A molecular approach to galactosemia

Heterogeneity of disease-causing variants in the Swedish galactosemia population: Identification of 16 novel GALT variants

The aim was to determine disease-causing variants in the GALT gene which codes for the enzyme galactose-1-phosphate uridylyltransferase. Loss of activity of this enzyme causes classical galactosemia-a life threatening, treatable disorder, included in the Swedish newborn screening program since 1967. A total of 66 patients with the disease are known in Sweden and 56 index patients were investigated. An additional two patients with Duarte galactosemia were included. The disease-causing variants were identified in all patients. As reported from other countries only a few variants frequently recur in severe disease. The two variants p.(Gln188Arg) (c.563A>G) and p.(Met142Lys) (c.425T>A) are present in several index patients whereas the remaining are found in one to three patients each. The most common variant, p.(Gln188Arg), has an allele frequency of 51% in the cohort. A total of 16 novel variants were found among the 33 different variants in the cohort. Two of these are synonymous variants affecting splicing, demonstrating the importance of the evaluation of synonymous variants at the cDNA level. Concise sentence: Galactosemia is a rare disease in Sweden and the disease-causing variants are heterogenous including two synonymous variants.

Pilot study of classic galactosemia: Neurodevelopmental impact and other complications urge neonatal screening in Egypt

Classic galactosemia is caused by deficiency of galactose-1-phosphate uridylyltransferase (GALT). It causes serious morbidity and mortality if left untreated. Screening for galactosemia is not included in Egyptian neonatal screening program. The study aimed to define clinical presentation and complications of galactosemia at Pediatric Hepatology Clinic, Cairo University, Egypt. Thus, the clinical presentation, course and outcome of 37 children with documented galactosemia was studied. Jaundice was the main presentation (67.6%). Other presentations included; convulsions (29.7%), motor retardation (24.3%), mental retardation (5.4%), microcephaly (5.4%), failure to thrive (16.2%), hepatomegaly (62.2%), splenomegaly (35.1%), vomiting (16.2%), diarrhea (8.1%), liver cell failure (10.8%), renal tubular acidosis (5.4%), cataract (5.4%), autoimmune hepatitis (2.7%), self-mutilation (2.7%), combined immune deficiency (2.7%) and kernicterus (2.7%). There was no correlation of residual enzyme activity to severity, clinical presentation, liver function tests, liver biopsy findings or outcome apart from highly significant correlation with repeated chest infections (P = 0.001). Duration to diagnosis and exposure to galactose in diet correlated with liver pathology severity i.e. hepatocyte necrosis (P = 0.003) and cytoskeleton damage (P = 0.003), but not to outcome. Galactosemia should be suspected in any child with liver, neurologic disease and/or immunodeficiency. Its complications are potentially preventable. Early detection is mandatory to prevent serious morbidity and mortality. Initiation of neonatal screening for galactosemia in Egypt is recommended.

Sweet and sour: an update on classic galactosemia

Classic galactosemia is a rare inherited disorder of galactose metabolism caused by deficient activity of galactose-1-phosphate uridylyltransferase (GALT), the second enzyme of the Leloir pathway. It presents in the newborn period as a life-threatening disease, whose clinical picture can be resolved by a galactose-restricted diet. The dietary treatment proves, however, insufficient in preventing severe long-term complications, such as cognitive, social and reproductive impairments. Classic galactosemia represents a heavy burden on patients' and their families' lives. After its first description in 1908 and despite intense research in the past century, the exact pathogenic mechanisms underlying galactosemia are still not fully understood. Recently, new important insights on molecular and cellular aspects of galactosemia have been gained, and should open new avenues for the development of novel therapeutic strategies. Moreover, an international galactosemia network has been established, which shall act as a platform for expertise and research in galactosemia. Herein are reviewed some of the latest developments in clinical practice and research findings on classic galactosemia, an enigmatic disorder with many unanswered questions warranting dedicated research.

