An unexpectedly high frequency of hypergalactosemia in an immigrant Bosnian population revealed by newborn screening

In Silico Structural Protein Evaluation of the Phenylalanine Hydroxylase p.(Tyr77His) Variant Associated with Benign Hyperphenylalaninemia as Identified through Mexican Newborn Screening

Hyperphenylalaninemia (HPA), which includes phenylketonuria (PKU), is a genetic autosomal recessive disorder arising from a deficiency in the enzyme named phenylalanine hydroxylase (PAH). Affected patients can experience severe and irreversible neurological impairments when phenylalanine (Phe) blood concentration exceeds 360 μmol/L (6 mg/dL). Here, we describe a female HPA patient who was born in Mexico to Cuban non-consanguineous parents and identified by newborn screening, and who bears the previously unreported PAH NM_000277.3(PAH):c.[229T>C];[1222C>T] or p.[Tyr77His];[Arg408Trp] genotype. At diagnosis, the patient showed a Phe blood level of 321 μmol/L (5.3 mg/dL), indicative of mild HPA. Neither of the PAH variants found in this patient had been previously reported in the mutational PAH spectrum of the Mexican population. The c.229T>C or p.(Tyr77His) PAH variant was previously related to mild HPA in the Swedish population. Our in silico structural analysis and molecular docking showed that mutated His 77 residue is located in the allosteric site of PAH at the interface of the two monomers. The PDBsum in silico tool predicted that this variant would cause minimal structural disturbance of the protein interface in the presence of Phe at the allosteric site. Docking studies revealed that these structural changes might be attenuated by the allosteric effect of Phe. Given the classic PKU phenotype conditioned by the "Celtic" or c.[1222C>T] or p.(Arg408Trp) PAH variant, which is the second variant in this patient, we propose that p.(Tyr77His) has a hypomorphic feature that could explain her mild HPA phenotype. Our results show the importance of following up on cases detected by NBS and the value of genetic studies and in silico tools that aid in the establishment of correct therapeutic strategies.

Galactokinase deficiency: a treatable cause of bilateral cataracts

Congenital cataract can be caused by several systemic diseases and differential diagnosis should be done between infections, genetic or metabolic diseases. We present a case of a 12-month-old girl with bilateral nuclear cataracts that was referred for investigation. Since she did not present a family history of congenital cataracts or metabolic diseases, and her physical examination was normal, a systemic evaluation was performed. Biochemical studies disclosed abnormal galactose metabolism signs. The diagnosis of galactokinase (GALK1) deficiency was considered and the study of the GALK1 gene allowed identifying a pathogenic genetic variant and a predictably pathogenic missense mutation, previously not described. Dietary measures were imposed with a good evolution.

Recommendations for newborn screening for galactokinase deficiency: A systematic review and evaluation of Dutch newborn screening data

INTRODUCTION

Galactokinase (GALK) deficiency causes cataract leading to severe developmental consequences unless treated early. Because of the easy prevention and rapid reversibility of cataract with treatment, the Dutch Health Council advised to include GALK deficiency in the Dutch newborn screening program. The aim of this study is to establish the optimal screening method and cut-off value (COV) for GALK deficiency screening by performing a systematic review of the literature of screening strategies and total galactose (TGAL) values and by evaluating TGAL values in the first week of life in a cohort of screened newborns in the Netherlands.

METHODS

Systematic literature search strategies in OVID MEDLINE and OVID EMBASE were developed and study selection, data collection and analyses were performed by two independent investigators. A range of TGAL values measured by the Quantase Neonatal Total Galactose screening assay in a cohort of Dutch newborns in 2007 was evaluated.

RESULTS

Eight publications were included in the systematic review. All four studies describing screening strategies used TGAL as the primary screening marker combined with galactose-1-phosphate uridyltransferase (GALT) measurement that is used for classical galactosemia screening. TGAL COVs of 2200 μmol/L, 1665 μmol/L and 1110 μmol/L blood resulted in positive predictive values (PPV) of 100%, 82% and 10% respectively. TGAL values measured in the newborn period were reported for 39 GALK deficiency patients with individual values ranging from 3963 to 8159 μmol/L blood and 2 group values with mean 8892 μmol/L blood (SD ± 5243) and 4856 μmol/L blood (SD ± 461). Dutch newborn screening data of 72,786 newborns from 2007 provided a median TGAL value of 110 μmol/L blood with a range of 30-2431 μmol/L blood.

CONCLUSION

Based on TGAL values measured in GALK deficiency patients reported in the literature and TGAL measurements in the Dutch cohort by newborn screening we suggest to perform the GALK screening with TGAL as a primary marker with a COV of 2500 μmol/L blood, combined with GALT enzyme activity measurement as used in the classical galactosemia screening, to ensure detection of GALK deficiency patients and minimize false positive referrals.

