J Clin Pathol: first published as 10.1136/jcp.16.4.293 on 1 July 1963. Downloaded from J. clin. Path. (1963), 16, 293 Inherited enzyme defects: a review T. HARGREAVES' From the Department of Chemical Pathology, St. George's Hospital Medical School, Hyde Park Corner, London, S. W.I Sir Archibald Garrod in 1909 pointed out that four The consequences of a genetic change and an diseases, albinism, alkaptonuria. cystinuria, and alteration in the quality or quantity of a protein will pentosuria, were present from birth, lasted through- depend on the role normally subserved by that out life without worsening or improvement, and protein; in the cases that we are considering this is were present in other members of the family (Garrod, derangement of enzymatic function. The derange- 1909). These diseases were described as 'inborn errors ments that can occur include a blocked metabolic of metabolism'. The original four diseases have sequence. In the reaction A---* B-- C--# D, if been expanded to over 50 and the actual site of the C--# D is blocked then a clinical disorder may biochemical abnormality has been elucidated in many result from a failure to produce D, e.g., hypo- of them. In this review we shall consider those glycaemia due to an inability to form glucose from diseases which are either known or thought to be glucose-6-phosphate in von Gierke's disease. The due to single enzyme defects. We have not con- precursor C may accumulate; for example, bilirubin sidered those diseases that are thought to be due to accumulates in the absence of glucuronyl transferase. renal tubular or intestinal abnormalities, for The remote precursers A and B may accumulate, for example, cystinuria. It is not yet known whether example, glycogen in von Gierke's disease. The block this type of disease is due to a specific enzyme between C and D may result in an increased utiliza- defect. tion of normally minor pathways, for example, in The fundamental discovery of Mendel (1901) con- phenylketonuria. The application of these metabolic cerning the inheritance of plant characters had just cycles will be reviewed in each section. been resurrected and given prominence at the time The subject has been covered generally in recent http://jcp.bmj.com/ when Garrod was most active. The transmission of reviews, for example, that of Stanbury, Wyngaarden, a single character as a governing agent for a single and Fredrickson (1960). enzyme and accordingly, as one step in intermediary metabolism, seems implicit in Garrod's writings but DISTURBANCES IN AMINO-ACID METABOLISM is never stated as such. The concept attained clear definition in the 'one gene-one enzyme' principle Recent advances in the past few years in the under- first clearly stated by Beadle (1945). The formulation standing of the biochemistry of the amino-acids on September 25, 2021 by guest. Protected copyright. emerged gradually from studies of eye colour in the coupled with the use of amino-acid chromatography fruit fly Drosophila and received extensive support in urine has helped to elucidate problems in the from work done on induced mutants of Neurospora disturbance of amino-acid metabolism and to crassa. The concept has been expressed as: 1 All demonstrate new cases of disturbed amino-acid biochemical processes in all organisms are under metabolism. genic control. 2 These biochemical processes are resolvable into series of individual stepwise reactions. 3 Each biochemical reaction is under the ultimate PHENYLKETONURIA In 1934 F0oling described 10 control of a different gene. 4 Mutation of a single patients, some of them siblings, who excreted phenyl gene results only in an alteration in the ability of the pyruvic acid and were mentally deficient. Jervis cell to carry out a single primary chemical reaction. (1953) demonstrated that phenylalanine hydroxylase It is now realized that the one gene-one enzyme of the liver was inactive in these patients. The concept does not require a one enzyme-one gene condition is inherited through a single autosomal corollary, and that the 'gene' is being replaced by recessive gene. It is required that both parents of a progressively smaller units of ultimate genetic patient have the defective form of one of the two control. genes controlling phenylalanine hydroxylase; an apparent reduction of phenylalanine hydroxylase IB.M.A. research scholar can be detected in the parents. On the average one 293 J Clin Pathol: first published as 10.1136/jcp.16.4.293 on 1 July 1963. Downloaded from 294 T. Hargreaves Albinism Phenylalanine I , Phenyl- Phenylalanine \\-CH.CH COOH Tyrosinase ketonuria hydroxylase -\NH Tyrosine 3,4 Dihydroxyphenylalanine 3,4 Dopaquinone--oMelanin HO/ CHCHCOOH HO NH. HO- -CH2.CH COOH O= -CH2CH2COC ?Tyrosinosis Tyrosine transam nase NH, NH., p-Hydroxyphenylpyruvic acid - 4 p-Hydroxyphenyllactic acid p hydroxy- - \ \-CHi.,CH COOH Tyrosinosis phenylpyruvic HO/ CH2C COOH HO-/ (Medes) acid oxidase \. O.