Molecular analysis of 11 galactosemia patients

AAV-mediated expression of galactose-1-phosphate uridyltransferase corrects defects of galactose metabolism in classic galactosemia patient fibroblasts

Classic galactosemia (CG) is a rare disorder of autosomal recessive inheritance. It is caused predominantly by point mutations as well as deletions in the gene encoding the enzyme galactose-1-phosphate uridyltransferase (GALT). The majority of the more than 350 mutations identified in the GALT gene cause a significant reduction in GALT enzyme activity resulting in the toxic buildup of galactose metabolites that in turn is associated with cellular stress and injury. Consequently, developing a therapeutic strategy that reverses both the oxidative and ER stress in CG cells may be helpful in combating this disease. Recombinant adeno-associated virus (AAV)-mediated gene therapy to restore GALT activity offers the potential to address the unmet medical needs of galactosemia patients. Here, utilizing fibroblasts derived from CG patients we demonstrated that AAV-mediated augmentation of GALT protein and activity resulted in the prevention of ER and oxidative stress. We also demonstrate that these CG patient fibroblasts exhibit reduced CD109 and TGFβRII protein levels and that these effectors of cellular homeostasis could be restored following AAV-mediated expression of GALT. Finally, we show initial in vivo proof-of-concept restoration of galactose metabolism in a GALT knockout mouse model following treatment with AAV-GALT.

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.

Guideline for the Evaluation of Cholestatic Jaundice in Infants: Joint Recommendations of the North American Society for Pediatric Gastroenterology, Hepatology, and Nutrition and the European Society for Pediatric Gastroenterology, Hepatology, and Nutrition

Cholestatic jaundice in infancy affects approximately 1 in every 2500 term infants and is infrequently recognized by primary providers in the setting of physiologic jaundice. Cholestatic jaundice is always pathologic and indicates hepatobiliary dysfunction. Early detection by the primary care physician and timely referrals to the pediatric gastroenterologist/hepatologist are important contributors to optimal treatment and prognosis. The most common causes of cholestatic jaundice in the first months of life are biliary atresia (25%-40%) followed by an expanding list of monogenic disorders (25%), along with many unknown or multifactorial (eg, parenteral nutrition-related) causes, each of which may have time-sensitive and distinct treatment plans. Thus, these guidelines can have an essential role for the evaluation of neonatal cholestasis to optimize care. The recommendations from this clinical practice guideline are based upon review and analysis of published literature and the combined experience of the authors. The committee recommends that any infant noted to be jaundiced after 2 weeks of age be evaluated for cholestasis with measurement of total and direct serum bilirubin, and that an elevated serum direct bilirubin level (direct bilirubin levels >1.0 mg/dL or >17 μmol/L) warrants timely consideration for evaluation and referral to a pediatric gastroenterologist or hepatologist. Of note, current differential diagnostic plans now incorporate consideration of modern broad-based next-generation DNA sequencing technologies in the proper clinical context. These recommendations are a general guideline and are not intended as a substitute for clinical judgment or as a protocol for the care of all infants with cholestasis. Broad implementation of these recommendations is expected to reduce the time to the diagnosis of pediatric liver diseases, including biliary atresia, leading to improved outcomes.

Black children deficient in galactose 1-phosphate uridyltransferase: correlation of activity and immunoreactive protein in erythrocytes and leukocytes

