A novel single-base substitution (380C>T) that activates a 5-base downstream cryptic splice-acceptor site within exon 5 in almost all transcripts in the human mitochondrial acetoacetyl-CoA thiolase gene

Mutation update on ACAT1 variants associated with mitochondrial acetoacetyl-CoA thiolase (T2) deficiency

Mitochondrial acetoacetyl‐CoA thiolase (T2, encoded by the ACAT1 gene) deficiency is an inherited disorder of ketone body and isoleucine metabolism. It typically manifests with episodic ketoacidosis. The presence of isoleucine‐derived metabolites is the key marker for biochemical diagnosis. To date, 105 ACAT1 variants have been reported in 149 T2‐deficient patients. The 56 disease‐associated missense ACAT1 variants have been mapped onto the crystal structure of T2. Almost all these missense variants concern residues that are completely or partially buried in the T2 structure. Such variants are expected to cause T2 deficiency by having lower in vivo T2 activity because of lower folding efficiency and/or stability. Expression and activity data of 30 disease‐associated missense ACAT1 variants have been measured by expressing them in human SV40‐transformed fibroblasts. Only two variants (p.Cys126Ser and p.Tyr219His) appear to have equal stability as wild‐type. For these variants, which are inactive, the side chains point into the active site. In patients with T2 deficiency, the genotype does not correlate with the clinical phenotype but exerts a considerable effect on the biochemical phenotype. This could be related to variable remaining residual T2 activity in vivo and has important clinical implications concerning disease management and newborn screening.

Recent advances in understanding beta-ketothiolase (mitochondrial acetoacetyl-CoA thiolase, T2) deficiency

Beta-ketothiolase (mitochondrial acetoacetyl-CoA thiolase, T2) deficiency (OMIM #203750, *607809) is an inborn error of metabolism that affects isoleucine catabolism and ketone body metabolism. This disorder is clinically characterized by intermittent ketoacidotic crises under ketogenic stresses. In addition to a previous 26-case series, four series of T2-deficient patients were recently reported from different regions. In these series, most T2-deficient patients developed their first ketoacidotic crises between the ages of 6 months and 3 years. Most patients experienced less than three metabolic crises. Newborn screening (NBS) for T2 deficiency is performed in some countries but some T2-deficient patients have been missed by NBS. Therefore, T2 deficiency should be considered in patients with severe metabolic acidosis, even in regions where NBS for T2 deficiency is performed. Neurological manifestations, especially extrapyramidal manifestations, can occur as sequelae to severe metabolic acidosis; however, this can also occur in patients without any apparent metabolic crisis or before the onset of metabolic crisis.

A novel mutation (c.121‑13T>A) in the polypyrimidine tract of the splice acceptor site of intron 2 causes exon 3 skipping in mitochondrial acetoacetyl-CoA thiolase gene

Mitochondrial acetoacetyl-CoA thiolase (T2) (gene symbol: ACAT1) deficiency is an autosomal recessive disorder affecting isoleucine catabolism and ketone body utilization. In this study, mutational analysis of an Indian T2-deficient patient revealed a homozygous mutation (c.121‑13T>A) located at the polypyrimidine tract of the splice acceptor site of intron 2, and exon 3 skipping was identified by cDNA analysis using cycloheximide. We made three mutant constructs (c.121‑13T>A, T>C, and T>G substitutions) followed by making a wild-type minigene construct that included an ACAT1 segment from exon 2 to 4 for a splicing experiment. The minigene splicing experiment demonstrated that exon 3 skipping was induced not only by c.121‑13T>A mutation, but also by the other two substitutions. It was difficult to predict the effect of these mutations on splicing using in silico tools, as predictions of different tools were inconsistent with each other. The minigene splicing experiment remains the most reliable method to unravel splicing abnormalities.

Single-nucleotide substitution T to A in the polypyrimidine stretch at the splice acceptor site of intron 9 causes exon 10 skipping in the ACAT1 gene

Background

β‐ketothiolase (T2, gene symbol ACAT1) deficiency is an autosomal recessive disorder, affecting isoleucine and ketone body metabolism. We encountered a patient (GK03) with T2 deficiency whose T2 mRNA level was <10% of the control, but in whom a previous routine cDNA analysis had failed to find any mutations. Genomic PCR‐direct sequencing showed homozygosity for c.941‐9T>A in the polypyrimidine stretch at the splice acceptor site of intron 9 of ACAT1. Initially, we regarded this variant as not being disease‐causing by a method of predicting the effect of splicing using in silico tools. However, based on other findings of exon 10 splicing, we eventually hypothesized that this mutation causes exon 10 skipping.