Genetic Candidate Variants in Two Multigenerational Families with Childhood Apraxia of Speech

Childhood apraxia of speech (CAS) is a severe and socially debilitating form of speech sound disorder with suspected genetic involvement, but the genetic etiology is not yet well understood. Very few known or putative causal genes have been identified to date, e.g., FOXP2 and BCL11A. Building a knowledge base of the genetic etiology of CAS will make it possible to identify infants at genetic risk and motivate the development of effective very early intervention programs. We investigated the genetic etiology of CAS in two large multigenerational families with familial CAS. Complementary genomic methods included Markov chain Monte Carlo linkage analysis, copy-number analysis, identity-by-descent sharing, and exome sequencing with variant filtering. No overlaps in regions with positive evidence of linkage between the two families were found. In one family, linkage analysis detected two chromosomal regions of interest, 5p15.1-p14.1, and 17p13.1-q11.1, inherited separately from the two founders. Single-point linkage analysis of selected variants identified CDH18 as a primary gene of interest and additionally, MYO10, NIPBL, GLP2R, NCOR1, FLCN, SMCR8, NEK8, and ANKRD12, possibly with additive effects. Linkage analysis in the second family detected five regions with LOD scores approaching the highest values possible in the family. A gene of interest was C4orf21 (ZGRF1) on 4q25-q28.2. Evidence for previously described causal copy-number variations and validated or suspected genes was not found. Results are consistent with a heterogeneous CAS etiology, as is expected in many neurogenic disorders. Future studies will investigate genome variants in these and other families with CAS.

Functional and structural impact of the most prevalent missense mutations in classic galactosemia

Galactose-1-phosphate uridylyltransferase (GALT) is a key enzyme in galactose metabolism, particularly important in the neonatal period due to ingestion of galactose-containing milk. GALT deficiency results in the genetic disorder classic galactosemia, whose pathophysiology is still not fully elucidated. Whereas classic galactosemia has been hypothesized to result from GALT misfolding, a thorough functional-structural characterization of GALT most prevalent variants was still lacking, hampering the development of alternative therapeutic approaches. The aim of this study was to investigate the structural-functional effects of nine GALT mutations, four of which account for the vast majority of the mutations identified in galactosemic patients. Several methodologies were employed to evaluate the mutations' impact on GALT function, on the protein secondary and tertiary structures, and on the aggregation propensity. The major structural effect concerns disturbed propensity for aggregation, particularly striking for the p.Q188R variant, resulting from the most frequent (∼60%) allele at a worldwide scale. The absence of major effects at the secondary and tertiary structure levels suggests that the disturbed aggregation results from subtle perturbations causing a higher and/or longer exposure of hydrophobic residues in the variants as compared to WT GALT. The results herein described indicate a possible benefit from introducing proteostasis regulators and/or chemical/pharmacological chaperones to prevent the accumulation of protein aggregates, in new avenues of therapeutic research for classic galactosemia.

Deficits in sequential processing manifest in motor and linguistic tasks in a multigenerational family with childhood apraxia of speech

The purpose of this study was to evaluate a global deficit in sequential processing as candidate endophenotypein a family with familial childhood apraxia of speech (CAS). Of 10 adults and 13 children in a three-generational family with speech sound disorder (SSD) consistent with CAS, 3 adults and 6 children had past or present SSD diagnoses. Two preschoolers with unremediated CAS showed a high number of sequencing errors during single-word production. Performance on tasks with high sequential processing loads differentiated between the affected and unaffected family members, whereas there were no group differences in tasks with low processing loads. Adults with a history of SSD produced more sequencing errors during nonword and multisyllabic real word imitation, compared to those without such a history. Results are consistent with a global deficit in sequential processing that influences speech development as well as cognitive and linguistic processing.

Frequency distribution of Q188R, N314D, Duarte 1, and Duarte 2 GALT variant alleles in an Indian galactosemia population

Classical galactosemia is a genetic disorder caused by mutations in the galactose-1-phosphate uridyltransferase (GALT) gene. The Q188R and N314D mutations are the most frequently cited GALT gene mutations. N314D is further associated with two variants, Duarte 1 and Duarte 2. Nevertheless, no reports are available on the clinical and molecular spectrum of galactosemia from the Indian population. The present study was designed to establish the frequency of these two most common mutations and their variants in Indian galactosemia patients so as to determine a single most common mutation/polymorphism for establishing the DNA-based diagnosis of galactosemia. Three alleles were found to be present at a frequency of 0.036 (Q188R), 0.40 (N314D), and 0.39 (D2); no D1 alleles were found. A significantly higher frequency of the Duarte 2 allele in our population suggests the presence of a milder form of galactosemia, which can be well managed by early diagnosis and dietary management.

A case of classical galactosemia: identification and characterization of 3 distinct mutations in galactose-1-phosphate uridyl transferase (GALT) gene in a single family

Galactosemia is an autosomal recessive disorder of galactose metabolism. In the very first instance of its kind from India, the authors report the presence of three different galatose-1-phosphate uridyl transferase (GALT) gene mutations, associated with galactosemia, in a single Indian family. One of the three mutations, S307X, is a novel mutation (GenBank Accession number GQ355273) and is of nonsense nature causing the truncation of the GALT protein resulting in the decreased enzyme activity. The authors have also emphasized the importance of introduction of new born screening program for galactosemia and its genetic analysis in select settings across the country.