-Ii0 OH Homr entisic acid Alkaptonuria Homogentisic acid oxidase HO/_-\\HO I\CH ,COOH Maleylacetoacetic acid FIG. 1. Metabolic blocks in thle oxida(tioni of I plhenylalanine ani(l tYrosine. Fumarylacetoacetic acid http://jcp.bmj.com/ Fumaric acid t- acetoacetic acid out of four offspring from two heterozygous parents Mitoma, Auld, and Udenfriend (1957) showed that has both genes defective, and in these offspring two protein fractions are involved in this reaction, a phenylketonuria results. The disability is first labile fraction I found in liver and a more stable manifest several weeks after birth, initially by fraction II found in kidney and heart. The system on September 25, 2021 by guest. Protected copyright. elevation of the plasma phenylalanine level and by required N.A.D. or N.A.D.P. and iron together with the excretion of phenylpyruvic acid. After six an unknown but necessary coenzyme which can be months, retardation of mental development is replaced by tetrahydrofolic acid. Fraction I is deficient apparent. Many of the children have blond hair, in phenylketonuria. blue eyes and fair skin, and a few show a tendency The deficiency of phenylalanine hydroxylase to develop dermatitis or eczema. The majority of causes an excessive accumulation of phenylalanine patients have an I.Q. of 30 or less. in the blood and cerebrospinal fluid. The excessive Pathogenesis Phenylalanine is converted to tyro- phenylalanine is converted by its transaminase to sine by the enzyme system 'phenylalanine hydrox- phenylpyruvic acid, and this in turn is converted to ylase' in the liver alone; the reaction can be repre- phenyllactic acid, phenylacetic acid, and phenyl- sented as acetyl glutamine, which are excreted in the urine. o-Hydroxyphenylacetic acid, m-hydroxyphenylacetic L-phenylalanine.02±+N.A.D.P.H.-1 +H+--+- and ++ H20- acid, and indole products derived from tyrosine L-tyrosine+-N.A.D.P. tryptophan are also found in the urine of such 'The following abbreviations are used The excessive phenylalanine inhibits the A.T.P. Adenosine Triphosphate patients. N.A.D. Nicotinamide-adenine dinucleotide normal pathways of tyrosine metabolism, there is a N.A.D.P. Nicotinamide-adenine dinucleotide phosphate decreased formation of melanine accounting for U.D.P.G.A. Uridine diphosphate glucuronic acid U.D.P. Uridine 5'-pyrophosphate the blond hair and blue eyes; plasma adrenaline J Clin Pathol: first published as 10.1136/jcp.16.4.293 on 1 July 1963. Downloaded from Inherited enzyme defects: a review 295 levels are low in phenylketonurics. The excessive bolism of phenylalanine and tyrosine, cannot be phenylalanine or one of its products possibly metabolized; it accumulates and is excreted in the damages the central nervous system; the patients are urine. If the urine is allowed to stand it becomes mentally retarded and have epileptic seizures and dark and this sign leads to early recognition of the abnormal electroencephalograms. disease. Boedeker (1859) first diagnosed the disease The condition can be diagnosed by the presence with certainty and used the property of avid oxygen of phenylpyruvic acid in the urine and an increase uptake to give the substance a name 'alkapton'. of phenylalanine in the plasma. Heterozygotes can Garrod in 1909 wrote of the probable defect: 'We be detected by phenylalanine tolerance tests. The may further conceive that the splitting of the benzene homozygous condition can be ameliorated by giving ring in normal metabolism is the work of a special the infants a diet low in phenylalanine, which has no enzyme, that in congenital alkaptonuria this enzyme effect on older children, demonstrating that early is wanting, whilst in disease its working may be diagnosis of the disease is essential. partially or even completely inhibited'. The con- dition is compatible with long life. Except for the TYROSINOSiS This is a very rare inborn error of dark-coloured urine there are no clinical manifesta- metabolism described by Medes (1932). Her patient tions until the second or third decade when was a male Russian Jew who excreted large quantities ochronosis begins to appear. Black pigments are of p-hydroxyphenylpyruvic acid and other tyrosine deposited in various parts of the body, and eventually metabolites but had no clinical symptoms. Medes, the patients develop deforming arthritis in middle Berglund, and Lohmann (1927) found an excess of age. p-hydroxyphenylpyruvic acid in the urine of a Garrod (1902) first described the inheritance of patient with myasthenia gravis, and Felix, Leonhardi. alkaptonuria and presented evidence that the con- and Glasenapp (1951) made similar observations on dition was congenital and familial. He suggested two patients with liver disease. The amount of that it was inherited as a single recessive gene. p-hydroxyphenylpyruvic acid excreted was consider- A study of the cases reported since that time has ably less than 50 mg. per day and it is probable that supported the view that the defect is due to a single these cases are of a different type of metabolic recessive autosomal gene, and gains further support abnormality from the case described by Medes.