These results demonstrate for the first time a partial dominant negative effect caused by a naturally occurring mutation in human galactose-1-phosphate uridylyltransferase.

Functional requirements of the active site position 185 in the human enzyme galactose-1-phosphate uridylyltransferase

The active site of galactose-1-phosphate uridylyltransferase (GALT) includes a HPH sequence that has been conserved in all species examined from Escherichia coli to humans. The crystal structure of the E. coli enzyme suggests that this proline is important in positioning the active site histidine (His-166) near the substrate. To examine the role of this proline in the homologous human sequence, we have performed saturating mutagenesis at Pro-185 within human GALT and characterized each resultant mutant enzyme using a yeast expression system. Activity analyses in crude lysates indicated that only proline at position 185 produced wild-type levels of activity, although five other amino acids, Ala, Gly, Ser, Gln, and Glu, all produced partially active enzymes. Western blot analyses of the GALT proteins in these lysates demonstrated that abundance varied from 9-118% of wild-type and was independent of activity. All five active mutant proteins were purified and characterized with regard to specific activity, apparent Km for both substrates, and temperature-dependence of activity. Finally, modeling of these mutations onto the conserved E. coli active site structure was performed. Together, these results provide functional evidence demonstrating the critical role of Pro-185 in facilitating the transferase reaction.

Heterodimer formation and activity in the human enzyme galactose-1-phosphate uridylyltransferase

One of the fundamental questions concerning expression and function of dimeric enzymes involves the impact of naturally occurring mutations on subunit assembly and heterodimer activity. This question is of particular interest for the human enzyme galactose-l-phosphate uridylyl-transferase (GALT), impairment of which results in the inherited metabolic disorder galactosemia, because many if not most patients studied to date are compound heterozygotes rather than true molecular homozygotes. Furthermore, the broad range of phenotypic severity observed in these patients raises the possibility that allelic combination, not just allelic constitution, may play some role in determining outcome. In the work described herein, we have selected two distinct naturally occurring null mutations of GALT, Q188R and R333W, and asked the questions (i) what are the impacts of these mutations on subunit assembly, and (ii) if heterodimers do form, are they active? To answer these questions, we have established a yeast system for the coexpression of epitope-tagged alleles of human GALT and investigated both the extent of specific GALT subunit interactions and the activity of defined heterodimer pools. We have found that both homodimers and heterodimers do form involving each of the mutant subunits tested and that both heterodimer pools retain substantial enzymatic activity. These results are significant not only in terms of their implications for furthering our understanding of galactosemia and GALT holoenzyme structure-function relationships but also because the system described may serve as a model for similar studies of other complexes composed of multiple subunits.

Characterization of the N314D allele of human galactose-1-phosphate uridylyltransferase using a yeast expression system

Transferase-deficiency galactosemia is an inborn error of metabolism resulting from impairment of the enzyme galactose-1-phosphate uridylyltransferase (GALT), which normally catalyzes the second step of the Leloir pathway of galactose metabolism. Several recent studies have linked a previously reported substitution, N314D (asn to asp at position 314), with both the Duarte and Los Angeles (LA) variant alleles of GALT. While both variants demonstrate similar mobility shifts relative to the normal enzyme on isoelectric focusing (IEF) gels, one (Duarte) is associated with diminished activity, while the other (LA) is associated with greater than normal activity. Therefore, although the concordance rates between N314D and both of these phenotypes are compelling, the question remains as to whether N314D alone is sufficient to cause either or both variants. To address the question of precisely what properties of variant GALT can be attributed to the N314D substitution alone, we have modeled both the wildtype and N314D-GALT alleles in a previously defined yeast expression system, and characterized each with respect to activity, abundance, subunit interaction, and mobility on isoelectric focusing gels. Our results indicate that the N314D subunit dimerizes well both with wildtype GALT and with itself and that the N314D substitution is sufficient to confer the expected shift of IEF banding pattern associated with both the Duarte and LA variant proteins isolated from human cells. However, our results also suggest that N314D-GALT retains full specific activity, thereby calling into question the suggestion that N314D encodes the Duarte variant of GALT.

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.

