Linda Tyfield

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. Hum Mutat 13:417–430, 1999.

Galactosaemia and allelic variation at the galactose-1-phosphate uridyltransferase gene: a complex relationship between genotype and phenotype.

Abstract More than 160 different base changes have been described at the galactose-1-phosphate uridyltransferase gene and most of these are associated with a disease phenotype. Q188R is the most common mutation in north European populations and those predominantly of European descent. K285N is much rarer but in some countries of east/central Europe it is the second most common mutation. In some populations of northern Europe these two mutations can be found on 70%–80% of mutant chromosomes. Both mutations appear to be associated with a complete loss in enzyme activity and thus, a more severe biochemical phenotype. A single amino acid substitution, N314D, is found on both Duarte 1 and Duarte 2 alleles. Additional base changes that are different on each distinguish D1 from D2 alleles. Whether the differences in galactose-1-phosphate uridyltransferase enzyme activities are associated with the additional molecular changes that distinguish D1 and D2 alleles remains unclear. S135L is found almost exclusively in galactosaemic individuals of African origin. Despite early diagnosis and treatment and adherence to a lactose free diet neurological complications, poor growth and reduced fertility are frequently observed in affected individuals. Conclusion Allelic variation at the galactose-1-phosphate uridyltransferase gene undoubtedly plays a role in defining the biochemical and clinical phenotype. However, clinical galactosaemia is a complex trait in which multiple developmental and metabolic pathways are involved. Ultimately the phenotype is beyond the control of the single gene itself.

Late onset immune pancytopenia following bone marrow transplantation

Summary A 17‐year‐old boy developed autoimmune pancytopenia in the absence of chronic graft‐versus‐host disease 170 d after allogeneic bone marrow transplantation (BMT) from his HLA identical brother. The anaemia and thrombocytopenia responded to conventional immunosuppressive treatment, but the neutropenia was refractory to this and to splenectomy and subsequent removal of splenic remnant.

Persistent hyperthyrotropinaemia since the neonatal period in clinically euthyroid children.

We describe three children, now aged between 5 and 6 years, with a persistent mild hyperthyrotropinaemia since the neonatal period and normal levels of thyroid hormones. The increased thyroid stimulating hormone concentration is not artefactual and is not caused by antibody interferences. Their growth and development is normal and none has received thyroid hormone replacement. We believe that they have compensated hypothyroidism, and that before the advent of screening for congenital hypothyroidism these children would have presented in mid-childhood with juvenile hypothyroidism.

Biochemical control, genetic analysis and magnetic resonance imaging in patients with phenylketonuria

Thirteen patients with phenylketonuria, detected by neonatal screening and started on diet within 16 days of age, were investigated between 10 and 18 years of age by magnetic resonance imaging (MRI) of the brain. Biochemical control was assessed from: (1) the life time blood phenylalanine (Phe) control (as determined from (a) the mean yearly exposure to Phe; (b) the accumulated time for each patient that Phe was <120 μmol/l; (c)>400 μmol/l; (d)>800 μmol/l; and (e)>1200 μmol/l); and (2) the blood Phe control over the 5 years prior to imaging (assessed for each patient by the mean yearly Phe exposure over that period). In all patients the phenylalanine hydroxylase gene locus was studied using restriction fragment length polymorphism haplotypes and mutant genes were screened for a variety of specific mutations which have been reported in other European populations or in populations of north European descent. Two patients had significant abnormalities of cerebral white matter. Although both showed poor biochemical control this did not reach statistical significance when compared to those with normal imaging. DNA haplotype patterns could be assigned to 11 patients and mutant genes were identified in 12. One patient with abnormal imaging and 4 patients without abnormalities had mutations on both chromosomes identified. In these 5 patients there was significant correlation between their genotype and biochemical control. Mutations resulting in residual in vitro enzyme activity were associated with normal imaging.

Prediction of multiple hypermutable codons in the human PAH gene: codon 280 contains recurrent mutations in Quebec and other populations.

The predicted mutability profile (MUTPRED) of the phenylalanine hydroxylase (PAH) gene shows that the 48 CpG sites (template and atemplate strands) are either empty of known mutations (7 sites), harbour “PKU” alleles involving CpG doublets (16 sites), or contain mutations that do not involve a C→ T or G→ A substitution in the doublet. These hypermutable sites harbour 32 different mutations in association with at least 66 different haplotypes and hyperphenylalaninemia. The E280K mutation in exon 7 of the PAH gene is a cause of phenylketonuria. It occurs on four different haplotypes in Europeans and on haplotypes 1 and 2 in Quebec. Whereas a single recombination event could explain the two haplotype associations in Quebec, the mutation does involve a CpG dinucleotide. By analyzing multiallelic markers 5′ (STR) and 3′ (VNTR) to the E280K allele on 12 mutant and 30 normal chromosomes, we conclude that recurrent mutation is the likely origin of E280K in Quebec. The PAH mutation databse shows that the allele accounts for 1.5% of PKU chromosomes worlwide. Hum Mutat 9:316–321, 1997.

Alterations in sulphur-containing amino acids in rat brain extracts on storage

Abstract Difficulties encountered in the amino acid analysis of deproteinised brain tissue extracts are described. Wide variations in some sulphur-containing amino acids occurred as a result of storage at —20°. Increases in the oxidised glutathione and cysteic acid peaks and the appearance of a peak identified as the mixed disulphide of reduced glutathione and cysteine were noted even after 24 h. Attempts have been made to establish the source of cystine molecules which have appeared on storage of the tissue extracts. A simple pH adjustment of the extract to 2.5 before freezing has prevented these changes.