Organic acidurias: a review. Part 2

Spectrum of Organic Aciduria Diseases in Tunisia: A 35-year Retrospective Study

Background

Organic aciduria diseases (OADs) occur worldwide, with differences in prevalence and patterns between populations.

Objectives

To describe the spectrum of OADs identified in Tunisia over a 35-years period.

Materials and Methods

This retrospective study included patients who were diagnosed with OADs between 1987 and 2022 in the Laboratory of Biochemistry, Rabta Hospital, Tunisia. Organic acids were analyzed using gas chromatography-mass spectrometry.

Results

A total of 30,670 urine samples were analyzed for OADs, of which 471 were positive for OADs. The estimated incidence of OADs in Tunisia was 6.78 per 100,000 live births. Methylmalonic (n = 146) and propionic (n = 90) acidurias were the most common OADs (estimated incidence: 2.10 and 1.30 per 100,000 live births, respectively). There were 54 cases of L-2-hydroxyglutatric acidurias and 30 cases of pyroglutamic acidurias, which makes it one of the highest in the world. The main clinical features were hypotonia (65%) and feeding difficulties (41%). Age at diagnosis was highly variable, ranging from 1 day to 49 years. Only 27% of the patients were diagnosed within the first month of life. The prevalence of OADs was highest in the Center-East and Southeast regions.

Conclusions

In Tunisia, OADs are relatively frequent, but there are shortcomings regarding the diagnosis of these disorders. The frequency and health/social impact of these disorders warrant the need for implementing newborn screening programs and suitable patient management.

N-acylethanolamines as novel alcohol dehydrogenase 3 substrates

N-acylethanolamines (NAEs) are members of the fatty acid amide family. The NAEs have been proposed to serve as metabolic precursors to N-acylglycines (NAGs). The sequential oxidation of the NAEs by an alcohol dehydrogenase and an aldehyde dehydrogenase would yield the N-acylglycinals and/or the NAGs. Alcohol dehydrogenase 3 (ADH3) is one enzyme that might catalyze this reaction. To define a potential role for ADH3 in NAE catabolism, we synthesized a set of NAEs and evaluated these as ADH3 substrates. NAEs were oxidized by ADH3, yielding the N-acylglycinals as the product. The (V/K)(app) values for the NAEs included here were low relative to cinnamyl alcohol. Our data show that the NAEs can serve as alcohol dehydrogenase substrates.

Urine organic acid analysis for inherited metabolic disease using gas chromatography-mass spectrometry

Urine organic acid analysis is an essential component of the workup of the patient suspected to have an inborn error of metabolism (IEM). Urine contains several hundred different organic acids, which arise from a multitude of different sources including both normal and abnormal metabolism. They may also arise from drugs and drug metabolism or from xenobiotics and dietary supplements. In addition to the diagnosis of inborn errors of metabolism, the identification of organic acids in a urine sample has a wide range of potential applications, including toxicology and poisonings. The method described below extracts the acidic fraction from urine samples, derivatizes the extracted compounds, and identifies intermediate metabolites by GC-MS. The method utilizes electron impact ionization gas chromatography-mass spectrometry (GC-MS) with total ion collection.

Glutaric aciduria type II [corrected] and brain tumors: a case report and review of the literature

Heritable diseases associated with childhood tumors are sometimes defined as a probable etiologic factor or a coincidence. First of all, we must know the actual number of patients. Herein a case with medulloblastoma associated with glutaric aciduria type II [corrected] is reported for this purpose. A 5-year-old boy was admitted with nausea, vomiting, and lethargy. In medical history, consanguinity and siblings with mental-motor retardation and epilepsy are remarkable. Growth retardation, macrocephaly, lethargy, tremor, bilateral nistagmus, and papilledema were prominent features in physical examination. Noncontrast computed tomography of the brain showed a hyper dense mass in the cerebellar vermis. Gross total resection was made and the histopathology of the tumor was medulloblastoma. Besides medical history and physical findings, radiologic white matter changes in the subcortical, periventricular regions, bilateral basal ganglia, and caudate nuclei in magnetic resonance images other than tumor led us to investigate the child for glutaric aciduria type II [corrected]. The level of the 2-OH glutaric acid was determined as being 12-fold high in the urine. Chemo-radiotherapy was performed after surgery. Our case was the third patient with medulloblastoma in the literature and is still alive with no evidence of the disease 19 months after the initial diagnosis.