Identification and functional analysis of three distinct mutations in the human galactose-1-phosphate uridyltransferase gene associated with galactosemia in a single family

We have identified three mutations associated with transferase-deficiency galactosemia in a three-generation family including affected members in two generations and have modeled all three mutations in a yeast-expression system. A sequence of pedigree, biochemical, and molecular analyses of the galactose-1-phosphate uridyltransferase (GALT) enzyme and genetic locus in both affected and carrier individuals revealed three distinct base substitutions in this family, two (Q188R and S135L) that had been reported previously and one (V151A) that was novel. Biochemical analyses of red-blood-cell lysates from the relevant family members suggested that each of these mutations was associated with dramatic impairment of GALT activity in these cells. While this observation was consistent with our previous findings concerning the Q188R mutation expressed both in humans and in a yeast-model system, it was at odds with a report by Reichardt and colleagues, indicating that in their COS cell-expression system the S135L substitution behaved as a neural polymorphism. To address this apparent paradox, as well as to investigate the functional significance of the newly identified V151A substitution, all three mutations were recreated by site-directed mutagenesis of the otherwise wild-type human GALT sequence and were expressed both individually and in the appropriate allelic combinations in a GALT-deficient strain of the yeast Saccharomyces cerevisiae.(ABSTRACT TRUNCATED AT 250 WORDS)

Serum-responsive expression from the murine thymidine kinase promoter is specifically disrupted in a transformed cell line

Thymidine kinase (TK) gene expression is controlled in normal cells at both the transcriptional and posttranscriptional levels. Together, these regulatory systems mediate the 20-50-fold induction of TK mRNA observed as cells traverse the G1-S boundary of the cell cycle. Previously, we have reported that a "Yi" protein complex was observed to bind the mouse TK promoter in a cell cycle-dependent manner in nontransformed cells (Q-P. Dou, J. L. Fridovich-Keil, and A. B. Pardee, Proc. Natl. Acad. Sci. USA, 88: 1157-1161, 1991) and bound constitutively in transformed cells (D. W. Bradley, Q-P. Dou, J. L. Fridovich-Keil, and A. B. Pardee, Proc. Natl. Acad. Sci. USA, 87: 9310-9314, 1990). Nonetheless, TK mRNA levels in these cells continue to exhibit a marked S-specific induction (> 10 fold), raising the question: what is the status of TK promoter-mediated, as opposed to posttranscriptional, gene regulation in these transformed cells? To address this question, we have used cell synchrony experiments involving both transformed and nontransformed cells stably transfected with a TK promoter-beta-globin reporter gene construct. We have found that, in marked contrast to the tight regulation of reporter gene expression observed in nontransformed cells (J. L. Fridovich-Keil, J. M. Gudas, Q-P. Dou, I. Bouvard, and A. B. Pardee, Cell Growth & Differ., 2: 67-76, 1991), reporter gene expression in the transformed cells is constitutive and, therefore, closely parallels the presence of Yi DNA-binding activity. These data are fully consistent with other recently published observations concerning differential controls of TK transcriptional and posttranscriptional regulation (J. M. Gudas, J. L. Fridovich-Keil, and A. B. Pardee, Cell Growth & Regul., 4: 421-430, 1993) and support the hypothesis that, in transformed cells, endogenous TK is regulated predominantly at the posttranscriptional level.

DNA sequences required for serum-responsive regulation of expression from the mouse thymidine kinase promoter

We have used site-specific mutagenesis and thymidine kinase (TK) promoter/reporter gene transfection experiments to investigate DNA sequences required for serum-responsive regulation of expression from the mouse thymidine kinase promoter. Mutations were targeted to each of three previously described protein binding domains (MT1, MT2, and MT3) upstream of the TK translation initiation site, as well as to sequences within the TK first exon in order to address each of the following three questions: (a) Do these sequences play any role in regulation? (b) Do all of these sites play the same role? and (c) If any controls are observed, do they act positively or negatively on gene expression? The results of these experiments indicated that, in the wild-type TK promoter, at least some of these sequences do play a role in regulation, that not all of these sites appear to play the same role, and that some of the targeted elements act positively on gene expression, whereas others appear to act negatively. In particular, mutagenesis of the Sp1 site within MT1 virtually eliminated promoter function, whereas mutations in either the MT2 site or the TK first exon rendered reporter gene expression nearly constitutive with respect to serum. Thus, both MT2 and sequences within the TK first exon appear to contain negatively acting elements. In contrast, mutation or deletion of the MT3 site produced a much less pronounced effect on reporter gene regulation. These results support recent observations from our laboratory (Q-P. Dou et al., manuscript in preparation) indicating that although the protein complexes that bind to these various sites are similar, they are not identical.(ABSTRACT TRUNCATED AT 250 WORDS)

