Morteza Pourfarzam

Clear correlation of genotype with disease phenotype in very-long-chain acyl-CoA dehydrogenase deficiency.

Very-long-chain acyl-CoA dehydrogenase (VLCAD) catalyzes the initial rate-limiting step in mitochondrial fatty acid beta-oxidation. VLCAD deficiency is clinically heterogenous, with three major phenotypes: a severe childhood form, with early onset, high mortality, and high incidence of cardiomyopathy; a milder childhood form, with later onset, usually with hypoketotic hypoglycemia as the main presenting feature, low mortality, and rare cardiomyopathy; and an adult form, with isolated skeletal muscle involvement, rhabdomyolysis, and myoglobinuria, usually triggered by exercise or fasting. To examine whether these different phenotypes are due to differences in the VLCAD genotype, we investigated 58 different mutations in 55 unrelated patients representing all known clinical phenotypes and correlated the mutation type with the clinical phenotype. Our results show a clear relationship between the nature of the mutation and the severity of disease. Patients with the severe childhood phenotype have mutations that result in no residual enzyme activity, whereas patients with the milder childhood and adult phenotypes have mutations that may result in residual enzyme activity. This clear genotype-phenotype relationship is in sharp contrast to what has been observed in medium-chain acyl-CoA dehydrogenase deficiency, in which no correlation between genotype and phenotype can be established.

Neonatal screening for medium-chain acyl-CoA dehydrogenase deficiency

Neonatal screening for medium-chain acyl-CoA dehydrogenase (MCAD) deficiency has not yet been introduced in the UK, primarily because of uncertainty about the natural history of the disorder and concerns about the specificity of the screening test. To obtain data on these issues, we did a retrospective study in which we analysed the concentrations of acylcarnitines in stored neonatal blood spots, and reviewed patients with high octanoylcarnitine concentrations at age 7-9 years. The high morbidity and mortality associated with the disorder, and the specificity of acylcarnitine analysis seen in our study support the introduction of screening for MCAD deficiency.

Biochemical, clinical and molecular findings in LCHAD and general mitochondrial trifunctional protein deficiency

SummaryGeneral mitochondrial trifunctional protein (TFP) deficiency leads to a wide clinical spectrum of disease ranging from severe neonatal/infantile cardiomyopathy and early death to mild chronic progressive sensorimotor poly-neuropathy with episodic rhabdomyolysis. Isolated long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency resulting from the common Glu510Gln mutation usually gives rise to a moderately severe phenotype with multiorgan involvement with high morbidity and mortality. However, isolated LCHAD deficiency can also be consistent with long-term survival in patients identified and treated from an early age. We present biochemical, clinical and mutation data in 9 patients spanning the full spectrum of disease. Fibroblast acylcarnitine profiling shows good correlation with clinical phenotype using the ratio C18(OH)/(C14(OH)+C12(OH)). This ratio shows a gradation of values, from high in four patients with severe neonatal disease (2.5±0.8), to low in two neuromyopathic patients (0.35, 0.2). Fibroblast fatty acid oxidation flux assays also show correlation with the patient phenotype, when expressed either as percentage residual activity with palmitate or as a ratio of percentage activity of myristate/oleate (M/O ratio). Fibroblasts from four patients with severe neonatal disease gave an M/O ratio of 4.0±0.6 compared to 1.97 and 1.62 in two neuromyopathic patients. Specific enzyme assay of LCHAD and long-chain 3-ketothiolase activity in patient cells shows lack of correlation with phenotype. These results show that measurements in intact cells, which allow all determinative and modifying cellular factors to be present, better reflect patient phenotype. Mutation analysis reveals a number of α- and β-subunit mutations. Peripheral sensorimotor polyneuropathy, often as the initial major presenting feature but usually later accompanied by episodic rhabdomyolysis, is a manifestation of mild TFP protein deficiency. The mild clinical presentation and relative difficulty in diagnosis suggest that this form of TFP is probably underdiagnosed.

