Cloning and mutational analysis of human malonyl-coenzyme A decarboxylase

Malonyl coenzyme A (CoA) decarboxylase (E.C.4. 1.1.9) catalyzes the conversion of malonyl CoA to acetyl CoA. The metabolic role of malonyl CoA decarboxylase has not been fully defined, but deficiency of the enzyme has been associated with mild mental retardation, seizures, hypotonia, cardiomyopathy, vomiting, hypoglycemia, metabolic acidosis, and malonic aciduria. Here we report the isolation and sequencing of the human gene encoding malonyl CoA decarboxylase, and the identification of a mutation causing malonyl CoA decarboxylase deficiency. Human malonyl CoA decarboxylase cDNA sequences were identified by homology to the goose gene, and the intron/exon boundaries were determined by direct sequencing of a PAC clone containing the entire human gene. The 1479 basepair human cDNA is 70 percent identical to the goose sequence, and the intron/exon boundaries are completely conserved between the two species. The genetic mutation underlying malonyl CoA decarboxylase deficiency was determined in a patient with clinical features of this defect, malonic aciduria, and markedly reduced malonyl CoA decarboxylase activity.

Hematological abnormalities and cholestatic liver disease in two patients with mevalonate kinase deficiency

We describe two patients with mevalonate kinase deficiency and prominent hematologic abnormalities and cholestatic liver disease. Patient R.B. was not anemic at birth, but developed petechiae and cutaneous extramedullary hematopoiesis, hepatosplenomegaly, leukocytosis, and recurrent febrile events without positive bacterial or viral cultures. Patient N.M. manifested minor anomalies, hepatosplenomegaly, anemia, thrombocytopenia, recurrent febrile crises, and facial rashes. Mevalonic aciduria was found by urinary organic acid analysis, and mevalonate kinase deficiency was documented in both. The clinical spectrum of normocytic hypoplastic anemia, leukocytosis, thrombocytopenia, and abnormal blood cell forms led to diagnoses of congenital infection, myelodysplastic syndromes, or chronic leukemia in these patients before recognition of mevalonate kinase deficiency. Mevalonate kinase deficiency represents a single-gene abnormality that may be associated with significant hematologic findings. Recognition of the variability of this disorder with some patients manifesting only mild neurologic findings, yet significant hepatosplenomegaly, normocytic anemia, thrombocytopenia, and leukocytosis is important for all specialists who need to be aware of this organic aciduria.

Two exon-skipping mutations as the molecular basis of succinic semialdehyde dehydrogenase deficiency (4-hydroxybutyric aciduria)

Succinic semialdehyde dehydrogenase (SSADH) deficiency, a rare metabolic disorder of 4-aminobutyric acid degradation, has been identified in approximately 150 patients. Affected individuals accumulate large quantities of 4-hydroxybutyric acid, a compound with a wide range of neuropharmacological activities, in physiological fluids. As a first step in beginning an investigation of the molecular genetics of SSADH deficiency, we have utilized SSADH cDNA and genomic sequences to identify two point mutations in the SSADH genes derived from four patients. These mutations, identified by standard methods of reverse transcription, PCR, dideoxy-chain termination, and cycle sequencing, alter highly conserved sequences at intron/exon boundaries and prevent the RNA-splicing apparatus from properly recognizing the normal splice junction. Each family segregated a mutation in a different splice site, resulting in exon skipping and, in one case, a frameshift and premature termination and, in the other case, an in-frame deletion in the resulting protein. Family members, including parents and siblings of these patients, were shown to be heterozygotes for the splicing abnormality, providing additional evidence for autosomal recessive inheritance. Our results provide the first evidence that 4-hydroxybutyric aciduria, resulting from SSADH deficiency, is the result of genetic defects in the human SSADH gene.

Identification of a familial mutation associated with GABA-transaminase deficiency disease

GABA-transaminase (GABA-T) deficiency disease is a rare recessive disorder characterized by abnormal development, seizures, and high levels of GABA in serum and cerebrospinal fluid. Although some patients are the offspring of consanguineous marriages, most are not. To identify the molecular basis of this disease, we have determined the sequence of human GABA-T cDNA. We have compared the GABA-T cDNA sequences in cultured cells derived from six healthy controls with those from a GABA-T-deficient patient and both parents. Our data indicate that GABA-T deficiency disease may result from an allele that encodes an R220K substitution.

