Duarte-1 (Los Angeles) and Duarte-2 (Duarte) variants in Germany: two new mutations in the GALT gene which cause a GALT activity decrease by 40-50% of normal in red cells

Clinical, molecular, and genetic evaluation of galactosemia in Turkish children

AIM

Galactosemia is a carbohydrate metabolism disorder with autosomal recessive inheritance. The most frequent enzyme deficiency is galactose-1-phosphate-uridylytransferase, which causes classic galactosemia. When the enzyme is absent, an infant cannot metabolize galactose-1-phosphate and it cumulates in liver, kidney, brain, tongue, lens, and skin. This study aimed to evaluate the clinical and molecular characteristics of patients with galactosemia, which is observed more frequently in our country than anywhere else in the world.

MATERIAL AND METHODS

This is a retrospective study that includes the moleculer and genetic charcteristics of 14 patient who were diagnosed as having galactosemia between January 2009 and January 2011.

RESULTS

Nine patients were male and 5 female. Consanguineous marriage was detected in the family history of 7 patients. One patient had a history of a deceased sibling with a confirmed diagnosis of galactosemia. The main reasons for admission to the hospital were jaundice in 9, hypoglycemia in 2, sepsis in 2, and elevated liver enzymes in 1 patient. The Beutler test was positive in all patients. The mean enzyme activity was 0.36±0.26 μmol/mL. Only 6 of our cases were diagnosed in the early period (first 15 days). Cataract was present in four patients. Q188R mutation was observed in 13 patients, and homozygote N314D and homozygote E340X mutations were observed in one patient. Three patients had impaired neurologic development according to the Denver Developmental Screening Test II.

CONCLUSION

The most common genetic abnormality was Q188R mutation. Only 43% of our patients's disease could be diagnosed at an early stage. We suggest that galactosemia should be included in the national newborn screening program in order to make earlier diagnoses.

Clinical profile and molecular characterization of Galactosemia in Brazil: identification of seven novel mutations

Background

Classical Galactosemia (CG) is an inborn error of galactose metabolism caused by the deficiency of the galactose-1-phosphate uridyltransferase enzyme. It is transmitted as an autosomal recessive disease and is typically characterized by neonatal galactose intolerance, with complications ranging from neonatal jaundice and liver failure to late complications, such as motor and reproductive dysfunctions. Galactosemia is also heterogeneous from a molecular standpoint, with hundreds of different mutations described in the GALT gene, some of them specific to certain populations, reflecting consequence of founder effect.

Methods

This study reviews the main clinical findings and depicts the spectrum of mutations identified in 19 patients with CG, six with Duarte Galactosemia and one with type 2 Galactosemia in Brazil. Some individuals were diagnosed through expanded newborn screening test, which is not available routinely to all newborns.

Results

The main classical Galactosemia mutations reported to date were identified in this study, as well as the Duarte variant and seven novel mutations - c.2 T > C (p.M1T), c.97C > A (p.R33S), c.217C > T (p.P73S), c.328 + 1G > A (IVS3 + 1G > A), c.377 + 4A > C (IVS4 + 4A > C), c.287_289delACA (p.N97del) and c.506A > C (p.Q169P). This was expected, given the high miscegenation of the Brazilian population.

Conclusions

This study expands the mutation spectrum in GALT gene and reinforces the importance of early diagnosis and introduction of dietary treatment, what is possible with the introduction of Galactosemia in neonatal screening programs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12881-016-0300-8) contains supplementary material, which is available to authorized users.

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.