Prevalence and phenotype of childhood apraxia of speech in youth with galactosemia

PURPOSE

In this article, the authors address the hypothesis that the severe and persistent speech disorder reported in persons with galactosemia meets contemporary diagnostic criteria for childhood apraxia of speech (CAS). A positive finding for CAS in this rare metabolic disorder has the potential to impact treatment of persons with galactosemia and inform explanatory perspectives on CAS in neurological, neurodevelopmental, and idiopathic contexts.

METHOD

Thirty-three youth with galactosemia and significant prior or persistent speech sound disorder were assessed in their homes in 17 states. Participants completed a protocol yielding information on their cognitive, structural, sensorimotor, language, speech, prosody, and voice status and function.

RESULTS

Eight of the 33 participants (24%) met contemporary diagnostic criteria for CAS. Two participants, 1 of whom was among the 8 with CAS, met criteria for ataxic or hyperkinetic dysarthria. Groupwise findings for the remaining 24 participants are consistent with a classification category termed Motor Speech Disorder-Not Otherwise Specified (Shriberg, Fourakis et al., 2010a).

CONCLUSION

The authors estimate the prevalence of CAS in galactosemia at 18 per hundred-180 times the estimated risk for idiopathic CAS. Findings support the need to study risk factors for the high occurrence of motor speech disorders in galactosemia despite early compliant dietary management.

The GALT rush: high carrier frequency of an unusual deletion mutation of the GALT gene in the Ashkenazi population

Classic galactosemia is an autosomal recessive disorder of galactose metabolism manifesting in the first weeks of life following exposure to a milk-based diet. Despite the benefit of avoidance of lactose, many patients suffer from long-term complications including neurological deficits and ovarian failure. To date, over 230 mutations have been described in the GALT gene resulting in galactosemia. Recently, an unusual mutation was characterized causing a 5.5 kb deletion, with a relatively high carrier rate in subjects of Ashkenazi Jewish (AJ) descent. The aim of this study was to estimate the carrier frequency of this mutation in the AJ population in Israel. For this purpose we developed a high-throughput methodology to genotype both normal and deleted alleles using a chip-based matrix-assisted laser desorption-time-of-flight (MALDI-TOF) mass spectrometer and Multiplex PCR. DNA samples of 760 anonymous AJ subjects were submitted for analysis, subsequently detecting six individuals heterozygous for the GALT deletion mutation, giving a carrier frequency of 1 in 127 (0.79%). Based on these results, we suggest that the method described here provides a basis for genetic screening and prenatal counseling and can potentially reduce the morbidity and mortality associated with delayed diagnosis of galactosemia in this patient population.

Longitudinal assessment of intellectual achievement in patients with classical galactosemia

OBJECTIVE

To conduct a longitudinal assessment of long-term cognitive outcome in patients with classical galactosemia.

METHODS

Inclusion criteria were (1) previous assessment of IQ dating back >10 years with tests being comparable with the recent German tests HAWIK-III and HAWIE-R, (2) absence of illnesses other than galactosemia, (3) absence of foreign language problems, (4) enzymatic-metabolic proof of classical galactosemia, (5) compliance with dietary therapy, and (6) written informed consent. Twenty-three patients who fulfilled these criteria were found. They underwent the first IQ test at a mean age of 11 +/- 5 years and the second 13.6 to 15.5 years later at a mean age of 26 +/- 5 years. Results were corrected for the obsolescence of test norms (Flynn effect).

RESULTS

Mean total IQ scores on the first and second tests were 78 +/- 14 and 73 +/- 15, respectively, and not significantly different. IQ scores in the average range were observed for 7 patients on the first test and for 5 patients on the second test. For 17 patients, the intraindividual IQ scores remained essentially unchanged. Five patients showed a decrease and 1 an increase of the IQ score over time. No consistent pattern of change was found with respect to performance or verbal IQ subscores or in achievements in the individual subtest.

CONCLUSIONS

The results confirm the presence of reduced cognitive ability in classical galactosemia and present evidence for an absence of substantial galactosemia-induced aggravation of this impairment with increasing age, at least in patients from 4 to 40 years of age. It remains to be clarified whether a reduction of cognitive function in galactosemia may be initiated by an in utero toxicity of endogenously formed galactose and which role such a process may play in the development of intellectual deficiencies that are later maintained throughout life.