Methods

cDNA analysis was performed using GK03's fibroblasts treated with/without cycloheximide (CHX), since exon 10 skipping caused a frameshift and nonsense‐mediated mRNA decay (NMD). Minigene splicing experiment was done to confirm aberrant splicing.

Results

cDNA analysis using fibroblasts cultured with cycloheximide indeed showed the occurrence of exon 10 skipping. A minigene splicing experiment clearly showed that the c.941‐9T>A mutant resulted in transcripts with exon 10 skipping. There are few reports describing that single‐nucleotide substitutions in polypyrimidine stretches of splice acceptor sites cause aberrant splicing.

Conclusion

We showed that c.941‐9T>A induces aberrant splicing in the ACAT1 gene. Our ability to predict the effects of mutations on splicing using in silico tools is still limited. cDNA analysis and minigene splicing experiments remain useful alternatives to reveal splice defects.

Exon 10 skipping in ACAT1 caused by a novel c.949G>A mutation located at an exonic splice enhancer site

Beta-ketothiolase deficiency, also known as mitochondrial acetoacetyl-CoA thiolase (T2) deficiency, is an autosomal recessive disease caused by mutations in the acetyl‑CoA acetyltransferase 1 (ACAT1) gene. A German T2‑deficient patient that developed a severe ketoacidotic episode at the age of 11 months, was revealed to be a compound heterozygote of a previously reported null mutation, c.472A>G (p.N158D) and a novel mutation, c.949G>A (p.D317N), in ACAT1. The c.949G>A mutation was suspected to cause aberrant splicing as it is located within an exonic splicing enhancer sequence (c. 947CTGACGC) that is a potential binding site for serine/arginine‑rich splicing factor 1. A mutation in this sequence, c.951C>T, results in exon 10 skipping. A minigene construct was synthesized that included exon 9‑truncated intron 9‑exon 10‑truncated intron 10‑exon 11, and the splicing of this minigene revealed that the c.949G>A mutant construct caused exon 10 skipping in a proportion of the transcripts. Furthermore, additional substitution of G for C at the first nucleotide of exon 10 (c.941G>C) abolished the effect of the c.949G>A mutation. Transient expression analysis of the c.949G>A mutant cDNA revealed no residual T2 activity in the mutated D317N enzyme. Therefore, c.949G>A (D317N) is a pathogenic missense mutation, and diminishes the effect of an exonic splicing enhancer and causes exon 10 skipping. The present study demonstrates that a missense mutation, or even a synonymous substitution, may disrupt enzyme function by interference with splicing.

A novel AVPR2 splice site mutation leads to partial X-linked nephrogenic diabetes insipidus in two brothers

X-linked nephrogenic diabetes insipidus (NDI, OMIM#304800) is caused by mutations in the arginine vasopressin (AVP, OMIM*192340) receptor type 2 (AVPR2, OMIM*300538) gene. A 20-month-old boy and his 8-year-old brother presented with polyuria, polydipsia, and failure to thrive. Both boys demonstrated partial DDAVP (1-desamino-8-D AVP or desmopressin) responses; thus, NDI diagnosis was delayed. While routine sequencing of AVPR2 showed a potential splice site variant, it was not until exome sequencing confirmed the AVPR2 splice site variant and did not reveal any more likely candidates that the patients' diagnosis was made and proper treatment was instituted. Both patients were hemizygous for two AVPR2 variants predicted in silico to affect AVPR2 messenger RNA (mRNA) splicing. A minigene assay revealed that the novel AVPR2 c.276A>G mutation creates a novel splice acceptor site leading to 5' truncation of AVPR2 exon 2 in HEK293 human kidney cells. Both patients have been treated with high-dose DDAVP with a remarkable improvement of their symptoms and accelerated linear growth and weight gain.