A recent study found a high prevalence of a missense mutation (S135L) in the gene for galactose 1-phosphate uridyltransferase (GALT) in black children with galactosemia (J Pediatr 1996; 128:89-95). In the present study, GALT activity and GALT protein content were measured in erythrocytes and leukocytes of eight black and seven white galactosemic (GALT-deficient) children, for correlation with the presence of the S135L and Q188R (highly prevalent in white galactosemic children) missense mutations. The S135L mutation was found in 9 of 16 alleles of black children but not in white children; the Q188R mutation was found in 10 of 14 alleles examined in white galactosemic children and in 4 of 16 alleles in black galactosemic children. The GALT activity was near zero in the erythrocytes of white and black galactosemic children (0.26 +/- 0.28 vs 0.33 +/- 0.25 mumol/hr per gram of hemoglobin, respectively; p = 0.61) (normal 17 to 26 mumol/hr per gram), and no correlation of erythrocyte activity with genotype was observed. The GALT activity was higher in the leukocytes of black galactosemic children compared with white children (5 +/- 6 vs 1 +/- 2 mumol/hr per gram, respectively) (normal 172 to 374 mumol/hr per gram), but the difference was not statistically significant (p = 0.11). Analysis by genotype revealed that the two S135L homozygotes had much more leukocyte activity (9 and 17 mumol/hr per gram) than Q188R homozygotes or than all non-S135L allelic genotypes. Compound heterozygotes (S135L/G) had intermediate activity. The GALT protein was not detectable by Western blot in the erythrocytes of either white or black galactosemic children, as determined by antibodies specific for both C- and N-terminal sequences. The GALT protein was undetectable in the leukocytes of white galactosemic children, but leukocytes from black galactosemic children with the S135L mutation contained reduced but readily detectable GALT protein. Erythrocyte galactose 1-phosphate levels were significantly lower in galactosemic children with an S135L mutant allele (1.1 +/- 0.2 gm/dl) compared with children who had other mutations (3.1 +/- 0.9 mg/dl; p = 0.0001). The correlation of protein content data with activity levels in the blood cells suggests that the S135L missense mutation affects the stability of GALT protein to produce a deficiency state.

Rat galactose-1-phosphate uridyltransferase coding sequence, transcription start site and genomic organization

Galactosemia is an inborn error of metabolism in humans due to deficient activity of the enzyme galactose-1-phosphate uridyl-transferase (GALT). Using a 1.3 kb Bam HI restriction fragment of the human GALT cDNA as a probe, three cDNAs, 786 bp, 265 bp and 1.4 kb were isolated from a rat liver cDNA library. The total rat GALT coding sequence obtained from these three clones was 1051 nt. Repeat cDNA library screening failed to isolate cDNA clones with additional 5'-coding sequence. Using the 786 bp cDNA as a probe, three genomic clones, RGG1, RGG2, and RGG3 were isolated from a rat genomic library. RGG2 and RGG3 contained additional 5'-coding sequence, missing from the cDNA clones, which was identified by comparison with the human cDNA sequence. The rat GALT sequence, from transcription start site to the polyadenylation tail, is 1254 nt and is divided into 11 exons which span over 3.5 kb of genomic sequence. Primer extension identified the transcription start site as 17 nt 5' to the translation start site. Rat GALT is 379 amino acids long, identical to human GALT, with a derived molecular weight of 43,312 Da similar to the 43,360 Da weight for human GALT. The derived rat amino acid sequence is 90% identical to the human sequence. The isolation of rat GALT genomic sequence will facilitate future investigations of the regulation of expression of the rat GALT gene.

Molecular characterization of two galactosemia mutations and one polymorphism: implications for structure-function analysis of human galactose-1-phosphate uridyltransferase

We report here the molecular characterization of two galactosemia mutations, L74P and F171S, and one polymorphism, S135L, in human galactose-1-phosphate uridyltransferase (GALT). Both galactosemia mutations result in reduced enzymatic activity when reconstructed in the cDNA and overexpressed. The polymorphism, in contrast, has near normal activity. Both mutations affect evolutionarily conserved residues, suggesting that they are functionally important, while the polymorphism occurs in a nonconserved domain which is presumably not critical for enzymatic function. The F171S mutation is close to the putative active-site nucleophile. Our data further support the notion of molecular heterogeneity of galactosemia and suggest that galactosemia mutations and GALT polymorphisms may be useful tools in highlighting different functional domains in human GALT.

Genetic basis of galactosemia

Classic galactosemia is an inborn error of galactose metabolism and results from deficiency of the ubiquitously expressed enzyme galactose-1-phosphate uridyltransferase (GALT). Nine missense mutations, three splicing mutations, three GALT protein polymorphisms, and one silent nucleotide substitution have been identified to date. Most of the disease-causing mutations are rare among patients. The most common mutation, Q188R, has a frequency of only one-fourth in the patient population examined. Three classes of disease-causing mutations have been reported: CRM+ missense mutations (the most common class), CRM- missense mutations, and splicing mutations. Thus, galactosemia is heterogeneous at the molecular level, which is noteworthy in light of the well-documented clinical variability observed in this disorder. It has also been shown that eight of nine galactosemia missense mutations occur in evolutionarily well-conserved domains, suggesting that they affect functionally and/or structurally important residues. In contrast, all protein polymorphisms alter variable amino acids which presumably are not important for the enzyme's function.