Outcomes of siblings with classical galactosemia

OBJECTIVES

To determine the long-term outcome of dietary intervention in siblings from 14 Irish families with classical galactosemia (McKusick 230400), an autosomal recessive disorder of carbohydrate metabolism and galactose-1-phosphate uridyltransferase (GALT) deficiency.

STUDY DESIGN

Outcomes in siblings on dietary galactose restriction were studied to evaluate whether birth order (ie, time of commencement of diet) and compliance with lactose-restricted diet (galactose intake > or < 20 mg /day), assessed by dietary recall and biochemical monitoring of galactose-1-phosphate [Gal-1-P] and galactitol values, affected outcomes. The outcome variables assessed were IQ, speech, and language assessment scores, neurologic examination results, and magnetic resonance imaging (MRI) of the brain.

RESULTS

There was a high incidence of complications in the overall group, particularly speech and language delay (77%) and low IQ (71%). There was no significant difference in outcome between earlier-treated and later-treated siblings or any correlation with mean Gal-1-P or galactitol values. In most cases, cerebral white matter disease was evident on MRI scanning, with evidence of progressive cerebellar degeneration seen in 2 highly compliant families.

CONCLUSION

The subjects with a higher galactose intake did not exhibit an increased incidence of complications; conversely, those who were very compliant with dietary restrictions did not have more favorable outcomes.

An updated review of the long-term neurological effects of galactosemia

Classical galactosemia is an autosomal recessive condition in which there is near total absence of the activity of galactose-1-phosphate uridyltransferase. Patients with this condition have substantial motor, cognitive, and psychiatric impairments despite dietary treatment. A characteristic pattern of biochemical abnormalities is observed in patients with this disorder. Galactose-1-phosphate, the substrate of galactose-1-phosphate uridyltransferase, accumulates within cells, and surplus galactose is reduced to galactitol or oxidized to galactonate. Using sophisticated mass spectrometry, these compounds as well as free galactose can be measured in plasma and in urine. It is clear that initiation of dietary restriction of galactose in the newborn period produces reversal of hepatic, renal, brain, and immune dysfunction, along with reduction of the accumulated galactose metabolites. However, the neurologist should be aware that chronic and progressive neurologic impairments occur even in patients spared these neonatal symptoms. The purpose of this review is to summarize current information about neurologic complications of galactosemia and what is known, and still unknown, about its pathophysiology.

Galactose-1-phosphate uridyl transferase gene mutations in women with premature ovarian failure

We determined the frequency of galactose-1-phosphate uridyl transferase gene mutations: Q188R, K285N, and the Duarte allelle in 86 patients with idiopathic premature ovarian failure (POF) and 95 controls. No association of the mutations with POF was found.

Cognitive Strengths and Weaknesses in Children and Adolescents Homozygous for the Galactosemia Q188R Mutation: A Descriptive Study

Children and adolescents (n = 25) with galactosemia homozygous for the common Q188R mutation (substitution of glutamine codon 188 with arginine) were group matched for sex and age with healthy control participants (n = 20). Participants were administered an abbreviated neuropsychological battery by a doctoral-level psychologist. Results indicate that children and adolescents with galactosemia function generally within the low average IQ range, with a small standard deviation (indicating a relatively homogeneous IQ profile), and have many features suggestive of left-hemisphere dysfunction. Word retrieval difficulties are a primary component of the galactosemia profile. In addition, participants with galactosemia have less well-developed executive functions. Child and parental reports of behavioral symptoms differ; parents reported that their children had more internalizing symptoms than the children with galactosemia self-reported. Cognitive complications in galactosemia appear to emerge even in well-treated children.

[Premature ovarian failure in galactosaemia: pathophysiology and clinical management]

Classic galactosaemia is a rare aetiology of premature ovarian failure. It is caused by galactose-1-phosphate uridyltransferase deficiency and leads to a severe disease in the newborn. This acute toxic syndrome will completely regress under a galactose-free diet, but some long-term complications, particularly hypergonadotropic hypogonadism in female patients, are frequently observed. Ovarian toxicity could be due to intracellular accumulation of galactose metabolites or to deficient glycosylation reactions. Moreover, the tremendous follicular decrease in the galactosaemic ovary could also involve programmed cell death (apoptosis). As the exact mechanisms of this ovarian injury are still unknown, there is no prevention of follicular loss, thus clinical management especially includes hormonal replacement therapy in order to prevent bone loss and cardiovascular risks and sometimes to allow patients to become pregnant.