Posttranscriptional control of thymidine kinase messenger RNA accumulation in cells released from G0-G1 phase blocks

In this study, we have utilized thymidine kinase (TK) mRNA induction as a model for investigating regulatory events at the G1-S boundary of the cell cycle. Using three independent methods for synchronizing diploid, nontumorigenic CHEF/18 cells, we found that the mechanism(s) underlying TK mRNA accumulation varied with the method of cell synchrony used. When cells were arrested by serum deprivation, both transcriptional and posttranscriptional controls contributed to the observed accumulation of TK mRNA at the G1-S boundary. When synchronized by isoleucine deprivation, mature TK mRNA and TK pre-mRNAs increased significantly at the G1-S boundary of the cell cycle with no detectable change in the rate of TK gene transcription. Following lovastatin treatment, which appears to arrest cells at a point very early in G1, posttranscriptional mechanisms were solely responsible for the subsequent accumulation of TK mRNA observed upon mevalonate repletion. We confirmed that transcriptional mechanisms were involved in TK mRNA regulation only when cells progressed from G0 into S phase using reporter genes transcribed from the heterologous human TK promoter. Taken together, these results indicate that posttranscriptional mechanism(s) are primarily responsible for regulating the abundance of TK mRNA during the cell cycle in CHEF/18 cells and further suggest uncoupling of transcriptional and posttranscriptional controls following different physiological conditions of cell cycle arrest.

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.

Down-regulation of tumor necrosis factor expression by pentoxifylline in cancer patients: a pilot study

The wasting syndrome (cachexia) characterized by anorexia, malaise, and weight loss is observed in many patients with cancer or chronic infection. The excessive levels of tumor necrosis factor-alpha (TNF)/cachectin reported in 50% of cancer patients exhibiting clinically active disease may therefore mediate, at least in part, the cachexia associated with malignancy. Pentoxifylline, a substituted methylxanthine approved for treatment of intermittent claudication, has been shown in preclinical studies to down-regulate TNF RNA expression as well as TNF activity. We report that pentoxifylline suppressed TNF RNA levels on all three occasions in patients with initially elevated levels of TNF RNA. Pentoxifylline did not suppress TNF RNA to subnormal levels in all five patients with initially normal TNF RNA levels. Four patients reported an increased sense of well-being, improved appetite and ability to perform the activities of daily living. Two of these five patients with normal TNF levels each had a weight gain of more than 5% after 3 weeks of pentoxifylline therapy suggesting that, although TNF may be important in the pathogenesis of cancer cachexia, other anorexia-producing cytokines that are potentially affected by pentoxifylline may also be involved. No severe adverse effects were observed. Taken together these findings suggest that pentoxifylline can down-regulate TNF expression and improve the sense of well-being in cancer patients. A larger study with a randomized, double-blind, placebo-controlled design and more sophisticated estimates of quality of life will be needed to confirm these observations.

The human galactose-1-phosphate uridyltransferase gene

Classical galactosemia is an inborn error of metabolism caused by a deficiency of galactose-1-phosphate uridyltransferase (GALT). Standard treatment with dietary galactose restriction will reverse the potentially lethal symptoms of the disease that are manifest in the newborn period. However, the long-term prognosis for these patients is variable. As a first step toward investigating the molecular basis for phenotypic variation in galactosemia, we have cloned and sequenced the entire gene for human galactose-1-phosphate uridyltransferase. This gene is organized into 11 exons spanning 4 kb. In exons 6, 9, and a portion of 10, there is a high degree of amino acid sequence conservation among Escherichia coli, yeast, mouse, and human. We have identified a number of nucleotide changes in the GALT genes of galactosemic patients that alter conserved amino acids. The most common of these is an A to G transition at nucleotide position 1470, converting a glutamine to an arginine at amino acid codon position 188 (Q188R).(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the murine thymidine kinase-encoding gene and analysis of transcription start point heterogeneity

