Peroxisomal disorders

Although peroxisomes were once considered a vestige, their importance in cellular metabolism is clearly established by the many inherited diseases that have been described in the past two decades. Unfortunately there is no definitive treatment for the various disorders, but based on the recognition of the biochemical abnormalities, prenatal testing and appropriate genetic counseling can be provided. It is essential for clinicians to be aware of this group of diseases, as diagnosis and further study of these patients are essential in understanding the basic etiologic mechanisms underlying these complex groups of disorders. Clearly, there is much to be learned about the relationship between biochemical abnormalities and the phenotypic variability of the peroxisomal disorders.

[Pre- and postnatal diagnosis of organoacidopathies]

Organoacidopathies are the most common life-threatening inborn errors of metabolism presenting acutely in the neonatal period. Early diagnosis rests on a high degree of suspicion. Clinical and laboratory findings are often nonspecific and can be misinterpreted. We present an algorithm for a quick and comprehensive diagnosis of these disorders using commonly available parameters. Different methods for the prenatal diagnosis of organoacidopathies are discussed and our experience with over 150 cases presented. The method of choice is the precise quantification of elevated levels of metabolites in amniotic fluid obtained by amniocentesis at 12-18 weeks of pregnancy. Quantification is best done by stable isotope dilution analysis with the addition of the labelled metabolite to the amniotic fluid. A positive prenatal diagnosis allows a decision of the family for a termination of pregnancy or the immediate institution of therapy after birth. The conduction of a prenatal diagnosis requires the knowledge of the exact diagnosis of a previously affected child.

[A staged plan for laboratory diagnosis of hereditary metabolic diseases]

When clinical evidence provides grounds for suspecting inborn errors of metabolism it is urgent to perform the necessary, relevant, specific laboratory investigations in good time and with a view to quality. Normally, the realization depends on individual initiatives and the use of laboratories mainly designed for pediatrics and human genetics. Consequently the results are equally a matter of chance. Nothing in this situation can be changed in principle by using the catalogue of services of the Society for Human Genetics of the GDR. Central administrative provisions are necessary to improve the present unsatisfactory situation. Proposals for regulations, division of responsibility and a graduated programme of parameters are discussed here with a view to establishing uniform procedures.

Enzyme-replacement therapy: problems and prospects

Several diseases can, at least in theory, be treated by the administration of an enzyme, the deficiency of which is the cause of the disease. Various attempts have been made to correct enzymatic deficiencies responsible for the clinical manifestation of diseases for which prevention cannot be achieved by modification of the diet or by supportive therapy with drugs. Except for treating certain digestive disorders, enzyme-replacement therapy has not yet found a broad application. In this review article a compilation is given of the problems and prospects of enzyme-replacement therapy in diseases caused by the deficiency of an enzyme.

[Hemolytic anemias caused by erythrocyte enzyme disorders]

The congenital alterations of red cell enzymes are one of the most frequent causes of red cell hemolysis. The enzyme defects have more important effects on the red cells than any other cells in the body with the same alteration. It is because red cells do not have a protein regenerating system, this meaning that they have to keep the enzyme level during the circulation time. In order to study this defect, they are classified as follows: a) defects of glycolytic pathway; b) defects of glutathionE++ pathway; c) defects of the nucleotidic metabolism. The most frequent defects are: the deficiency of: G6PD, of pyruvate kinase, of glucose phosphate isomerase and probably pyrimidine 5'-nucleotidase deficiency.

Disorders of glycogen metabolism of muscle

Glycogen is a crucial source of energy in the initial stages of muscle activity and during exercise of high intensity. There are 10 well-defined biochemical defects of glycogen metabolism expressed in muscle and affecting the following enzymes: alpha 1,4 glucosidase (glycogenesis type II), debrancher enzyme (III), brancher enzyme (IV), phosphorylase (V), phosphofructokinase (VII), phosphorylase b kinase (VIII), phosphoglycerate kinase (IX), phosphoglycerate mutase (X), lactate dehydrogenase (XI). These disorders cause two main syndromes: one characterized by exercise intolerance with cramps and myoglobinuria, the other by fixed weakness. However, there are examples of clinical and biochemical heterogeneity for each disease, and molecular genetic analysis is already showing evidence of genetic heterogeneity. Although our understanding of the biochemical errors has progressed considerably, the pathogenesis of symptoms and signs remains incomplete.