Frequencies of Q188R and N314D mutations and IVS5-24g>A intron variation in the galactose-1-phosphate uridyl transferase (GALT) gene in the Slovenian population

Numerous mutations in the galactose-1-phosphate uridyl transferase (GALT) gene have been found to impair GALT activity to different extent, causing galactosemia. This disorder exhibits considerable allelic heterogeneity in different populations and ethnic groups. The Q188R mutation accounts for 60-70% of classical galactosemia alleles in the Caucasian population. Individuals homoallelic for Q188R have a severe phenotype with complete loss of enzyme activity. Another form of GALT deficiency is Duarte galactosemia with N314D mutation associated alleles (Duarte-2). Although heterozygotes for classical galactosemia are asymptomatic at birth and Duarte galactosemia appears to be quite benign, there are some indications that these disorders can increase the risk of developing certain diseases later in life. The aim of our study was to analyze a healthy Slovenian population for the frequencies of Q188R and N314D mutations, and for the Duarte-2 indicative intronic variation IVS5-24G>A. DNA samples from 174 healthy subjects were analyzed for all three mutations by polymerase chain reaction and digestion with restriction enzymes. Allele frequencies for Q188R and N314D mutations and IVS5-24G>A intron variation were found to be 0.29%, 8.0% and 5.7%, respectively. These results correlate well with those reported for most other healthy Caucasian populations.

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.

Risk factors for premature ovarian failure in females with galactosemia

The risk for premature ovarian failure (POF) in females with galactosemia can be predicted by analyzing 3 areas of risk pathology: the patient's molecular genotype for galactose-1-phosphate uridyltransferase (GALT), alternate pathways for galactose metabolism, and the patient's environment at diagnosis and during treatment.

STUDY DESIGN

Retrospective cross-sectional information was collected on 53 females with classic galactosemia, and their ovarian function was analyzed by determination of serum follicle-stimulating hormone and luteinizing hormone levels and by clinical observation. The associations were analyzed between POF and the mutations in GALT, the highest erythrocyte galactose-1-phosphate (Gal-1-P) level at diagnosis, the age at which dietary treatment was initiated, mean erythrocyte Gal-1-P level during treatment, and whole-body carbon 13-labeled galactose oxidation to (13)CO(2).

RESULTS

The most prevalent mutation, Q188R, had a significant effect of genotype category (Q188R/Q188R, Q188R/Other, Other/Other) on POF (P =.04, Fisher exact test and an odds ratio of 8.3). Mean erythrocyte Gal-1-P level during treatment was a significant risk factor for POF (P =.04). Also, all patients studied with less than 5% total body oxidation of galactose to (13)CO(2) had POF, whereas those with more than 5% did not have POF (P =.008, Fisher exact test).

CONCLUSION

The development of POF in females with galactosemia is more likely if the patient's genotype is Q188R/Q188R, if the mean erythrocyte Gal-1-P is >3.5 mg/dL during therapy, and if the recovery of (13)CO(2) from whole-body (13)C-galactose oxidation is reduced below 5% of administered (13)C-galactose.

Galactose breath testing distinguishes variant and severe galactose-1-phosphate uridyltransferase genotypes

A galactose breath test that quantitates [1-(13)C]galactose conversion to 13CO2 provides information on the whole body galactose oxidative capacity. As there is little information on the relationship between whole body oxidation and the genotype in patients with galactosemia, we measured the 13CO2 excretion for 2 h after administration of [1-(13)C]galactose in 37 patients (3-48 y old) with galactose-1-phosphate uridyltransferase (GALT) deficiency and 20 control subjects (3-37 y old). Eleven patients with the common Q188R/Q188R genotype and no detectable erythrocyte GALT activity eliminated <2% of a bolus of [1-(13)C]galactose as 13CO2 compared with 8.47 to 28.23% in controls. This defines a severe metabolic phenotype. Seven patients with one Q188R allele and a second mutant allele such as L195P, E308K, V151A, M142K, or Q344K and one patient with a K285N/unknown genotype also released <2% as 13CO2 in 2 h. The presence of N314D or S135L as the second mutant allele does not impair total body galactose oxidation, as individuals with the GALT genotype of Q188R/N314D, K285N/N314D, and Q188R/S135L had normal 2-h galactose breath tests. Subjects with S135L/S135L, N314D/N314D, S135L/deltaT2359 as well as other rarer genotypes such as R258C/Y209C, E203K/IVSC-N314D, K285N/T138M, Q188R/D113N, S135L/F171S, R148W/N314D, and IVSC-N314D/N314D oxidized galactose comparable to controls. The dissociation of residual erythrocyte GALT activity and whole body galactose oxidative capacity is exemplified by blacks with a S135L/S135L genotype and absent erythrocyte GALT activity. An oral 2-h [1-(13)C]galactose breath test distinguishes severe and variant GALT genotypes and enables delineation of the extent of impaired galactose metabolism in an array of patients who possess diverse GALT mutations. It may prove to be useful in establishing whether a patient is capable of manifesting disease similar to patients with a Q188R/Q188R genotype.