3-Methylcrotonyl-coenzyme A carboxylase deficiency in Amish/Mennonite adults identified by detection of increased acylcarnitines in blood spots of their children

Isolated 3-methylcrotonyl coenzyme A carboxylase (MCC) deficiency was documented in four adult women from the Amish/Mennonite population of Lancaster County, Pennsylvania. Metabolic and enzymatic investigations in these individuals were instituted after the detection of abnormal acylcarnitine profiles in blood spots obtained from their newborn children, in whom MCC activity was normal.

HMG CoA lyase deficiency: identification of five causal point mutations in codons 41 and 42, including a frequent Saudi Arabian mutation, R41Q

The hereditary deficiency of 3-hydroxy-3-methylglutaryl (HMG) CoA lyase (HL; OMIM 246450 [http://www3.ncbi.nlm.nih. gov:80/htbin-post/Omim/dispmim?246450]) results in episodes of hypoketotic hypoglycemia and coma and is reported to be frequent and clinically severe in Saudi Arabia. We found genetic diversity among nine Saudi HL-deficient probands: six were homozygous for the missense mutation R41Q, and two were homozygous for the frameshift mutation F305fs(-2). In 32 non-Saudi HL-deficient probands, we found three R41Q alleles and also discovered four other deleterious point mutations in codons 41 and 42: R41X, D42E, D42G, and D42H. In purified mutant recombinant HL, all four missense mutations in codons 41 and 42 cause a marked decrease in HL activity. We developed a screening procedure for HL missense mutations that yields residual activity at levels comparable to those obtained using purified HL peptides. Codons 41 and 42 are important for normal HL catalysis and account for a disproportionate 21 (26%) of 82 of mutant alleles in our group of HL-deficient probands.

4-Hydroxybutyric acid and the clinical phenotype of succinic semialdehyde dehydrogenase deficiency, an inborn error of GABA metabolism

SSADH deficiency, a rare inborn error of human metabolism, disrupts the normal metabolism of the inhibitory neurotransmitter GABA. In response to the defect, physiologic fluids from patients accumulate GHB, a compound with numerous neuromodulatory properties. Clinical and bio-chemical findings in patients are contrasted with existing neuropharmacologic data on GHB in animals and men. We conclude that GHB contributes to the pathogenesis of SSADH deficiency; whether this effect is mediated by GHB, by GABA following metabolic interconversion, or via synergistic mechanisms by both compounds, remains to be determined. An animal model of SSADH deficiency should further define the role of GHB in the pathogenesis of SSADH deficiency, and provide a useful vehicle for the evaluation of new therapeutic intervention.

Characterization of the mevalonate kinase 5'-untranslated region provides evidence for coordinate regulation of cholesterol biosynthesis

Using a probe derived from the 5'-untranslated region of the human mevalonate kinase (MK) cDNA, we screened a lambda gt 11 genomic library and obtained a single clone containing the 5' untranslated region of the gene. Nucleotide sequencing identified several putative regulatory elements, including two Sp1 (GC box) elements and a CCAAT box. A canonical TATA box was not detected. Directly adjacent to one Sp1 element was a sterol regulatory element (SRE), 5'-CACCCCAG-3', which was a 7/8 base pair match to the consensus sequences identified in the genes encoding 3-hydroxy-3-methyl-glutaryl-coenzyme A synthase and reductase, and the LDL receptor. There was no Sp1 element upstream of the SRE. Northern blot analysis in human CRL1508T cells revealed that quantities of MK poly A+ RNA increased for cells grown in the presence of lipid-deficient calf serum, and further increased upon addition of 1 microM lovastatin. Primer extension analysis with human poly A+ RNA suggested at least 4 transcription initiation sites downstream from the CCAAT box. To assess sterol responsiveness of transcription initiation, a 1.4 kb genomic fragment upstream of the translational start site was fused to the pSV2cat vector for transient expression in COS-7 cells, with chloramphenicol acetyltransferase (CAT) as the reporter gene. This construct demonstrated modest levels of CAT expression which was induced > 2-fold when cells were grown in lipoprotein-deficient calf serum. Our data provide further evidence for coordinate regulation of cholesterol biosynthesis in response to sterol.