Expanded newborn screening of inherited metabolic disorders by tandem mass spectrometry: Clinical and laboratory aspects

Newborn screening started in the 1960s for the purpose of identifying phenylketonuric patients to begin early intervention and to prevent mental retardation in these patients. Soon thereafter, screening programs expanded to include additional genetic disorders added individually one at a time. In the 1980s, tandem mass spectrometry (MS/MS) was introduced in clinical laboratories, and in the 1990s, the technique was used for newborn screening. Unlike measuring one analyte at a time, MS/MS allows measurement of >40 analytes, in a few minutes with the use of a single assay. Currently, MS/MS is being used for the identification of several amino acid, organic acid and fatty acid disorders. Several states in the United States and many other countries are using MS/MS in newborn screening. However, there is a significant disparity among different newborn screening programs for disorders being screened by MS/MS and many other challenges are faced by the expanded newborn screening. It is anticipated that in the future the use of MS/MS in newborn screening will expand both at the analyte and geographic levels. Clinicians and laboratory scientists should become familiar with MS/MS, disorders being screened in their patients' population and the future of this emerging technology.

A biochemical perspective on the use of tandem mass spectrometry for newborn screening and clinical testing

The first newborn screen was a clinical test to detect a disorder of the biochemistry of the amino acid, phenylalanine. This disorder, known as phenylketonuria, produces profound mental retardation if not detected and treated early in life. Early screening programs relied on inexpensive population screening techniques that have all but been replaced by more accurate analytical methods such as tandem mass spectrometry (MS/MS). MS/MS enables a multianalyte approach for detecting biochemical disorders such that a metabolic profile is obtained rather than a single analyte measurement. The metabolic profile has clearly shown improvements in the detection of diseases such as phenylketonuria and several new disorders arising from errors in fatty acid oxidation and organic acid metabolism. MS/MS is a powerful tool for accessing the metabolic status of a newborn and can detect both inborn metabolic errors as well as examine the effect of acquired diseases or pharmacologic intervention on intermediary metabolism.

L-2-Hydroxyglutaric aciduria: identification of a mutant gene C14orf160, localized on chromosome 14q22.1

l-2-Hydroxyglutaric aciduria (l-2-HGA) is characterized by progressive deterioration of central nervous system function including epilepsy and macrocephaly in 50% of cases, and elevated levels of l-2-hydroxyglutaric acid in urine, blood and cerebrospinal fluid (CSF). Nuclear magnetic resonance imaging shows distinct abnormalities. We report the identification of a gene for l-2-HGA aciduria (MIM 236792) using homozygosity mapping. Nine homozygous mutations including three missense mutations, two nonsense mutations, two splice site mutations and two deletions were identified in the gene C14orf160, localized on chromosome 14q22.1, in 21 patients from one non-consanguineous and 14 consanguineous Turkish families. We propose to name the gene duranin. Duranin encodes a putative mitochondrial protein with homology to FAD-dependent oxidoreductases. The functional role of this enzyme in intermediary metabolism in humans remains to be established.

The application of tandem mass spectrometry to neonatal screening for inherited disorders of intermediary metabolism

This review is intended to serve as a practical guide for geneticists to current applications of tandem mass spectrometry to newborn screening. By making dried-blood spot analysis more sensitive, specific, reliable, and inclusive, tandem mass spectrometry has improved the newborn detection of inborn errors of metabolism. Its innate ability to detect and quantify multiple analytes from one prepared blood specimen in a single analysis permits broad recognition of amino acid, fatty acid, and organic acid disorders. An increasing number of newborn screening programs are either utilizing or conducting pilot studies with tandem mass spectrometry. It is therefore imperative that the genetics community be familiar with tandem mass spectrometric newborn screening.

Profiling of organic acids by capillary gas chromatography-mass spectrometry after direct methylation in urine using trimethyloxonium tetrafluoroborate

Trimethyloxonium tetrafluoroborate (TMO) is applied as derivatising reagent to transform urinary organic acids into their methyl esters. The method is suggested as an alternative to the use of diazomethane which is carcinogenic and explosive. In contrast to other methods avoiding diazomethane, such as derivatizations with acetyl chloride-methanol and boron trifluoride-methanol, which require an organic reaction medium and therefore an extraction of the organic acids from the urine, TMO efficiently reacts with the acids in an aqueous solution and can therefore be directly applied to native urine. The use of TMO simplifies and improves the sample preparation in the profile analysis of urinary organic acids by capillary GC-MS and hereby increases the speed of analysis. The method gives reproducible results which are comparable with the data obtained using conventional solid-phase extraction with strong anion-exchange cartridges prior to derivatisation.