Molecular heterogeneity of classical and Duarte galactosemia: mutation analysis by denaturing gradient gel electrophoresis

Classical galactosemia is caused by one common missense mutation (Q188R) and by several rare mutations in the galactose-1-phosphate uridyltransferase (GALT) gene. The most common variant of GALT, the Duarte variant, occurs as two types, Duarte-1 (D-1) and Duarte-2 (D-2), both of which carry the sequence change N314D. D-1 increases, whereas D-2 decreases GALT activity. To study the molecular genetics of classical and Duarte galactosemia, we analyzed the GALT mutations in 30 families with classical galactosemia, in 10 families with the D-2 variant and in 3 individuals carrying the D-1 allele by denaturing gradient gel electrophoresis (DGGE). DGGE detected 59 of the 60 classical galactosemia alleles. Q188R accounted for 60%, K285N accounted for 28% of these alleles. Eight novel candidate galactosemia mutations were found. On all D-2 alleles N314D occurred in cis with two intronic sequence changes, on the D-1 alleles in cis with a neutral mutation in exon 7. We conclude that the mutations causing galactosemia are highly heterogeneous and that K285N is a second common galactosemia mutation in our population.

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.

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

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

Outcomes analysis of verbal dyspraxia in classic galactosemia

PURPOSE

This study evaluates a genotype/phenotype relationship between developmental verbal dyspraxia (DVD) and the common, missense mutation of the galactose-1-phosphate uridyltransferase gene, Q188R, in patients with classic galactosemia (G/G).

METHODS

As part of this study, we devised a questionnaire for "speech problems" to be completed by the patient\'s clinician. To validate the questionnaire and determine its accuracy in detecting DVD, we analyzed questionnaire responses for 21 patients by testing them independently and directly for DVD through a speech pathologist blinded to the patients' genotype.

RESULTS

We found that the questionnaire had a sensitivity of 0.56 and a specificity of 0.75. We then calculated the prevalence of DVD for a larger set of 113 patients with G/G galactosemia whose biochemical phenotype, molecular genotypes, and clinical status were known. The prevalence of "speech problems" from raw data were 50 of 113 (44.2%). After adjusting for misclassification, 43 (38.1%) were classified as cases of DVD. Using multivariate, logistic, regression analyses we found a significant interaction between genotype and mean red blood cell (RBC) galactose-1-phosphate (Gal-1-P). When corrected, using mean RBC Gal-1-P < h 3.28 mg%, the Q188R/Q188R genotype was the best predictor of DVD. There was a significant risk (odds ratio = 9.6, p = 0.0504) of having DVD associated with homozygosity for Q188R compared with other genotypes.

CONCLUSIONS

We conclude that homozygosity for Q188R mutations in the GALT gene is a significant risk factor for DVD. However, poor metabolic control obviates this relationship as indicated by RBC Gal-1-P greater than 3.28 mg%.

Urine and plasma galactitol in patients with galactose-1-phosphate uridyltransferase deficiency galactosemia

Urinary excretion of galactitol was determined in 95 normals (N/N), 67 galactosemic (G/G), and 39 compound heterozygotes for the Duarte and galactosemia genotype (D/G). Galactitol excretion is age-dependent in both normal individuals and patients with classic galactosemia on lactose-restricted diets. In galactosemic patients who are homozygous for the Q188R mutation, urinary galactitol levels were fivefold to 10-fold higher than those of normal subjects of comparable age. All but a few patients with classic galactosemia with the Q188R mutation and another mutant G allele had urinary excretion comparable to the Q188R homozygous patients. African-American galactosemic patients with the S135L mutation of the galactose-1-phosphate uridyltransferase (GALT) gene also excreted abnormal quantities of galactitol. Most subjects with a Duarte allele and a G allele excrete normal amounts of the sugar alcohol. There is a correlation between galactitol excretion and red blood cell (RBC) galactose-1-phosphate (gal-1-P). Plasma galactitol was also elevated in galactosemic patients (3.4 to 23.2 micromol/L; undetectable in normal individuals). In contrast to the decrease in urinary galactitol with age, plasma levels remain in a narrow concentration range with no significant difference with age. Urine and plasma galactitol distinguish galactosemic patients from normals. In addition, urinary galactitol excretion may be an important parameter for the assessment of steady-state galactose metabolism in galactosemia.