We have determined the molecular organization and transcription start points (tsp) for the murine gene (TK) encoding thymidine kinase. The exon/intron structure and sequences present at the splice junctions of the mammalian TK genes have been highly conserved; however, the promoter sequences of these genes have diverged widely. Both the human and Chinese hamster TK promoter regions contain CCAAT and TATA consensus motifs, whereas the mouse promoter has neither element. This difference between species is reflected in that, unlike the hamster and human TK genes, transcription initiates from numerous specific tsp within a 100-bp region in the mouse TK gene. The complex pattern of tsp seen in the endogenous gene was not maintained in transfected cell lines containing TK promoter::beta-globin (HBB) fusions. Transcription from the murine TK:HBB fusion genes initiated from a small number of tsp that were clustered downstream from the ATG in hybrids containing TK coding sequences, and in the HBB 5' UTR in hybrids that did not. Few or no specific tsp were detected from the upstream sites used in the endogenous mouse TK gene.

Improved expression vectors for eukaryotic promoter/enhancer studies

We describe two transfectable vectors designed to facilitate the functional analysis of eukaryotic promoter/enhancer sequences. The first, pJFCAT1, is an improved chloramphenicol acetyltransferase (CAT) reporter gene expression vector with two features that distinguish it from the majority of other CAT vectors currently in use: 1) it carries a trimer cassette of the simian virus 40 major late polyadenylation site to block plasmid-initiated read-through expression of CAT, and 2) it includes the phage f1 origin of replication, permitting generation of single-stranded copies to serve as templates for oligonucleotide-directed mutagenesis or single-strand DNA sequencing. The promoterless pJFCAT1 directs little if any CAT activity in transfected mouse L cells and, therefore, may be particularly useful for the analysis of weak promoters whose activity is otherwise masked by background CAT expression. The second vector, pTAG-1, uses human beta-globin as a reporter gene and was designed to facilitate the analysis of reporter gene expression at the RNA level. Like pJFCAT1, pTAG-1 also includes the simian virus 40 polyadenylation site trimer cassette located just upstream of the promoter insertion site. We have used each of these vectors to study functional elements in the human and mouse thymidine kinase promoters.

Growth-responsive expression from the murine thymidine kinase promoter: genetic analysis of DNA sequences

As a first step toward elucidating the biochemical basis of gene regulation at the G1-S boundary of the cell cycle, we have identified regions of the murine thymidine kinase (TK) promoter sufficient to confer appropriately growth-responsive expression to a heterologous gene. Using a series of TK promoter-chloramphenicol acetyltransferase (CAT) gene fusion constructs, we have identified sequences located between -174 base pairs upstream and +159 base pairs downstream of the TK translation initiation site that are sufficient to drive efficient S phase-specific expression of the CAT reporter gene in transfected murine fibroblasts. Both deletion analysis and site-specific mutagenesis experiments indicated that an Sp1 consensus binding site is critical to the activity of this promoter. Synchronized populations of BALB/c 3T3 cells stably transfected with either TK promoter-CAT fusion constructs or TK promoter-beta-globin fusion constructs expressed their respective reporter genes in an S phase-specific manner following serum stimulation. In each case, reporter gene expression was reduced during quiescence and G1 and rose upon entry of cells into S phase. The TK sequences included in these constructs therefore contained information sufficient to confer S phase-specific regulation to these two reporter genes. These results set the stage for a more detailed analysis of the sequences and trans-acting factors responsible for regulating murine TK gene expression and may lead to insights into the control of proliferation in normal and transformed cells.