Analysis of organic acids, amino acids, and carnitine in dogs with lipid storage myopathy

Abnormal accumulations of lipid droplets, localized predominantly in histochemical type 1 fibers, were observed in fresh frozen sections of muscle biopsies from 25 dogs with myalgia, weakness, and muscle atrophy. Compared to controls, lactic acidemia, hyperalaninemia, lactic and pyruvic aciduria, variably increased urinary excretion of carnitine esters, and muscle carnitine deficiency were present. These findings support a metabolic block in oxidative metabolism resulting in lactic acidemia in dogs with lipid storage myopathy.

Developmental profile of mitochondrial glycine N-acyltransferase in human liver

OBJECTIVE

To study the developmental profile of glycine N-acyltransferase (GAT) in the livers of children of various ages and to compare the total and specific GAT activity with that of the adult control subjects.

METHODS

We measured the specific and the total mitochondrial activity of GAT in liver samples taken from 13 children 4 hours to 11 years of age. The samples were compared with those of control adults aged 24 to 40 years. Samples, either from liver-transplant donors or from autopsy, from those who died of a disorder not related to the liver, were obtained between 6 and 36 hours after death.

RESULTS

At 4 hours after birth, very low specific activity and the total liver mitochondrial activity were observed (0.19 mumol/min per milligram protein and 210 mumol/min), with a steady increase up to age 7 months (2.51 mumol/min per milligram protein and 812 mumol/min). The mean specific and total GAT activity in children (n = 5) aged 18 months to 11 years was 6.38 +/- 0.13 and 1389 +/- 43 and in control adults aged 24 to 40 years (n = 3) was 6.5 +/- 0.3 and 1461 +/- 71 mumol/min per milligram protein and mumol/min, respectively. These specific and total GAT activity values from children aged 18 months to 11 years were not statistically significant (by analysis of variance and Mann-Whitney test) in comparison with the corresponding activity values from the adult control subjects.

CONCLUSIONS

Our results indicate that up to age 7 months, children have only 5% to 40% of liver GAT-specific activity, whereas the peak activity is achieved at 18 months and remains constant until age 40 years. The delayed development of GAT in children may thus compromise the detoxification of various drugs and xenobiotics.

Methylmalonic and malonic aciduria in a dog with progressive encephalomyelopathy

A 12 week old female Labrador retriever dog with signs of progressive diffuse degeneration of the brain and spinal cord was found to have methylmalonic and malonic aciduria. Over a 5 month period, the dog developed neurologic signs compatible with disease of the central nervous system with predominant diffuse cerebral and right lateralizing brainstem deficits. Gross pathological examination of the brain showed that the lateral, third, and fourth ventricles of the brain were markedly enlarged and associated with white and grey matter atrophy. Syringomyelia and hydromyelia of the central canal into the dorsal funiculus of the spinal cord beginning at the level of the cervical intumescence and extending to the lumbar intumescence was also present. Significant biochemical abnormalities include methylmalonic and malonic aciduria, mild lactic and pyruvic aciduria. There was also accumulation of citric acid cycle intermediates including succinic, aconitic, and fumaric acids. Disordered fatty acid oxidation was suggested by increased excretion of adipic, ethylmalonic, suberic and sebacic acids. Neither ketoacidosis nor hyperammonemia were present, and serum cobalamin levels were normal. Overall, this dog demonstrates an inborn error of metabolism resulting in abnormal organic acid accumulation associated with a neurodegenerative disease.

Mid-second trimester organic acids in severe open neural tube defects

OBJECTIVE

To investigate amnioic fluid organic concentrations at 16 weeks gestation from normal fetuses and those affected by severe neural tube defects.

METHODS

Amniotic fluid collected from normal, anencephalic and severe open spina bifida fetuses was analysed for up to 24 organic acids by gas liquid chromatography.

RESULTS

Significant increase in organic acids similar to those observed in defects of phenylalanine and tyrosine metabolism were identified when compared with the normal fetuses. Significant differences in organic acid concentrations were also identified within the two neural tube defect groups.

CONCLUSION

A relationship between phenylalanine and tyrosine metabolism, closure rate of the neural tube and folate metabolism is proposed.