Classical galactosaemia in Chinese: A case report and review of disease incidence

We report a case of galactose-1-phosphate uridyl transferase (GALT) deficiency in a full-term Chinese neonate, who presented with atypical biochemical features of hyperammonaemia in addition to the classical presenting features of jaundice and lethargy after feeding. Red cell GALT activity was virtually absent in the patient while 50% of normal activity was found in parents and a sibling. Mutation screening excluded both Q188R and N314D as the causative mutation in GALT gene, which suggested a possible genetic segregation among ethnic groups. Data from a Taiwan screening program suggested that the incidence of the disease was approximately 1 in 400 000 in the Chinese population which was a sixth of that in Caucasian populations.

The biochemical role of glutamine 188 in human galactose-1-phosphate uridyltransferase

The substitution of arginine for glutamine at amino acid 188 (Q188R) ablates the function of human galactose-1-phosphate uridyltransferase (GALT) and is the most common mutation causing galactosemia in the white population. GALT catalyzes two consecutive reactions. The first reaction binds UDP-glucose (UDP-Glu), displaces glucose-1-phosphate (glu-1-P), and forms the UMP-GALT intermediate. In the second reaction, galactose-1-phosphate (gal-1-P) is bound, UDP-galactose (UDP-Gal) is released, and the free enzyme is recycled. In this study, we modeled glutamine, asparagine, and a common mutation arginine at amino acid 188 on the three-dimensional model of the Escherichia coli GALT-UMP protein crystal. We found that the amide group of the glutamine side chain could provide two hydrogen bonds to the phosphoryl oxygens of UMP with lengths of 2.52 and 2.82 A. Arginine and asparagine could provide only one hydrogen bond of 2. 52 and 3.02 A, respectively. To test this model, we purified recombinant human Gln188-, Arg188-, and Asn188-GALT and analyzed the first reaction in the absence of gal-1-P by quantitating glu-1-P released using enzyme-linked methods. Gln188-GALT displaced 80 +/- 7. 0 nmol glu-1-P/mg GALT/min in the first reaction. By contrast, both Arg188- and Asn188-GALT released more glu-1-P (170 +/- 8.0 and 129 +/- 28.4 nmol/mg GALT/min, respectively). The overall, double displacement reaction was quantitated in the presence of gal-1-P. Gln188-GALT produced 80,030 +/- 5,910 nmol glu-1-P/mg GALT/min, whereas the mutant Arg188- and Asn188-GALT released only 600 +/- 71. 2 and 2960 +/- 283.6 nmole glu-1-P/mg GALT/min, respectively. We conclude from these data that glutamine at position 188 stabilizes the UMP-GALT intermediate through hydrogen bonding and enables the double displacement of both glu-1-P and UDP-Gal. The substitution of arginine or asparagine at position 188 reduces hydrogen bonding and destabilizes UMP-GALT. The unstable UMP-GALT allows single displacement of glu-1-P with release of free GALT but impairs the subsequent binding of gal-1-P and displacement of UDP-Gal.

Elevation of erythrocyte redox potential linked to galactonate biosynthesis: elimination by Tolrestat

Alternate pathways of galactose metabolism were explored in erythrocytes from normal subjects and patients with galactose-1-phosphate uridylyltransferase (GALT) deficiency incubated with galactose. Micromolar quantities of galactonate accumulated in both normal and mutant cells linearly with time up to 5 hours and with concentrations of galactose up to 25 mmol/L. Galactitol also was found at levels less than one third of the galactonate level, while galactose-1-phosphate concentrations comparable to those of galactonate were found in galactosemic cells. Concomitant with the formation of these galactose metabolites, the erythrocyte redox potential based on measurement of lactate and pyruvate increased fourfold in both cell types. This was due to a 60% to 72% decrease in pyruvate and a 24% to 26% increase in lactate. The oxidation of galactose to galactonate, which is known to generate NADH, is the most likely explanation for the increase in the redox state. The aldose reductase inhibitor (ARI), Tolrestat (Wyeth Ayerst Research, Princeton, NJ), at 70 micromol/L inhibited the formation of both galactonate and galactitol in both cell types without affecting galactose-1-phosphate, and eliminated the increase in the redox potential as indicated by restoration of pyruvate and lactate levels to the levels obtained before exposure of the cells to galactose. A functioning galactonate pathway is a route of galactose disposal in patients with GALT deficiency, but by altering the cellular redox potential, it may also contribute to galactose toxicity.

