Impact of patient mutations on heterodimer formation and function in human galactose-1-P uridylyltransferase

Assessment of galactose-1-phosphate uridyltransferase activity in cells and tissues

Galactosemias are a family of autosomal recessive genetic disorders resulting from impaired enzymes of the Leloir pathway of galactose metabolism including galactokinase, galactose uridyltransferase, and UDP-galactose 4-epimerase that are critical for conversion of galactose into glucose-6-phosphate. To better understand pathophysiological mechanisms involved in galactosemia and develop novel therapies to address the unmet need in patients, it is important to develop reliable assays to measure the activity of the Leloir pathway enzymes. Here we describe in-depth methods for indirectly measuring galacose-1-phosphate uridyltransferase activity in cell culture and animal tissues.

Pathophysiology and targets for treatment in hereditary galactosemia: A systematic review of animal and cellular models

Since the first description of galactosemia in 1908 and despite decades of research, the pathophysiology is complex and not yet fully elucidated. Galactosemia is an inborn error of carbohydrate metabolism caused by deficient activity of any of the galactose metabolising enzymes. The current standard of care, a galactose-restricted diet, fails to prevent long-term complications. Studies in cellular and animal models in the past decades have led to an enormous progress and advancement of knowledge. Summarising current evidence in the pathophysiology underlying hereditary galactosemia may contribute to the identification of treatment targets for alternative therapies that may successfully prevent long-term complications. A systematic review of cellular and animal studies reporting on disease complications (clinical signs and/or biochemical findings) and/or treatment targets in hereditary galactosemia was performed. PubMed/MEDLINE, EMBASE, and Web of Science were searched, 46 original articles were included. Results revealed that Gal-1-P is not the sole pathophysiological agent responsible for the phenotype observed in galactosemia. Other currently described contributing factors include accumulation of galactose metabolites, uridine diphosphate (UDP)-hexose alterations and subsequent impaired glycosylation, endoplasmic reticulum (ER) stress, altered signalling pathways, and oxidative stress. galactokinase (GALK) inhibitors, UDP-glucose pyrophosphorylase (UGP) up-regulation, uridine supplementation, ER stress reducers, antioxidants and pharmacological chaperones have been studied, showing rescue of biochemical and/or clinical symptoms in galactosemia. Promising co-adjuvant therapies include antioxidant therapy and UGP up-regulation. This systematic review provides an overview of the scattered information resulting from animal and cellular studies performed in the past decades, summarising the complex pathophysiological mechanisms underlying hereditary galactosemia and providing insights on potential treatment targets.

Endogenous galactose formation in galactose-1-phosphate uridyltransferase deficiency

Patients with classical galactosaemia (galactose-1-phosphate uridyltransferase (GALT) deficiency) manifest clinical complications despite strict dietary galactose restriction. Therefore the significance of endogenous galactose production has been assessed. Previous in vivo studies primarily focused on patients homozygous for the most common genetic variant Q188R but little is known about other genetic variants. In the present study the endogenous galactose release in a group of non-Q188R homozygous galactosaemic patients (n = 17; 4-34 years) exhibiting comparably low residual GALT activity in red blood cells was investigated. Primed continuous infusion studies with D-[1-(13)C]galactose as substrate were conducted under post-absorptive conditions and in good metabolic control. The results demonstrate that all patients exhibiting residual GALT activity of <1.5% of control showed a comparable pathological pattern of increased endogenous galactose release irrespective of the underlying genetic variations. Possible implications of the findings towards a more differentiated dietary regimen in galactosaemia are discussed.

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.

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.

A Drosophila melanogaster model of classic galactosemia

Classic galactosemia is a potentially lethal disorder that results from profound impairment of galactose-1-phosphate uridylyltransferase (GALT). Despite decades of research, the underlying pathophysiology of classic galactosemia remains unclear, in part owing to the lack of an appropriate animal model. Here, we report the establishment of a Drosophila melanogaster model of classic galactosemia; this is the first whole-animal genetic model to mimic aspects of the patient phenotype. Analogous to humans, GALT-deficient D. melanogaster survive under conditions of galactose restriction, but accumulate elevated levels of galactose-1-phosphate and succumb during larval development following galactose exposure. As in patients, the potentially lethal damage is reversible if dietary galactose restriction is initiated early in life. GALT-deficient Drosophila also exhibit locomotor complications despite dietary galactose restriction, and both the acute and long-term complications can be rescued by transgenic expression of human GALT. Using this new Drosophila model, we have begun to dissect the timing, extent and mechanism(s) of galactose sensitivity in the absence of GALT activity.

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.

Wild-type Cu/Zn superoxide dismutase (SOD1) does not facilitate, but impedes the formation of protein aggregates of amyotrophic lateral sclerosis causing mutant SOD1

Aggregation of Cu/Zn superoxide dismutase (SOD1) is a hallmark of a subset of familial amyotrophic lateral sclerosis (ALS) cases. The expression of wild-type SOD1 [SOD(hWT)] surprisingly exacerbates the phenotype of mutant SOD1 in vivo. Here we studied whether SOD1(hWT) may affect mutant SOD1 aggregation by employing fluorescence microscopy techniques combined with lifetime-based Förster resonance energy transfer (FRET). Only a very minor fraction of SOD1(hWT) was observed in aggregates induced by mutant SOD1(G37R), SOD1(G85R) or SOD1(G93C). Quite in contrast, co-expression of SOD(hWT) reduced the amount of mutant SOD1 in the aggregate fraction. Furthermore, we did not detect endogenous mouse SOD1 in aggregates formed by mutant SOD1 in two distinct mutant SOD1 mouse lines. The hypothesis that SOD1(WT) is able to keep mutant SOD1 variants in a soluble state is supported by the increased presence of heterodimers upon SOD1(hWT) co-expression. Therefore we propose that SOD1(WT) contributes to disease by heterodimerization with mutant SOD1 forms.

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.

Homology modeling studies on human galactose-1-phosphate uridylyltransferase and on its galactosemia-related mutant Q188R provide an explanation of molecular effects of the mutation on homo- and heterodimers

We have created theoretical models of the three-dimensional dimeric structure of human galactose-1-phosphate uridylyltransferase as well as of homo- and heterodimers carrying the Q188R mutation by using comparative modeling procedures. These mutants are associated to the most frequent form of the genetic disease galactosemia. We have analyzed the impact of this mutation both on enzyme-substrate interactions as well as on interchain interactions in the heterodimers and in the homodimer. We suggest a molecular explanation for the altered function, caused by different enzyme-substrate interactions, and for the partial dominant negative effect of the mutant allele that is present in heterozygotes for this gene, related to a substantial loss of interchain hydrogen bonds. These results can be considered a starting point for a more extensive characterization at the molecular level of the other mutations linked to this genetic disease.