CT and MR of the brain in the diagnosis of organic acidemias. Experiences from 107 patients

The results of CT and/or MRI of the brain in 107 patients with different types of organic acidemia are presented. The CSF spaces were wide in more than two-thirds of the patients, in 46 slightly-to-moderately and in 26 markedly-to-severely dilated. Marked widening of the operculae was found in all 5 patients with glutaric acidemia type 1, but open opercula was also found in other organic acidemias. White matter changes were found in about half the patients, in 28 mildly-to-moderately pronounced, in another 28 marked or severe. Basal ganglia or central pathway pathology was seen in a total of 34 patients, i.e. 32%. These changes in 25 patients involved the caudate and/or lentiform nuclei: in 14 cases the T2 signal was increased and volume loss was present, in 9 cases increased T2 signal with preserved volume was found (in one of these the changes were transient). In 2 patients, both with ethylmalonic aciduria (cause unknown), only small high T2 spots were seen in the caudate heads and the putamina. In 4 patients, all suffering from methylmalonic acidemia, only the globus pallidus was affected. In 3 patients, all with beta-ketothiolase deficiency, high T2 intensity changes were seen only in the postero-lateral putamina. The remaining 8 patients represent a variety of different locations of lesions. The CT or MRI findings in many patients with organic acidemias should alert the radiologist that a neurometabolic disorder may be present; in some cases the location and appearance of the lesions may even suggest the correct diagnosis.

Unusual presentations of propionic acidemia

The files of 25 patients with propionic acidemia (PA), followed by the Inborn Errors of Metabolism Service (IEMS) at King Faisal Specialist Hospital and Research Centre (KFSH & RC) from 1990 to 1993, were studied retrospectively. In 14 patients PA presented acutely with acidosis, hyperammonemia and thrombocytopenia, while in 11 patients the presentation of the disease was unusual. In the latter group, two neonates with PA initially appeared as a primarily hyperammonemic metabolic disease. In two other neonates the vomiting was so severe that they were diagnosed as intestinal obstruction in referral hospitals. The presentation in three infants was primarily as an immune disorder. In four infants, PA appeared as an acute or chronic encephalopathy, i.e. as a silent organic acidemia, with few other findings of the disease. The clinical picture of PA includes facial and nipple dysmorphia, severe hypotonia and vomiting. Severe thrombocytopenia is the hallmark of the metabolic crisis. In one patient it was noticed late and caused intracranial hemorrhage, while in three others intracranial bleeding caused death. The prognosis in PA remained grave despite rigorous treatment. Only seven of the 25 PA patients remained to have a normal life-style, while eight patients expired. The diagnosis is readily achieved by urine gas chromatography/mass spectrometry (GC/MS), by tandem mass spectrometry (MS/MS), or by enzyme analysis of fibroblasts. While there may be both examiner- and patient-related reasons for the variations in the presentation of PA, one other reason may be the heterogeneity of the molecular defect in propionyl-CoA carboxylase.

A study of the response to protein-modified diets for propionic acidemia in twelve patients

The charts of twelve patients with propionic acidemia (PA) were analyzed retrospectively for the outcome of dietary treatment. The diet was prescribed for each patient based on their individual needs. We established good diet compliance in four PA patients who were alive and well with healthy life-styles at the time of this report. The main difficulty causing non-compliance was the parents' inability to take care of the patient due to other family responsibilities. In two cases, the parents believed their children to be cured and they abandoned diet and medications, resulting in the sudden death of both children. In four cases, the parents were unable to cope with the demands of the diet and medications, while complications and severe classic PA provided no hope for two patients.

A new patient with alpha-ketoglutaric aciduria and progressive extrapyramidal tract disease

A 4.5-year-old boy with chronic progressive encephalopathy is described. The clinical presentation initially included seizures and hypotonia which later evolved into severe extrapyramidal disease and dementia. The gas chromatography/mass spectrometry (GC/MS) analysis of urine indicated that alpha-ketoglutarate was increased 210 times and aconitic acid 80 times. No disturbance of acid/base balance, lactic acid or ammonia metabolism accompanied this clinical picture. The fibroblasts contained 29% of normal alpha-ketoglutarate dehydrogenase activity, while the activity of another mitochondrial marker enzyme, glutamate dehydrogenase, was normal. The neuroimaging studies revealed bilateral striatal necrosis. The clinical and biochemical findings were almost identical to two previously reported patients. Experience with this patient emphasizes the need for detailed organic acid biochemical investigation in any progressive encephalopathy and that extrapyramidal tract signs should evoke the possibility of alpha-ketoglutaric aciduria, among other 'neurologic organic acidemias'.