Progression through the cell cycle: an overview

Tissues in adults can be maintained at constant mass or they can increase or decrease in size because of imbalances of synthetic and degradative processes acting at the cellular and molecular levels. Some size changes are caused by physiologic conditions to which the tissue must adjust. Alternatively, the balance may be distorted in favor of net tissue increase in pathologic situations such as cancer. Strict regulatory mechanisms are required to keep proliferation responsive to the organism's needs; these mechanisms may be defective in disease. Net tissue proliferation requires repeated rounds of cell duplication in excess of that necessary to counterbalance cell death. Duplication of a cell requires a net doubling of its every molecule and structure. The myriad of molecular events required for cell proliferation such as DNA duplication and its partitioning at mitosis are tightly regulated in normal cells. One may conceive of two classes of molecules: those required for "housekeeping," which constitute the cell's structural and functional machinery, and those such as growth factors, their receptors, and second messengers involved in signal transduction responsible for regulating the activities of the housekeeping molecules. These molecular events and the cascade of processes that control them can be organized within the sequence of the cell cycle. In this brief overview, we illustrate these issues with a few examples taken from very recent discoveries of novel proteins that appear to have major regulatory roles. Most of these results have been obtained with mammalian fibroblasts, but some have originated with discoveries made using two very different yeasts.(ABSTRACT TRUNCATED AT 250 WORDS)

Transformed and nontransformed cells differ in stability and cell cycle regulation of a binding activity to the murine thymidine kinase promoter

A DNA binding activity to an upstream region of the murine thymidine kinase gene is regulated differently in a transformed and nontransformed cell line pair. Differences in regulation were observed (i) after serum levels were reduced, (ii) when serum levels were returned to initial high levels, and (iii) while protein synthesis was inhibited. After reduction of serum levels, the binding activity was unstable in nontransformed BALB/c 3T3 clone A31 cells but was significantly more stable in benzo[a]pyrene-transformed BALB/c 3T3 cells. After serum concentration was returned to high levels, the kinetic pattern of the binding activity differed between nontransformed and transformed cells. While protein synthesis was inhibited, the binding activity was unstable in nontransformed cells and stable in transformed cells. Partial inhibition of protein synthesis--a more stringent condition to test instability--prevented the induction of the binding activity in nontransformed cells. Previously, the labile protein hypothesis set forth the criterion that a protein regulating the onset of DNA synthesis should be unstable in nontransformed cells and stable in transformed cells. The DNA binding activity described here satisfies this criterion.

Domains of beta-tubulin essential for conserved functions in vivo

The relationship between the primary sequence of tubulins and their properties in cells was studied by gene transfection experiments. Previously, we studied a chimeric beta-tubulin formed from chicken beta-tubulin-2 sequences in the amino-terminal portion and the highly divergent Saccharomyces cerevisiae TUB2 sequences in the carboxy-terminal 25% of the molecule. In the cytoplasm of cultured animal cells, this protein incorporates into all microtubule structures and assembles with the same efficiency as endogenous tubulin. We show that the protein products of chimeric genes with an increasing proportion of yeast sequence, extending 5' of the carboxy-terminal 25%, are abnormal in two ways. First, they assemble with a significantly lower efficiency than the original chimeric protein or the endogenous tubulins. Second, they are less stable in the cytoplasm. The results suggest that the position of the yeast sequences is crucial in determining the properties of the molecule. Results of analyses of 1 deletion mutation and 10 linker insertions in the original chimeric tubulin suggest that those changes made outside the carboxyl terminus completely disrupt assembly activity, while those made in the carboxyl terminus do not.

A chicken-yeast chimeric beta-tubulin protein is incorporated into mouse microtubules in vivo

The role of divergent primary sequences in restricting tubulin function was tested in vivo by a gene transfection experiment. A chicken-yeast chimeric beta-tubulin DNA was introduced into 3T3 cells using the transfection vector pSV2. The 5' end of this gene, from chicken, is similar but not identical with that of mouse beta-tubulins; the 3' end, from yeast, contains a carboxyl terminus that is very different from other known beta-tubulin sequences. The chimeric protein is incorporated efficiently into each of the microtubule structures and each of the microtubules in the host cells. The presence of the protein has no apparent effect on either growth rate or cell morphology. The results suggest that the divergent sequences in this chimeric tubulin molecule place no restrictions on its activities in mouse cells.