Clinical evaluation and mutational analysis of GALK and GALE genes in patients with galactosemia in Greece: one novel mutation and two rare cases

BACKGROUND

Deficiencies of galactokinase (GALK) and UDP-epimerase (GALE) are implicated with galactose metabolic disorders. The aim of the study was the identification of mutations in GALK and GALE genes and clinical evaluation of patients.

METHODS

Five patients with GALK and five with GALE deficiency were picked up via the Neonatal Screening Program. Additionally, two females, 4 years old, were referred with late diagnosed galactosemia, as rare cases. Mutational analysis was conducted via Sanger sequencing, while in silico analysis tools were utilized for the novel mutation. Psychomotor and speech development tests were performed, as well.

RESULTS

The mutation p.Pro28Thr was identified in both alleles in GALK-deficient patients of Roma (gypsy) origin, whereas the novel p.Asn39Ser was detected in two non-Roma patients. In GALE-deficient patients benign and/or likely benign mutations were found. Psychomotor and speech delay were determined in the Roma GALK patients. In each of the late diagnosed females, four mutations were identified in all galactosemia-related genes.

CONCLUSIONS

The mutational spectrums of GALE- and GALK-deficient patients in Greece are presented for the first time along with a clinical evaluation. Mutational analysis in all galactosemia-related genes of symptomatic patients is highly recommended for future cases.

Inherited Congenital Cataract: A Guide to Suspect the Genetic Etiology in the Cataract Genesis

Cataracts are the principal cause of treatable blindness worldwide. Inherited congenital cataract (CC) shows all types of inheritance patterns in a syndromic and nonsyndromic form. There are more than 100 genes associated with cataract with a predominance of autosomal dominant inheritance. A cataract is defined as an opacity of the lens producing a variation of the refractive index of the lens. This variation derives from modifications in the lens structure resulting in light scattering, frequently a consequence of a significant concentration of high-molecular-weight protein aggregates. The aim of this review is to introduce a guide to identify the gene involved in inherited CC. Due to the manifold clinical and genetic heterogeneity, we discarded the cataract phenotype as a cardinal sign; a 4-group classification with the genes implicated in inherited CC is proposed. We consider that this classification will assist in identifying the probable gene involved in inherited CC.

Early cataract formation due to galactokinase deficiency: impact of newborn screening

BACKGROUND AND AIMS

Galactokinase (GALK) deficiency is an autosomal recessive disorder causing cataract formation that can be prevented or mitigated by early diagnosis and galactose-restricted diet. The aim of this retrospective study was to explore whether GALK-deficiency meets the criteria for neonatal mass screening programs.

METHODS

From 2000 until 2010, the Screening Laboratory Hannover performed newborn screening in 1,950,927 infants from Germany for galactosemia by measuring galactose-1-phosphate-uridyl-transferase and total galactose concentration (free galactose plus galactose-1-phosphate), including automatic screening for GALK deficiency.

RESULTS

Eleven cases were found with elevated galactose levels accompanied by normal transferase activity. Nine of 11 cases were informative; the diagnosis was established by demonstrating deficient activity of the GALK enzyme in erythrocytes. To our knowledge, screening did not produce any false negative results. All patients were treated with a galactose-restricted diet from the neonatal period or infancy. Three of nine patients suffered from congenital cataracts or eventual development of cataracts, despite normal galactose concentrations in blood.

CONCLUSIONS

Newborn screening for GALK deficiency prevents or at least mitigates cataract formation. As screening for GALK deficiency is technically simple, it seems to be reasonable to include this disorder in routine screening programs by simultaneous determination of transferase activity and quantification of galactose plus galactose-1-phosphate in dried blood spots.

Features and outcome of galactokinase deficiency in children diagnosed by newborn screening