Post-translational regulation of mevalonate kinase by intermediates of the cholesterol and nonsterol isoprene biosynthetic pathways

To assess the potential for feedback inhibition by isoprene intermediates in the cholesterol and nonsterol isoprene biosynthetic pathway, we expressed human cDNAs encoding mevalonate kinase (MKase), phosphomevalonate kinase (PMKase), and mevalonate diphosphate decarboxylase (MDDase) as fusion proteins in Escherichia coli DH5alpha, and purified these proteins by affinity chromatography. Several phosphorylated and non-phosphorylated isoprenes were analyzed as inhibitors of the enzymes using a standard spectrophotometric assay. Of the three proteins, only MKase was inhibited through competitive interaction at the ATP-binding site. The intermediates studied (and their relative inhibitory capacity) were: geranylgeranyl-diphosphate (GGPP, C20) > farnesyl-diphosphate (FPP, C15) > geranyl-diphosphate (GPP, C10) > isopentenyl-diphosphate (IPP, C5) > or = 3,3-dimethylallyl-diphosphate (DMAPP, C5) > farnesol (C15) > dolichol-phosphate (DP, C(80-100)). Mevalonate-diphosphate, geraniol, and dolichol were not inhibitors. Our data further define the spectrum of physiologic inhibitors of MKase, and provide the first evidence for feedback inhibition of MKase by a nonsterol isoprene produced by the branched pathway, dolichol-phosphate. These results provide additional evidence that MKase may occupy a central regulatory role in the control of cholesterol and nonsterol isoprene biosynthesis.

Identification of an active site alanine in mevalonate kinase through characterization of a novel mutation in mevalonate kinase deficiency

Sequencing of polymerase chain reaction-amplified cDNAs from cultured cells of three patients with mevalonate kinase deficiency revealed a G --> A transversion at nucleotide 1000 of the coding region, converting alanine to threonine at position 334 (A334T). To characterize this defect, we expressed wild-type and mutant cDNAs in Escherichia coli as the glutathione S-transferase fusion proteins, with purification by affinity chromatography. SDS-polyacrylamide gel electrophoresis analysis for wild-type and mutant fusion proteins indicated an expected molecular mass of 42-43 kDa. Kinetic characterization of the wild-type fusion protein yielded Km values of 150 +/- 23 and 440 +/- 190 microM (mean +/- S.E.) for substrates (RS)-mevalonate and ATP, respectively. Expressed wild-type mevalonate kinase (MKase) had a maximum velocity of 13.6 +/- 1.4 units/mg of protein (n = 22, +/-S.E.), whereas the A334T mutation yielded an enzyme with average Vmax of 0.26 +/- 0.02 unit/mg of protein (n = 6, +/-S.E.), representing a decrease to 1.4% of control Vmax. Restriction digestion with HhaI, in conjunction with direct sequencing of cDNAs, revealed that two patients were homozygous and one heterozygous for the A334T allele, establishing autosomal recessive inheritance within families. Although the A334T enzyme had a normal Km for ATP of 680 +/- 226 microM (n = 3, +/-S.E.), the Michaelis constant for (RS)-mevalonate was increased >30-fold to 4623 +/- 1167 microM (n = 4, +/-S.E.) under standard assay conditions. Comparable kinetic results were obtained using extracts of lymphoblasts, which were homozygous for the A334T allele. Alanine 334 is invariant in MKase from bacteria to man and located in a glycine-rich region postulated to have homology with ATP-binding sequences. Our results indicate that the bacterial expression system for human MKase will provide a useful model system in which to analyze inherited mutations and identify the first active site residue in MKase associated with stabilization of mevalonate binding.

R-flurbiprofen chemoprevention and treatment of intestinal adenomas in the APC(Min)/+ mouse model: implications for prophylaxis and treatment of colon cancer

