C. R. Scriver

THIAMINE-RESPONSIVE MAPLE-SYRUP-URINE DISEASE

Abstract The rare inborn errors of metabolism are likely to be genetically heterogeneous. A new form of maple-syrup-urine disease in which the hyperaminoacidaemia is completely corrected by thiamine hydrochloride (10 mg. per day) without recourse to dietary restriction, illustrates this hypothesis. This trait is another example of vitamin-responsive hereditary metabolic disease.

Maple syrup urine disease: interrelations between branched-chain amino-, oxo- and hydroxyacids; implications for treatment; associations with CNS dysmyelination.

SummaryFour patients with classical maple syrup urine disease were treated for up to 5885 days per patient with a relaxed protocol allowing branched-chain amino acid levels in plasma to rise about 5 times the normal mean value. The patients have had satisfactory development and lifestyle. They spent 318 days in hospital during 19 937 aggregate treatment days. Plasma levels of leucine and the corresponding 2-oxo acid were shown to be elevated disproportionately relative to the other branched-chain metabolites. Levels of isoleucine and valine were lower than those of leucine apparently because of runout into alternative metabolite pools, namely theR metabolites for isoleucine and the hydroxyacid for valine. The chronic accumulation of branched-chain 2-oxo acid(s) in our patients was associated with chronic dysmyelinating changes in CNS visible by imaging. Another patient with a thiamine-responsive variant of maple syrup urine disease had five acute crises incurring 29 days in hospital in a total of 6910 treatment days. However, she did not have chronic metabolic dyshomeostasis (her average plasma amino acid values were normal) and she had no evidence of dysmyelination. A relaxed treatment protocol for patients with maple syrup urine disease may benefit them in quality of life, but it apparently exacts a cost in metabolic control and CNS pathology.

Newborn Urine Screening Experience with over one Million Infants in the Quebec Network of Genetic Medicine

SummaryWe screened urine for chemical individuality in over 1 million newborn infants, by various chromatographic (thin-layer), chemical and spectrophotometric methods, 12 procedures in all. The programme is part of the Quebec Network of Genetic Medicine. Voluntary urine screening began in 1971 and has evolved with changes in choice of tests and times of sample collection. Urine samples were collected on filter paper at either 5, 14 or 21 days after birth; results were best with the 21-day test. Compliance is over 94% with the latter and over 98% with requests for repeat samples. Screening is centralized in one laboratory; follow-up diagnosis, counselling and management are done at four regional centres. Incidence of phenotypes ranged from 1:4300 live births (for expressed cystinuria alleles) to 1 per million (for hyperargininaemia). Over 20 inherited Mendelian disorders were identified. 30 patients required aggressive medical management. We show how this programme can be used for neuroblastoma screening.

Missense mutations in the phenylalanine hydroxylase gene (PAH) can cause accelerated proteolytic turnover of PAH enzyme: A mechanism underlying phenylketonuria

Phenylketonuria (PKU; McKusick 261600) and related forms of hyperphenylalaninaemia (HPA) are caused by defective activity of the hepatic enzyme phenylalanine hydroxylase (PAH; EC 1.14.16.1). Over 380 alleles of the PAH gene are known, of which over 60% are missense mutations (http ://www.mcgill.ca./pahdb Nowacki et al 1998). The question of how a pathogenic PAH missense mutation exerts its effects on PAH enzyme activity is of abiding interest. Over 40 PAH missense alleles, each identified in persons with PKU/HPA, have been expressed in transiently transfected mammalian cells. Most cause reduced levels of PAH protein, with corresponding reductions in enzyme activity, in lysates of these cells (reviewed by Waters et al 1998a). This has often been loosely ascribed to instability of mutant PAH enzyme, or simply presumed to reflect enzyme degradation. However, enzyme abundance is a function of the rates of both synthesis and turnover, and previous studies have not examined turnover of PAH protein. We have tested the hypothesis that some, perhaps many, PKU-associated missense mutations in the PAH gene are likely to accelerate proteolytic turnover of PAH enzyme. We have now analysed effects of six different PAH missense mutations upon PAH turnover rates, and factors involved in proteolytic degradation of mutant and wild-type PAH enzyme.

USE OF DITHIOTHREITOL TO CORRECT CYSTINE STORAGE IN CULTURED CYSTINOTIC FIBROBLASTS

Abstract Dithiothreitol (D.T.T.) removes cystine from cystinotic fibroblasts. The reagent is not toxic to cells at low concentrations (0.1 m M ). A cystinotic patient tolerated D.T.T. intravenously for 10 days with apparent reduction of cystine storage in rectal mucosa. He died of uraemia 1 month later.