Methylmalonic acidaemia: Examination of genotype and biochemical data in 32 patients belonging to mut, cblA or cblB complementation group

SummaryMethylmalonic acidaemia (MMA) is a genetic disorder caused by defects in methylmalonyl-CoA mutase or in any of the different proteins involved in the synthesis of adenosylcobalamin. The aim of this work was to examine the biochemical and clinical phenotype of 32 MMA patients according to their genotype, and to study the mutant mRNA stability by real-time PCR analysis. Using cellular and biochemical methods, we classified our patient cohort as having the MMA forms mut (n = 19), cblA (n = 9) and cblB (n = 4). All the mut0 and some of the cblB patients had the most severe clinical and biochemical manifestations, displaying non-inducible propionate incorporation in the presence of hydroxocobalamin (OHCbl) in vitro and high plasma odd-numbered long-chain fatty acid (OLCFA) concentrations under dietary therapy. In contrast, mut− and cblA patients exhibited a milder phenotype with propionate incorporation enhanced by OHCbl and normal OLCFA levels under dietary therapy. No missense mutations identified in the MUT gene, including mut0 and mut− changes, affected mRNA stability. A new sequence variation (c.562G>C) in the MMAA gene was identified. Most of the cblA patients carried premature termination codons (PTC) in both alleles. Interestingly, the transcripts containing the PTC mutations were insensitive to nonsense-mediated decay (NMD).

Fatty acid oxidation in peripheral blood cells : characterization and use for the diagnosis of defects of fatty acid oxidation

ABSTRACT: Disorders of mitochondrial fatty acid oxidation are increasingly recognized as an important group of inborn errors of metabolism that are associated with a significant, but easily preventable, morbidity and mortality in children. However, diagnosis is often delayed because there is no easily applied method that detects all defects. Therefore, we have characterized the acylcarnitine intermediates of fatty acid oxidation in peripheral blood cells from healthy control volunteers and patients with four different defects. After selective permeabilization with histone II AS, a novel permeabilizing agent, the cells were incubated with [U-14C]hexadecanoate and β-oxidation flux and the acylcarnitine esters formed were measured. Blood cells from the control population produced large amounts of 3-hydroxyacylcarnitines and 2-enoylcarnitine esters, in addition to saturated acylcarnitine esters. This result is different from that found in other tissues (fibroblasts and muscle), where only saturated acylcarnitine esters could be detected. In blood cells from patients with defects of enzymes involved in long-chain fatty acid oxidation, flux was significantly reduced at 15 to 20% of control values (7.1 ± 2.3 nmol C2 units formed per minute per International Unit of citrate synthase activity). There was a characteristic accumulation of acylcarnitines that was pathognomonic for the site of the defect. Thus, analysis of β-oxidation intermediates from blood cells allows unequivocal identification of the four most common β-oxidation defects.

Mitochondrial fatty acid beta-oxidation in the retinal pigment epithelium.

Pigmentary retinopathy is an important feature of long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency, a disorder of mitochondrial fatty acid β-oxidation. Pathogenesis of this complication remains unknown. The retinal pigment epithelium (RPE) is affected early in this retinopathy. We wanted to determine whether there is evidence of mitochondrial fatty acid β-oxidation in the RPE cells. Fatty acid oxidation was measured from cultured porcine RPE cells by incubating them with [U-13C]-hexadecanoic acid. Acylcarnitine esters were analyzed by tandem mass spectrometry. The activity of LCHAD and carnitine uptake capacity were measured from the cultured cells. Antibodies to the human mitochondrial trifunctional protein (MTP) containing LCHAD activity were used to analyze the expression of the MTP in the cultured RPE cell lysate and in human retinal sections by immunoblotting and immunohistochemistry. Fatty acid oxidation analysis showed normal chain shortening of hexadecanoic acid and production of acetylcarnitine in cultured RPE cells. Immunoblotting revealed expression of the MTP and enzyme assay showed the activity of LCHAD in the RPE cells. RPE cells were also capable of carnitine uptake. Positive labeling to the MTP antibodies was detected in the RPE, photoreceptors, and ganglion cells. The results give strong in vitro evidence for the presence of mitochondrial fatty acid β-oxidation in RPE cells and the expression of the MTP in the RPE and other layers of the retina. Further studies are required to clarify whether this pathway acts also in vivo in the retina.