Other hereditary diseases and the liver

In this chapter, an abbreviated account is presented on the subject of hereditary diseases and the liver. However, it is incomplete because Alagille syndrome, storage disorders, alpha-1-antitrypsin deficiency and Wilson disease are not included as they appear in other chapters of this volume. Biliary atresia is omitted because all available evidence does not support any significant genetic association. Molecular biological techniques have enabled linkage of several liver cholestatic disorders to chromosomal loci, and further characterization of the canalicular bile salt transporter (cBST) will advance our understanding of pathogenetic mechanisms involved in benign and progressive cholestatic syndromes. Disorders that have been treated as separate entities may have common 'roots', exemplified by the concept of the ductal plate malformation in fibropolycystic disease. Whereas the majority of disorders referred to in this chapter present early in life, there are several that are associated with liver failure in the neonatal period, which makes early recognition particularly important. Liver transplantation offers a cure for many hereditary disorders affecting the liver but it is not applicable to all.

Urinary galactonate in patients with galactosemia: quantitation by nuclear magnetic resonance spectroscopy

Although numerous reports have appeared showing high levels of galactitol in the urine of patients with galactose-1-phosphate uridylyltransferase deficiency, little attention has been paid to measurement of urinary galactonate. Herein we explored the use of 1H and 13C nuclear magnetic resonance, which required only the concentration of urine without derivatization, to detect and quantitate urinary galactonate. We report that transferase deficient infants, as well as adults on galactose restricted diets excrete significant amounts of galactonate, whereas none is detected in the urine of normal subjects. Galactose-toxic infants were found to excrete large amounts of galactonate, which decreased when the lactose-free diet was instituted. We also found that normal individuals subjected to an oral galactose load also excrete high levels of galactonate for at least 4 h after galactose ingestion. Our data provide evidence that the first reaction in the oxidative pathway of galactose metabolism described in rat liver in 1966 is activated in patients with a variety of galactose-1-phosphate uridylyltransferase gene mutations even while on a lactose-restricted diet. In both patients and normal individuals, flux through the alternate galactonate pathway appears to be related to the body galactose burden.

A prevalent mutation for galactosemia among black Americans

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Galactosemia unsolved

Classic galactosemia is an enigmatic disorder that presents the challenge of unraveling the basis of the long-term complications of mental disability, speech defects, ovarian failure and neurologic syndromes which occur despite a galactose-restricted diet. A complete understanding of the pathobiochemistry and molecular genetics, and evaluation of the present theories for the poor long-term outcome, continuous intoxication, critical metabolite depletion and in utero damage is needed in order to design new therapeutic strategies. Answering this urgent question of how to treat galactosemic patients mandates enhanced clinical and basic research efforts.

Prenatal diagnosis of galactosemia

The experience from three different European centres with the prenatal diagnosis of galactose-1-phosphate-uridyltransferase (GALT) deficiency is presented and the question whether or not there is a need for prenatal diagnosis of this disorder is discussed. Most prenatal diagnoses (n = 50) have been performed by assay of GALT activity in cultured amniotic fluid cells. The assay used is reliable and clearly distinguishes homozygous affected fetuses (n = 11; 0%-2.3% of mean control enzyme activity) from non-(homozygous)-affected fetuses. The GALT assay for cultured amniocytes was adapted to assay the enzyme directly in chorionic villi. The experience with chorionic villi comprises 23 cases with 5 affected fetuses (0%-4.2% of mean control enzyme activity). In 36 cases galactitol was determined in amniotic fluid supernatant by gas chromatography-mass spectrometry. This method also differentiated affected (n = 11; galactitol 5.9-10.6 mumol/l) and unaffected pregnancies (galactitol 0.23-1.6 mumol/l) clearly and has the advantage of providing a result within a day or two after amniocentesis. Prenatal diagnosis of galactosemia is undertaken rarely and sometimes for the wrong reasons, but it should perhaps be considered more seriously until better methods of treatment are established.

Partial deficiency of galactose-1-phosphate uridyltransferase

In screening programmes testing newborns for galactose-1-phosphate uridyltransferase and/or galactose, partial enzyme deficiency is frequently discovered. This is shown for one laboratory in Switzerland where 104 newborns were singled out from a total of 476,000. Of these, 72 had partial transferase deficiency below 9 mumol/h per g Hb and were assumed to be compound heterozygotes for "classical" galactosemia and the Duarte variant. Present day management of compound heterozygotes consisting of lactose-free diet for 4 months is described and discussed in the light of published opinion. The appropiateness of this pragmatic approach can at present not be judged according to objective criteria.