Galactokinase deficiency (GALK-D), an autosomal recessive disorder in the Leloir pathway, results in accumulation of galactose, galactitol, and galactonate and leads to early onset of juvenile bilateral cataract. Highest incidence of GALK-D is found in Romani populations. The migration wave due to the Yugoslavian civil war has changed the spectrum of inborn errors of metabolism within Europe. Hence, newborn screening (NBS) in the Berlin region, performed from 1991 until 2010 in 683,675 neonates, revealed an increased incidence of GALK-D of 1:40,000, comparable to that of galactose-1-phosphate-uridyltransferase deficiency. A total of 44% of GALK-D patients were of Romani origin. All patients of Bosnian or Serbian origin were homozygous for the Romani founder mutation p.P28T. Detection of GALK-D by NBS and early start of galactose-restricted diet resulted in regression or prevention of cataracts. Slight cataracts without visual impairment occurred in 50% of the patients, 56% of whom were noncompliant. Further clinical symptoms, e.g., hypoglycemia, mental retardation, microcephaly, and failure to thrive, were associated with noncompliance. With treatment, galactose in blood decreased from 8,892 ± 5,243 to 36.5 ± 49.3 μmol/l, galactose in urine from 31,820 ± 32,103 to 30.0 ± 36.1 μmol/mmol creatinine, galactitol in RBC from 1,584 ± 584 to 12.3 ± 9.4 μmol/l, and galactitol in urine from 11,724 ± 4,496 to 236 ± 116 μmol/mmol creatinine. This is the first presentation of outcome and clinical features in GALK-D patients diagnosed by NBS. As our data suggest, GALK-D should be considered for inclusion in NBS in populations expected to have substantial numbers of GALK-D carriers, e.g., Yugoslavian immigrants.

Molecular and biochemical characterization of the GALK1 gene in Korean patients with galactokinase deficiency

Galactokinase (GALK) deficiency is an autosomal recessive disorder characterized by elevation of blood galactose concentration and diminished galactose-1-phosphate, leading to the production of galactitol. To investigate the molecular defects of GALK1 gene and the biochemical characteristics of their mutant proteins, PCR-direct sequencing and in vitro expression analysis in Cos7 cells were performed in five Korean patients with GALK deficiency galactosemia. Four missense mutations (p.G137R, p.R256W, p.R277Q, and p.V281M) and one small insertion (c.850_851insG) were identified. Among four patients with severely reduced GALK activity, two were found to be homozygotes for p.R256W and the other two were compound heterozygotes for different molecular defects (p.G137R/p.R277Q and p.V281M/c.850_851insG). One Patient with moderately decreased GALK activity was heterozygous for p.R256W. Expression analysis in Cos7 cells confirmed that each of the mutations resulted in reduction of GALK activity and caused GALK deficiency.

Molecular structure of human galactokinase: implications for type II galactosemia

Galactokinase functions in the Leloir pathway for galactose metabolism by catalyzing the MgATP-dependent phosphorylation of the C-1 hydroxyl group of alpha-D-galactose. The enzyme is known to belong to the GHMP superfamily of small molecule kinases and has attracted significant research attention for well over 40 years. Approximately 20 mutations have now been identified in human galactokinase, which result in the diseased state referred to as Type II galactosemia. Here we report the three-dimensional architecture of human galactokinase with bound alpha-D-galactose and Mg-AMPPNP. The overall fold of the molecule can be described in terms of two domains with the active site wedged between them. The N-terminal domain is dominated by a six-stranded mixed beta-sheet whereas the C-terminal motif contains six alpha-helices and two layers of anti-parallel beta-sheet. Those residues specifically involved in sugar binding include Arg37, Glu43, His44, Asp46, Gly183, Asp186, and Tyr236. The C-1 hydroxyl group of alpha-D-galactose sits within 3.3 A of the gamma-phosphorus of the nucleotide and 3.4 A of the guanidinium group of Arg37. The carboxylate side chain of Asp186 lies within approximately 3.2 A of the C-2 hydroxyl group of alpha-D-galactose and the guanidinium group of Arg37. Both Arg37 and Asp186 are strictly conserved among both prokaryotic and eukaryotic galactokinases. In addition to providing molecular insight into the active site geometry of the enzyme, the model also provides a structural framework upon which to more fully understand the consequences of the those mutations known to give rise to Type II galactosemia.

Functional analysis of disease-causing mutations in human galactokinase

Galactokinase (EC 2.7.1.6) catalyzes the first committed step in the catabolism of galactose. The sugar is phosphorylated at position 1 at the expense of ATP. Lack of fully functional galactokinase is one cause of the inherited disease galactosemia, the main clinical manifestation of which is early onset cataracts. Human galactokinase (GALK1) was expressed in and purified from Escherichia coli. The recombinant enzyme was both soluble and active. Product inhibition studies showed that the most likely kinetic mechanism of the enzyme was an ordered ternary complex one in which ATP is the first substrate to bind. The lack of a solvent kinetic isotope effect suggests that proton transfer is unlikely to be involved in the rate determining step of catalysis. Ten mutations that are known to cause galactosemia were constructed and expressed in E. coli. Of these, five (P28T, V32M, G36R, T288M and A384P) were insoluble following induction and could not be studied further. Four of the remainder (H44Y, R68C, G346S and G349S) were all less active than the wild-type enzyme. One mutant (A198V) had kinetic properties that were essentially wild-type. These results are discussed both in terms of galactokinase structure-function relationships and how these functional changes may relate to the causes of galactosemia.