3-Methylglutaconic aciduria: A marker for as yet unspecified disorders and the relevance of prenatal diagnosis in a ‘new’ type (‘type 4’)

SummaryThe Mendelian disorder known as 3-methylgutaconic aciduria (McKusick 250950) gives evidence of allelic and locus heterogeneity. Type 1 has a mild clinical phenotype and confirmed 3-methylgutaconyl-CoA hydratase deficiency; inheritance is autosomal recessive. Other forms have major clinical manifestations and subdivide into X-linked (type 2), a form in Iraqi Jews with optic atrophy (so-called type 3); and untyped (putative autosomal recessive) forms without identified enzyme defects. In the latter, 3-methylglutaconic aciduria may simply be a marker for another metabolic disorder.We describe a male proband with 3-methylglutaconic aciduria designated here as ‘type 4’ (autosomal recessive, with severe psychomotor phenotype and cerebellar dysgenesis). He is the offspring of Italian consanguineous parents. Born with congenital malformations, he has been followed for 18 years, showing profound developmental delay and cerebellar dysgenesis. Measures of hydratase activity in cultured fibroblasts from the proband and 11 additional patients (two with type 1 disease, 9 with either type 2 or an unspecified form) revealed deficient enzyme activity in type 1 cases and normal activity in the proband and the other 11 cases. Two of the untyped cases probably have 3-methylglutaconic aciduria of the type described here.Prenatal diagnosis in the form described here may be feasible by analysis of amniotic fluid metabolites in pregnancies at risk if the mother does not entirely remove elevated concentrations. A female sibling of the proband had normal metabolite values in amniotic fluid. Postnatal follow-up confirmed absence of the disease. We give the normal values for amniotic fluid and results on these additional fetuses at risk (none affected).

The Salience of Garrod’s ‘Molecular Groupings’ and ‘Inborn Factors in Disease’

SummaryGarrods important second book,Inborn Factors in Disease (1931), was about inherited predisposition to disease. Chemical and metabolic individuality, which are the modalities of predisposition, originated in ‘molecular groupings’ (proteins) in Garrods view of life. Such ‘groupings’ as interlocus molecular hybrids, allelic complementation and expressions of modifier genes, can assume variant expression in heterozygotes. Here, it is shown that genetic variation in such ‘molecular groupings’ has clinical relevance, for example (1) in reproductive counselling for thalassaemia; (2) in heterozygosity where the affected enzymes are normally homopolymeric; (3) in clinical severity of ‘monogenic’ disease (e.g. familial hypercholesterolaemia and muscular dystrophy) when variation is not explained by allelic heterogeneity. The associated chemical individuality in each case can be used to identify risk and thus as a mode of predictive medicine.

Ornithine Loading Did Not Prevent Induced Hyper ammonemia in a Patient with Hyperornithinemia- Hyperammonemia-Homocitrullinuria Syndrome

ABSTRACT: Impairment of urea cycle function in hyperornithinemia- hyperammonemia-homocitrullinuria syndrome is presumably caused, in some patients, by deficient transport of ornithine from cytoplasm into mitochondria. We studied the effect of L-ornithine on L-alanine-induced hyperammonemia in a French-Canadian proband with the syndrome by giving: i) a 90-min intravenous alanine load (6.6 mmol/kg) together with ornithine (1.1 mmol/kg); ii) an intravenous ornithine bolus (0.3 mmol/kg) followed by ornithine infusion (1.1 mmol/kg) 90 min prior to loading with alanine and ornithine; iii) ornithine supplementation per os (1 g, four times daily × 2 wk) prior to loading with alanine and ornithine. Blood ammonia increased from high normal values to 975, 990, and 750μmol/liter (normal <70) and urinary orotic acid from trace to 539, 494, and 1296 μmol/mmol creatinine (normal 5-11) after the respective loads. Plasma alanine peaked at 1.56-4.24 mmol/ liter and ornithine at 1.29-1.95 mmol/liter, but other amino acids were stable. Therefore, ornithine loading did not protect this hyperornithinemia-hyperammonemia-homocitrullinuria patient from hyperammonemia induced by amino-nitrogen loading. Renal fractional excretion of citrulline, lysine, ornithine, glycine, alanine, and tyrosine increased more than 3-fold during ornithine priming, whereas all amino acids were excreted in excess after alanine + ornithine loads; homocitrulline excretion remained unchanged; some urine collections indicated “negative reabsorption” (i.e. apparent secretion) of lysine, histidine, and citrulline. Dietary supplementation with ornithine could deplete lysine pools by impairing lysine reabsorption.

Hereditary tyrosinaemia type II in a consanguineous Ashkenazi Jewish family: Intrafamilial variation in phenotype; absence of parental phenotype effects on the fetus

SummaryWe describe an Ashkenazi Jewish family in which two adults, offspring of consanguineous parents, have persistent hypertyrosinaemia (770–1110 µmol/L; normal < 110 µmol/L). The metabolic disorder in this family is apparently due to hepatic cytosolic tyrosine aminotransferase deficiency (hereditary tyrosinaemia, type II; McKusick, 276600), because it is associated with the oculocutaneous manifestations of Richner-Hanhart syndrome. The association of this syndrome with hereditary tyrosinaemia type II is presumed to be constant. It is not in this family. The affected female sib (age 41 years) has hypertyrosinaemia and oculocutaneous signs; the brother (age 39 years) has hypertyrosinaemia but no oculocutaneous disease. Both sibs have two children; none has signs of a metabolic fetopathy. Maternal hypertyrosinaemia and maternal hyperphenylalaninaemia evidently constitute different risk factors for the fetus. Paternal hypertyrosinaemia is apparently not a risk to male infertility.

Prospective ascertainment of complete and partial serum biotinidase deficiency in the newborn

SummaryWe screened 163 000 newborn filter-paper blood samples for serum biotinidase deficiency (McKusick 25326) and found 15 probands: three had complete deficiency (incidence 18.4 cases per million live births, 95% confidence interval 4–54 cases per million); the others had partial deficiency. The positive predictive value of the test for either form of biotinidase deficiency was 9.86%. We found seasonal variation in biotinidase activity in filter-paper blood samples. The cost per test was Can.