A histochemical principle bearing on correctness of diagnosis in storage diseases

A principle is proposed which may help pathologists avoid errors in diagnosis of storage diseases. Tissues from patients in whom a tentative diagnosis of a metabolic disorder has been made often store a number of metabolites in the cells. The presence of these metabolites can occur in single-enzyme or activator defects as a result of the following causes: (a) deposition of metabolites situated near the main substrate of the defective enzyme in the catabolic path, and compounds which were changed after they were deposited; (b) presence of multiple substrates for this enzyme; (c) co-deposition of molecules bound to the main substrates; (d) existence of multiple substrates for a single defective activator molecule. In contrast to these causes, variability in processes not associated with a single-enzyme or activator deficiency may be due to the following: (e) inhibition of multiple hydrolases by drugs or metabolites; (f) localization of substrates and hydrolases in different compartments; (g) multiple enzyme deficiencies; (h) concentration of metabolites beyond the catabolic capacity of cells. According to the proposed principle, diagnosis of storage disease resulting from a single enzyme deficiency can be negated if a wide-range histochemical test shows that the main substrate of a deficient enzyme is not present in some primary storage cells. The validity of the principle and possible pitfalls are discussed.

[Enzyme deficiency in the lymphocytes of tobacco smokers]

In the peripheral blood lymphocytes of 85 men aged 18 to 42 years, smoking cigarettes by 2 to 25 years, the intracellular enzymes activity having varying localization, has been determined by the use of semiquantitative histochemical methods. The lymphocytes from subjects smoking not more than 10 years the increased activity of acid phosphatase, leucyl aminopeptidase, and lactic dehydrogenase as well as decreased activity of N-acetyl-beta-D-glucosaminidase has been stated. In contrast, in subjects smoking more than 10 years the intracellular activity of acid phosphatase, beta-glucuronidase, N-acetyl-beta-D-glucosaminidase and lactic dehydrogenase in lymphocytes has been diminished. Above enzymatic deficiencies could represent the biochemical basis of functional alterations of lymphocytes from smokers observed by numerous authors.

The enzymatic deficiencies in lymphocytes of smokers

In the peripheral blood lymphocytes of 85 men aged 18 to 42 years having smoked cigarettes for 2 to 25 years, the activity of various intracellular enzymes was determined using the semiquantitative histochemical methods. The lymphocytes of people having smoked for less than 10 years were characterized by increased acid phosphatase (AP), leucine aminopeptidase (LAP) and lactic dehydrogenase (LDH) activities whereas that of N-acetyl-beta-D-glucosaminidase (NAG) were lowered. In subjects having smoked for more than 10 years lower acid phosphatase, beta-glucuronidase, N-acetyl-beta-D-glucosaminidase and lactic dehydrogenase activities have been observed. According to the authors' suggestions, the functional lymphocyte abnormalities noted in smokers by numerous investigators may be associated with the intracellular enzymatic deficiencies.

[Hypoglycemias in childhood. Pathophysiologic reflections and examples of hypoglycemias in hereditary disorders of carbohydrate metabolism]

For the understanding and interpretation of hypoglycemia it is important to know the many complex endocrine and metabolic regulations in the homoeostasis of blood glucose. Glucose-absorption, distribution and availability, glycolysis, production and utilization of glycogen as well as gluconeogenesis are important steps of this homoeostasis, and hypoglycemia always reflects a disturbance in it. When blood glucose is low the availability of energy for the brain is decreased if no alternative energy sources like lactate or ketones are provided. Hypoglycemia is more often in the neonatal period than in later childhood. The causes can be divided into different groups according to pathogenetic mechanisms. Within each group again many singular defects are known. Fructose-1,6-diphosphatase deficiency, hereditary fructose intolerance, glycogenosis type I and so called "ketotic hypoglycemia" are given as examples to elucidate special clinical and biochemical aspects of inborn errors of carbohydrate metabolism.

[Metabolic defects with hypoketotic hypoglycemia]

Metabolic defects resulting in hypoketotic hypoglycemia can lead to hepato-encephalopathy and can be lethal. Recognition of the association of hypoglycemia with hypoketonemia is essential for efficient diagnostic and therapeutic procedures. The pattern of urinary excretion of organic acids is useful in differential diagnosis between the possible metabolic defects, viz. carnitine deficiency, carnitine palmitoyl transferase deficiency, medium-chain, long-chain and multiple acyl-CoA dehydrogenase deficiencies, and HMG-CoA lyase deficiency. These (except for carnitine deficiency) can be confirmed by enzyme activity measurements in cultured fibroblasts and tissue biopsies and prenatally. Treatment is available for all of them except some cases of multiple acyl-CoA dehydrogenase deficiency. Genetic counselling of the families must be based on a precise biochemical diagnosis.