Comparative frequency and severity of hypoglycemia in selected organic acidemias, branched chain amino acidemia, and disorders of fructose metabolism

The Institution's experience with hypoglycemia in different types of organic acidemias, branched chain amino acidemia (MSUD), and disorders of fructose metabolism was reviewed retrospectively. The charts of 144 patients who were followed for 1-5 years were studied for the severity and frequency of hypoglycemia. The patients were mainly Saudi; however, 10-25% were from neighboring countries. Therefore, the observations pertain to the genetic groups in the Arabian peninsula. Organic acidemias which primarily manifest with neurologic signs, such as 4-hydroxybutyric aciduria, infantile onset 3-methylglutaconic aciduria, and glutaric aciduria type 1 never showed hypoglycemia. Patients with beta-ketothiolase deficiency, biotinidase deficiency, or intermittent or intermediate MSUD, also did not have hypoglycemia during metabolic crisis. Hypoglycemia was rare and mild among neonates with classic MSUD, ethylmalonic aciduria, and isovaleric acidemia. Less than 50% of the patients with MSUD older than 8 months, pyruvate carboxylase deficiency, methylmalonic acidemia, or propionic acidemia had hypoglycemia during metabolic crisis. On the other hand, patients with 3-hydroxy-3-methyl glutaryl-CoA lyase deficiency, holocarboxylase synthetase deficiency, medium or long-chain acyl-CoA dehydrogenase deficiency, neonatal onset 3-methylglutaconic aciduria, glutaric aciduria type 2, and disorders of fructose metabolism invariably had moderate-to-severe hypoglycemia associated with metabolic crisis. The purpose of this report is to provide the pediatrician, particularly in the Middle East, with a diagnostic guideline to the identification and management of different types of organic acidemias, based on co-existing hypoglycemia.

3-Ketothiolase deficiency: a review and four new patients with neurologic symptoms

3-Ketothiolase deficiency (3KTD) manifests with intermittent acidosis and is due to deficiency of mitochondrial 2-methylacetoacetate thiolase. Only 22 patients have been previously reported. Although its variable clinical presentation is recognized, the associated neurological findings have not been detailed. We report four new patients all with significant neurological symptoms. Three patients were examined with MRI of the brain which showed increased T2 intensity within the posterior lateral part of the putamen bilaterally. In two the MRI was otherwise normal; in one delayed myelination was also seen. These MRI putaminal findings may be typical enough to suggest the diagnosis of 3KTD. Two of the three had abnormal EEGs; one had an abnormal VEP. 3KTD can thus occur as an organic acidemia associated with encephalopathy.

Emergency presentations of patients with methylmalonic acidemia, propionic acidemia and branched chain amino acidemia (MSUD)

The charts of 16 patients with branched chain amino acidemia (MSUD) who had 48 emergency room (ER) visits, of 10 patients with propionic acidemia (PA) who had 57 ER visit, and of 13 patients with methylmalonic acidemia (MMA) who had 154 ER visits, were reviewed retrospectively for the most common clinical presentations, physical and laboratory findings. The most common clinical presentation was acute or chronic vomiting and the most common physical finding was dehydration. When hypoglycemia was found, the mental status of 55% of patients with MSUD and MMA and 20% of patients with PA, was alert. Mixed acid/base disturbance, i.e. alkalosis caused by vomiting mixed with metabolic acidosis caused by the disease, was present in 30% of MSUD, in 33% of PA, and 45% of MMA. There was no relationship between acidosis detected by the blood pH and mental status of the patients. A good correlation between base excess < -5 and serum bicarbonate < 21 mmol/l was found. Blood cultures were positive for bacteria and fungi in 15% of the visits with MSUD, in 23% with PA, and 3% with MMA. Patients with positive blood cultures did not necessarily have a temperature > 39 degrees C nor hypothermia. The results suggest that the mental status of the patients should not detract the ER physician from obtaining blood pH, gases and glucose and in all instances a blood culture should be secured, even if the patient has no fever.