So doctor, what exactly is wrong with my muscles? Glutaric aciduria type II presenting in a teenager

Late-onset glutaric aciduria type II (GAII) is a rare but treatable cause of profound proximal myopathy. GAII is caused by defects in intra-mitochondrial acyl-CoA dehydrogenation due to deficiency in one of three molecules: the alpha or beta subunits of the electron transport flavoprotein (ETFA; OMIM 231680, ETFB; OMIM 130410), or ETF-dehydrogenase (ETFDH; OMIM 231675). This case report illustrates that GAII may present in the teenage years and may not be associated with hypoglycaemia. It outlines some important diagnostic conundrums faced in diagnosing and managing juvenile onset myopathies. Mutational analysis from this patient revealed two mutations of the ETF-DH gene: EFTDH-334C>T/His122Tyr and EFTDH-1366C>A/Pro456Thr (OMIM 231675). An outline of this rare but important disease, its clinical characteristics and diagnostic methodology are given.

Lipid-storage myopathy and respiratory insufficiency due to ETFQO mutations in a patient with late-onset multiple acyl-CoA dehydrogenation deficiency

Summary: We report a patient with lipid-storage myopathy due to multiple acyl-CoA dehydrogenation deficiency (MADD). Molecular genetic analysis showed that she was compound heterozygous for mutations in the gene for electron transfer flavoprotein:ubiquinone oxidoreductase (ETFQO). Despite a good initial response to treatment, she developed respiratory insufficiency at age 14 years and has required long-term overnight ventilation. Thus, MADD is one of the few conditions that can cause a myopathy with weakness of the respiratory muscles out of proportion to the limb muscles.

New developments in neonatal screening

There have been significant advances in the methods available for the detection and investigation of individuals with inherited metabolic disorders. The burgeoning of molecular biology in recent years and the discovery of new classes of inherited metabolic disorders, such as inborn errors of fat oxidation, are well known. What is perhaps less well recognised, is that there have been comparable advances in analysis and laboratory automation. The combination of the development of generic analytical technology of great power and sophistication, and the discovery of new treatable metabolic diseases detectable in the newborn period, has, we believe, resulted in a sea change in neonatal screening. Neonatal screening for metabolic diseases became feasible as a result of the pioneering work of Guthrie. He developed a simple, robust, and effective technique for the detection of elevated concentrations of phenylalanine in blood—the “Guthrie Test”— which was used to detect phenylketonuria by means of a semi-quantitative microbiological bioassay.1 Screening for this disorder (PKU; phenylalanine hydroxylase deficiency; McKusick 261600) was subsequently implemented in the UK in 1969.2The specimens used are dried blood spots which are usually collected by means of a heel-prick on the sixth day of life, although later sampling, up to 14 days, can occur. This form of specimen collection has become almost universal. Quantitative assays specific for phenylalanine have been introduced by some centres, driven by the desire to introduce a degree of automation.3 4 An alternative approach is to use chromatographic methods, and this has been adopted by some centres.5 The proponents of this technology contend that other inherited disorders of amino acid metabolism can be detected and that improved prognosis may be achieved by early diagnosis. However, chromatographic methods are time consuming and difficult to automate, particularly in respect of the recognition of abnormal chromatograms. None …

Analysis of mitochondrial fatty acid oxidation intermediates by tandem mass spectrometry from intact mitochondria prepared from homogenates of cultured fibroblasts, skeletal muscle cells, and fresh muscle.

Defects of mitochondrial fatty acid β-oxidation are an important group of inherited metabolic disorders in children. Despite improved screening opportunities, diagnosis of these disorders is not often straightforward and requires enzyme analyses. Because therapy is effective in many of these disorders, rapid diagnosis is essential. We report a technique that allows analysis of fatty acid oxidation not only in cultured cells (fibroblasts, myoblasts, and myotubes) but also in fresh muscle homogenate. Fatty acid oxidation analysis was performed by incubating fresh muscle homogenate or harvested cultured cells with stable isotopically labeled palmitate. The intermediates generated were analyzed by tandem mass spectrometry. Results of patients with seven different β-oxidation disorders were compared with controls. Acylcarnitine intermediates in patient samples could be easily differentiated from the control samples. The acylcarnitine profile of each β-oxidation defect was compatible with localization of the enzyme defect. Both in patient and control samples, the same pattern of intermediates could be detected in fibroblasts, muscle cells, and fresh muscle homogenate. The procedure described allowed correct diagnosis of all the β-oxidation defects studied. Utilization of fresh muscle samples reduces the delay in diagnosis related to tissue culture and is useful in diagnostic of patients with neuromuscular phenotype. Measurement of fatty acid oxidation intermediates from myoblasts or myotubes is an additional tool in investigating pathogenetic mechanisms of myopathy in β-oxidation defects.