We used the C57BL/6J-APC(Min)/+ mouse (Min mouse) to evaluate the chemopreventive effects of R-flurbiprofen (R-FB), the noncyclooxygenase-inhibiting enantiomer of FB. Weanling Min mice were administered 6 weeks of oral treatment with R-FB using 2.5-25 mg/kg of R-FB once per day (q.d.), 2.5-10 mg/kg of R-FB twice per day (b.i.d.), and 5 mg/kg of R-FB b.i.d. challenged with a high saturated fat diet. At necropsy we determined tumor and ulcer numbers, tumor size, and plasma levels of R- and S-FB. A linear dose response was observed from 2.5 to 10 mg/kg of R-FB, regardless of whether the drug was administered as a single or divided dose. Reductions in tumor number were significant (P < or = 0.02) for doses of R-FB from 2.5 to 25 mg/kg/day. A dose of 5 mg/kg R-FB b.i.d. was able to overcome the doubling in tumor number associated with the high saturated fat diet. At 20 and 25 mg/kg/day R-FB, we obtained the maximum response with up to 90% inhibition of total tumor number. At these doses, however, there was toxicity and animal deaths. This toxicity was associated with ulceration, presumably resulting from the in vivo epimerization of R- to S-FB that occurs in the mouse. Thus, we evaluated the oral pharmacokinetics of R-FB and its conversion to S-FB in wild-type mice. These kinetics experiments revealed inversion rates of 7.3 and 11.0% for the 2.5 and 10 mg/kg R-FB doses, respectively. S-FB administered alone (0.5 and 2.0 mg/kg q.d.), in doses mimicking the concentrations of S-FB associated with the R to S epimerization of the doses of R-FB used in our experiments, had little or no antitumor efficacy (P > 0.05). Thus, we conclude that R-FB itself, not the S-FB resulting from epimerization in the mouse, inhibits adenoma formation in the Min mouse. In humans, where there is no R to S epimerization, it is possible that larger doses of R-FB can be used without causing cyclooxygenase inhibition and its resulting ulcerogenicity and other side effects. To assess the effect of R-FB on established adenomas, we allowed 40 Min mice to remain untreated until 70 days of age (the time of necropsy in the previous experiments) and then treated them for an additional 42 days with 10 mg/kg R-FB q.d. or 5 mg/kg R-FB b.i.d.. Both drug-treated groups demonstrated tumor numbers significantly less than that of the vehicle control (P < 0.01). Our results suggest that prophylaxis and treatment trials of R-FB should be extended to humans.

Endogenous natriuretic factors 7: biospecificity of a natriuretic gamma-tocopherol metabolite LLU-alpha

The structural elucidation and mechanism of action of a potential component, LLU-alpha, of what is possibly a multifactorial complex known as "natriuretic hormone" was recently reported [Wechter, W.J. et al. (1996a) Proc. Natl. Acad. Sci. U.S.A. 93: 6002-6007]. "Natriuretic hormone," a long-sought factor, is believed to regulate extracellular fluid volume and consequently be pathomimetic for hypertension, cirrhosis, congestive heart failure and other volume expanded states. The studies reported herein further characterize LLU-alpha. The precursor of the endogenous LLU-alpha was demonstrated to be gamma-tocopherol by radiolabeling studies. The pharmacokinetics of infused rac-LLU-alpha proved to be biphasic (half-lives: 12 min and 6 h). Specificity of the inhibition of the 70 pS potassium channel of the thick ascending limb of the loop of Henle was examined with the natural S-enantiomer being the most potent known inhibitor whereas the analogous alpha-tocopherol metabolite, rac-5-Me-LLU-alpha, showed no inhibition. Rac-LLU-alpha does not inhibit two isozymes of the Na+/K+-ATPase. LLU-alpha is natriuretic acting via inhibition of the 70 pS potassium channel and not Na+/K+-ATPase, the assumed mechanism of action of the "natriuretic hormone." LLU-alpha, a metabolite of a vitamin, if it were found to play a role in the regulation of extracellular fluid volume, would be the second example of a vitamin acting as a precursor for a hormone. Of considerable interest is the fact that this manuscript reports the first biological activity of gamma-tocopherol, a member of the vitamin E complex.

The clinical phenotype of succinic semialdehyde dehydrogenase deficiency (4-hydroxybutyric aciduria): case reports of 23 new patients

OBJECTIVES

To further define the clinical spectrum of the disease for pediatric and metabolic specialists, and to suggest that the general pediatrician and pediatric neurologist consider succinic semialdehyde dehydrogenase (SSADH) deficiency in the differential diagnosis of patients with (idiopathic) mental retardation and emphasize the need for accurate, quantitative organic acid analysis in such patients.