The biochemical basis of mitochondrial diseases

Dysfunctioning of human mitochondria is found in a rapidly increasing number of patients. The mitochondrial system for energy transduction is very vulnerable to damage by genetic and environmental factors. A primary mitochondrial disease is caused by a genetic defect in a mitochondrial enzyme or translocator. More than 60 mitochondrial enzyme deficiencies have been reported. Secondary mitochondrial defects are caused by lack of compounds to enable a proper mitochondrial function or by inhibition of that function. This may result from malnutrition, circulatory or hormonal disturbances, viral infection, poisoning, or an extramitochondrial error of metabolism. Once mitochondrial ATP synthesis decreases, secondary mitochondrial lesions may be generated further, due to changes in synthesis and degradation of mitochondrial phospholipids and proteins, to mitochondrial antibody formation following massive degradation, to accumulation of toxic products as excess acyl-CoA, to the depletion of Krebs cycle intermediates, and to the increase of free radical formation and lipid peroxidation.

Functional hemizygosity in the human genome: direct estimate from twelve erythrocyte enzyme loci

Cord blood samples from 2020 unrelated newborns were screened for levels of enzyme activity for twelve enzymes. The level of enzymatic activity for 100 determinations were consistent with the existence of an enzyme-deficiency allele. The frequency of deficiency alleles in the Black population (0.0071) was four times higher (after removal of the G6PD*A- variant) than in the Caucasian sample (0.0016). These frequencies are approximately double the frequency of rare electrophoretic mobility variants at similar loci in the same population. Given the number of functionally important loci in the human genome, these enzyme deficiency variants could constitute a significant health burden.

Peroxisomal disorders: clinical characterization

The peroxisomal disorders can be divided into three classes: firstly, those in which the activity of only one single enzyme is reduced; secondly, those in which the activities of multiple peroxisomal enzymes are deficient and also the number of peroxisomes is reduced; and thirdly, those in which the activities of multiple peroxisomal enzymes are lacking and at the same time the number of peroxisomes is normal at least in liver tissue. The cerebro-hepato-renal syndrome of Zellweger is the prototype of peroxisomal disorders of the second group. Clinical distinction between Zellweger syndrome and neonatal adrenoleukodystrophy or infantile Refsum disease can be impossible. The clinical abnormalities that should give rise to suspicion for the presence of a peroxisomal disorder and urge the necessity of further biochemical studies are proposed.

Haemoglobinopathies, thalassaemias and enzymopathies in Saudi Arabia: the present status

The presence of the sickle cell (Hb S) gene in Saudi Arabia was first reported by Lehmann et al. in 1963 [Nature 198, pp. 492-493]. Later, Hb S, alpha- and beta-thalassaemia, glucose-6-phosphate dehydrogenase deficiency and other enzymopathies were shown to occur at a variable prevalence in different regions of the country. Recent studies using restriction endonucleases have revealed alpha-globin gene arrangement and beta-globin gene polymorphism in the Saudi population. Interactions between abnormal genes are commonly encountered which often influence the clinical manifestations of sickle cell disease. In this paper, we present recent findings and discuss the status of haemoglobinopathies, thalassaemias and enzymopathies in Saudi Arabia.

Zellweger syndrome: biochemical procedures in diagnosis, prevention and treatment

In patients with cerebro-hepato-renal (Zellweger) syndrome, the absence of peroxisomes results in an impairment of metabolic processes in which peroxisomes are normally involved. These include the catabolism of very long chain (greater than C22) fatty acids, the biosynthesis of ether-phospholipids and of bile acids, the catabolism of phytanic acid and the catabolism of pipecolic acid. Many diagnostic tests for Zellweger syndrome have become available in recent years. In classic Zellweger syndrome abnormal C27-bile acids, very long chain fatty acids, dicarboxylic acids and pipecolic acid accumulate in the plasma of the patients. Moreover, depending upon the diet, plasma phytanic acid concentrations may be elevated. In platelets the activity of acyl-CoA: dihydroxyacetone phosphate acyltransferase is deficient; in erythrocytes from young (less than 4 months) patients the plasmalogen content of the phospholipids is decreased. In cultured fibroblasts from skin and from chorionic villus and cultured amniotic fluid cells from Zellweger patients the plasmalogen level is lowered; there is a decreased activity of acyl-CoA: dihydroxyacetone phosphate acyltransferase, alkyl dihydroxyacetonephosphate synthase and phytanic acid oxidase; the de novo biosynthesis of plasmalogens and the peroxisomal beta-oxidation of fatty acids are impaired and the intracellular localization of catalase is abnormal. Dietary treatment of patients with Zellweger syndrome has not so far resulted in an objective clinical improvement. As Zellweger syndrome is usually fatal in early life, prenatal diagnosis of the disease is important.