Frequencies of the Los Angeles and Duarte galactose-1-phosphate uridyltransferase variant alleles in the Hungarian population

A klasszikus galactosaemia autoszomális recesszív módon öröklődő anyagcsere-betegség, amelynek hátterében a galaktóz-1-foszfát-uridil-transzferáz (GALT) enzim defektusa áll. Hazánkban a betegséget az újszülöttkori tömegszűrés keretében, 1976 óta szűrik. A p.N314D mutáció asszociál a GALT enzim Los Angeles és a Duarte variánsaival, attól függően, hogy a mutáció mely polimorfizmusokkal öröklődik együtt cis helyzetben. Célkitűzés: Munkánk célja volt a magyar egészséges populációban a p.N314D mutáció, valamint a Los Angeles és Duarte variánsok gyakoriságát meghatározni. Módszerek: A 100 donor személyből izolált genomiális DNS-mintát polimeráz láncreakció során amplifikáltuk, majd restrikciós endonukleázzal emésztettük. Eredmények: A p.N314D mutáció, a Los Angeles variáns és a Duarte variáns allélgyakorisága az általunk vizsgált populációban 11,5%, 2,5%, illetve 9% volt. Következtetés: A magyar populációban a Los Angeles és Duarte varánsok allélgyakorisága jól korrelál más kaukázusi populációkban detektált gyakoriságokkal.

Origins, distribution and expression of the Duarte-2 (D2) allele of galactose-1-phosphate uridylyltransferase

Duarte galactosemia is a mild to asymptomatic condition that results from partial impairment of galactose-1-phosphate uridylyltransferase (GALT). Patients with Duarte galactosemia demonstrate reduced GALT activity and carry one profoundly impaired GALT allele (G) along with a second, partially impaired GALT allele (Duarte-2, D2). Molecular studies reveal at least five sequence changes on D2 alleles: a p.N314D missense substitution, three intronic base changes and a 4 bp deletion in the 5′ proximal sequence. The four non-coding sequence changes are unique to D2. The p.N314D substitution, however, is not; it is found together with a silent polymorphism, p.L218(TTA), on functionally normal Duarte-1 alleles (D1, also called Los Angeles or LA alleles). The HapMap database reveals that p.N314D is a common human variant, and cross-species comparisons implicate D314 as the ancestral allele. The p.N314D substitution is also functionally neutral in mammalian cell and yeast expression studies. In contrast, the 4 bp 5′ deletion characteristic of D2 alleles appears to be functionally impaired in reporter gene transfection studies. Here we present allele-specific qRT–PCR evidence that D2 alleles express less mRNA in vivo than their wild-type counterparts; the difference is small but statistically significant. Furthermore, we characterize the prevalence of the 4 bp deletion in GG, NN and DG populations; the deletion appears exclusive to D2 alleles. Combined, these data strongly implicate the 4 bp 5′ deletion as a causal mutation in Duarte galactosemia and suggest that direct tests for this deletion, as proposed here, could enhance or supplant current tests, which define D2 alleles on the basis of the presence and absence of linked coding sequence polymorphisms.

Xanthurenic aciduria due to a mutation in KYNU encoding kynureninase

Massive urinary excretion of xanthurenic acid, 3-hydroxykynurenine and kynurenine, known as xanthurenic aciduria or hydroxykynureninuria, in a young Somali boy suggested kynureninase deficiency. Mutation analysis of KYNU encoding kynureninase of the index case revealed homozygosity for a c.593 A > G substitution leading to a threonine-to-alanine (T198A) shift. A younger brother was found to have a similar excretion pattern and the same genotype. At present, neither of the two boys has symptoms of niacin deficiency. This is the first report linking xanthurenic aciduria to a mutation in the gene encoding kynureninase.

Identification of sequence variation in the galactose-1-phosphate uridyl transferase gene by dHPLC

Transferase-deficient galactosaemia is an inherited disorder of carbohydrate metabolism, caused by mutation at the galactose-1-phosphate uridyl transferase (GALT) locus. A denaturing high performance liquid chromatography (dHPLC) method was developed for variant scanning of the GALT gene. The method unequivocally identified the Duarte D1, D2, Q188R, and K285N GALT alleles and associated polymorphisms. Length polymorphism in an intronic Alu repeat was characterised and a novel Single Nucleotide Polymorphism (IVS10nt-322g-->t) associated with the D1 allele was identified.

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.