PATIENTS

The clinical features of 23 patients (20 families) with SSADH deficiency (4-hydroxybutyric acid-uria) are presented. The age at diagnosis ranged from 3 months to 25 years in the 11 male and 12 female patients; consanguinity was noted in 39% of families.

OUTCOME MEASUREMENTS

The following abnormalities were observed (frequency in 23 patients): motor delay, including fine-motor skills, 78%; language delay, 78%; hypotonia, 74%; mental delay, 74%; seizures, 48%; decreased or absent reflexes, 39%; ataxia, 30%; behavioral problems, 30%; hyperkinesis, 30%; neonatal problems, 26%; and electroencephalographic abnormalities, 26%. Associated findings included psychoses, cranial magnetic resonance or computed tomographic abnormalities, and ocular problems in 22% or less of patients. Therapy with vigabatrin proved beneficial to varying degrees in 35% of the patients. Normal early development was noted in 30% of patients.

CONCLUSIONS

Our data imply that two groups of patients with SSADH deficiency exist, differentiated by the course of early development. Our recommendation would be that accurate, quantitative organic acid analysis in an appropriate specialist laboratory be requested for any patients presenting with two or more features of mental, motor, or language delay and hypotonia of unknown cause. Such analyses are the only definitive way to diagnose SSADH deficiency; the diagnosis can be confirmed by determination of enzyme activity in white cells from whole blood. We think that increased use of organic acid determination will lead to increased diagnosis of SSADH deficiency and a more accurate representation of disease frequency. As additional patients are identified, we should have a better understanding of both the metabolic and clinical profiles of SSADH deficiency.

Regulatory adaptation of isoprenoid biosynthesis and the LDL receptor pathway in fibroblasts from patients with mevalonate kinase deficiency

In a search for the pathophysiologic mechanisms, we estimated isoprenoid synthesis and concentration, cellular growth, and the activity of the LDL receptor pathway in fibroblasts from patients with mevalonate kinase deficiency (MKD), a severe multisystemic disorder of cholesterol and non-sterol isoprenoid biosynthesis. In response to different concentrations of LDL and non-lipoprotein-bound cholesterol, MKD cells partially counteracted their enzyme defect by increased activities of 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase (results from earlier studies) and the LDL receptor pathway, responses similar to the pharmacologic effects seen upon administration of HMG-CoA reductase inhibitors. Rates of N-linked protein glycosylation, estimated as the amount of [14C]galactose-labeled macromolecules secreted into cell culture medium, were significantly decreased in MKD fibroblasts in comparison with control cells which may indicate alterations in the dolichol or dolichol phosphate pool. In response to exogenous cholesterol, the major feedback inhibitor of isoprenoid biosynthesis, growth velocities of MKD fibroblasts declined in comparison with control cells, further suggesting an impairment of non-sterol isoprenoid biosynthesis in MKD. Our results suggest an imbalance in the multilevel regulation of the biosynthesis of cholesterol and non-sterol isoprenoids in MKD, representing an additional causative factor responsible for the pre- and postnatal pathology of MKD.

Molecular cloning of human phosphomevalonate kinase and identification of a consensus peroxisomal targeting sequence

Two overlapping cDNAs which encode human liver phosphomevalonate kinase (PMKase) were isolated. The human PMKase cDNAs predict a 191-amino acid protein with a molecular weight of 21,862, consistent with previous reports for mammalian PMKase (Mr = 21,000-22,500). Further verification of the clones was obtained by expression of PMKase activity in bacteria using a composite 1024-base pair cDNA clone. Northern blot analysis of several human tissues revealed a doublet of transcripts at approximately 1 kilobase (kb) in heart, liver, skeletal muscle, kidney, and pancreas and lower but detectable transcript levels in brain, placenta, and lung. Analysis of transcripts from human lymphoblasts subcultured in lipid-depleted sera (LDS) and LDS supplemented with lovastatin indicated that PMKase gene expression is subject to regulation by sterol at the level of transcription. Southern blotting indicated that PMKase is a single copy gene covering less than 15 kb in the human genome. The human PMKase amino acid sequence contains a consensus peroxisomal targeting sequence (PTS-1), Ser-Arg-Leu, at the C terminus of the protein. This is the first report of a cholesterol biosynthetic protein which contains a consensus PTS-1, providing further evidence for the concept that early cholesterol and nonsterol isoprenoid biosynthesis may occur in the peroxisome.