Glutaric aciduria yype 1: First reported cases in three Saudi patients

The clinical and biochemical findings in three patients with glutaric aciduri Type 1 (GAT1) are presented. They had a normal postnatal period of three to 14 months. They developed sudden and severe encephalopathy following an infection or trauma (patient 3) that gradually progressed to severe dystonia, choreathetosis, spastic quadriplegia and mental retardation. Neuroradiologic studies of the brain revealed while matter disease and frontotemporal lobe hypoplasia. The urine findings by gas chromatography/mass spectrometry (GC)/(MS) were characteristic of GAT1. Since GAT1 is an organic acidemia without intermittent acidotic attacks, but primarily manifests with progressive encephalopathy, it is important to recognize the potential of its existence among handicapped children in chronic care facilities. The good clinical response in two of the patients urges early diagnosis in subsequent newborn siblings of the families with the disease. The diagnosis of three patients in less than two years indicate the need for neonatal screening for the recognition of this disease, among other treatable metabolic diseases, in Saudi Arabia.

Macrocephaly, dystonia, and bilateral temporal arachnoid cysts: glutaric aciduria type 1

Two siblings presented with macrocephaly, psychomotor delay, and progressive dystonia. The initial diagnosis was of hydrocephalus and bilateral temporal cerebrospinal fluid collections. Following ventriculoperitoneal shunting, the patients showed only modest neurological improvement. Metabolic investigations performed later in the course of the disease disclosed increased levels of glutaric acid in the urine and decreased levels of serum carnitine, which were confirmatory of glutaric aciduria type 1. The association of macrocephaly, dystonia, and bilateral temporal arachnoid cysts, shown either by computed tomography or magnetic resonance imaging, seems to be diagnostic of glutaric aciduria type 1. The authors report these two cases as they think they might be of interest to neurosurgeons.

Analysis of urinary organic acids by liquid chromatography-atmospheric pressure chemical ionization mass spectrometry

We developed a new method for the rapid determination of urinary organic acids using liquid chromatography-atmospheric pressure chemical ionization mass spectrometry. Mass spectra of authentic organic acids obtained in the negative-ion mode showed intense [M-H]- ions with some fragment ions. Urine samples of patients with methylmalonic aciduria, ornithine transcarbamylase deficiency, and phenylketonuria were extracted using anion-exchange columns. The mass chromatograms of the extracts showed some dominant peaks of abnormal metabolites characteristic of each disorder. This is a useful method for the analysis of urinary organic acids for the diagnosis of organic aciduria, because the sample preparation is simple.

A patient with propionic acidemia managed with continuous insulin infusion and total parenteral nutrition

An infant newly diagnosed with propionic acidemic coma was managed successfully with total parenteral nutrition (TPN) and continuous infusion of insulin. The urinary excretion of 3-hydroxypropionic acid was reduced to 3% of the admission value in 4 days, gradually decreasing to 1.5% in 16 days. The treatment did not prevent a prolonged episode of thrombocytopenia. The infant tolerated TPN well, except for continued hyper-lactic acidemia (2 to 4 times normal). Metabolic acidosis and mild hyperammonemia recurred only when the patient had sepsis secondary to Candida albicans and Staphylococcus aureus infection.

Neurometabolic diseases at a national referral center: five years experience at the King Faisal Specialist Hospital and Research Centre

Of 910 children under the age of 12 years referred to the national center, we were able to assign an etiology to 473 (52%). This group consisted of lysosomal storage diseases (32%), amino acid disorders (14%), organic acid disorders (16%), various chromosome abnormalities and syndromes with dysmorphia or brain dysgenesis (26%), and various other metabolic diseases (12%). While such amino acidemias as branched-chain amino acidemia (MSUD) in classic and intermediate forms (44%) and hyperphenylalaninemia (PKU) due to 6-pyruvoyltetrahydropterin synthase deficiency (6PTSD) (19%) were common, classic PKU was rare (16%). Methylmalonic acidemia (31%), 3-hydroxy-3-methylglutaryl-coenzyme A lyase deficiency (16%), and propionic acidemia (12%) were encountered more frequently than other types of organic acidemias. When compared to the number of referred Down's syndrome patients, the results suggested the birth of at least one infant with neurometabolic disease per 50 births. Five tribes of Saudi Arabia constituted 26% of the diagnosable neurometabolic diseases. Diseases such as MSUD, 6 PTSD, Sanfilippo syndrome type B, methylmalonic acidemia, homocystinuria, GM2 gangliosidosis Sandhoff variant, infantile central nervous system spongy degeneration (Canavan disease), and neuraminidase deficiency showed definite tribal occurrence. In addition, 32% to 42% of the definitely diagnosed, and 25% to 87% of patients with probable neurometabolic disease but without a definable etiology, had more than one sibling affected. It is concluded that the many rare autosomal diseases of Saudi Arabia are due to "founder effect" created by marriages occurring within tribal and extended family boundaries.