CONCLUSION

We present here a unique case of partial X-linked NDI due to an AVPR2 splice site mutation; patients with diabetes insipidus of unknown etiology may harbor splice site mutations that are initially underestimated in their pathogenicity on sequence analysis.

WHAT IS KNOWN

• X-linked nephrogenic diabetes insipidus is caused by AVPR2 mutations, and disease severity can vary depending on the functional effect of the mutation. What is New: • We demonstrate here that a splice site mutation in AVPR2 leads to partial X-linked NDI in two brothers. • Treatment with high-dose DDAVP led to improvement of polyuria and polydipsia, weight gain, and growth.

Inborn errors of ketone body utilization

Succinyl-CoA:3-ketoacid CoA transferase (SCOT) deficiency and mitochondrial acetoacetyl-CoA thiolase (beta-ketothiolase or T2) deficiency are classified as autosomal recessive disorders of ketone body utilization characterized by intermittent ketoacidosis. Patients with mutations retaining no residual activity on analysis of expression of mutant cDNA are designated as severe genotype, and patients with at least one mutation retaining significant residual activity, as mild genotype. Permanent ketosis is a pathognomonic characteristic of SCOT-deficient patients with severe genotype. Patients with mild genotype, however, may not have permanent ketosis, although they may develop severe ketoacidotic episodes similar to patients with severe genotype. Permanent ketosis has not been reported in T2 deficiency. In T2-deficient patients with severe genotype, biochemical diagnosis is done on urinary organic acid analysis and blood acylcarnitine analysis to observe characteristic findings during both ketoacidosis and non-episodic conditions. In Japan, however, it was found that T2-deficient patients with mild genotype are common, and typical profiles were not identified on these analyses. Based on a clinical study of ketone body utilization disorders both in Japan and worldwide, we have developed guidelines for disease diagnosis and treatment. These diseases are treatable by avoiding fasting and by providing early infusion of glucose, which enable the patients to grow without sequelae.

Ketone body metabolism and its defects

Acetoacetate (AcAc) and 3-hydroxybutyrate (3HB), the two main ketone bodies of humans, are important vectors of energy transport from the liver to extrahepatic tissues, especially during fasting, when glucose supply is low. Blood total ketone body (TKB) levels should be evaluated in the context of clinical history, such as fasting time and ketogenic stresses. Blood TKB should also be evaluated in parallel with blood glucose and free fatty acids (FFA). The FFA/TKB ratio is especially useful for evaluation of ketone body metabolism. Defects in ketogenesis include mitochondrial HMG-CoA synthase (mHS) deficiency and HMG-CoA lyase (HL) deficiency. mHS deficiency should be considered in non-ketotic hypoglycemia if a fatty acid beta-oxidation defect is suspected, but cannot be confirmed. Patients with HL deficiency can develop hypoglycemic crises and neurological symptoms even in adolescents and adults. Succinyl-CoA-3-oxoacid CoA transferase (SCOT) deficiency and beta-ketothiolase (T2) deficiency are two defects in ketolysis. Permanent ketosis is pathognomonic for SCOT deficiency. However, patients with "mild" SCOT mutations may have nonketotic periods. T2-deficient patients with "mild" mutations may have normal blood acylcarnitine profiles even in ketoacidotic crises. T2 deficient patients cannot be detected in a reliable manner by newborn screening using acylcarnitines. We review recent data on clinical presentation, metabolite profiles and the course of these diseases in adults, including in pregnancy.

Three Japanese Patients with Beta-Ketothiolase Deficiency Who Share a Mutation, c.431A>C (H144P) in ACAT1 : Subtle Abnormality in Urinary Organic Acid Analysis and Blood Acylcarnitine Analysis Using Tandem Mass Spectrometry

Mitochondrial acetoacetyl-CoA thiolase (T2) deficiency affects both isoleucine catabolism and ketone body metabolism. The disorder is characterized by intermittent ketoacidotic episodes. We report three Japanese patients. One patient (GK69) experienced two ketoacidotic episodes at the age of 9 months and 3 years, and no further episodes until the age of 25 years. She had two uncomplicated pregnancies. GK69 was a compound heterozygote of the c.431A>C (H144P) and c.1168T>C (S390P) mutations in T2 (ACAT1) gene. She was not suspected of having T2 deficiency during her childhood, but she was diagnosed as T2 deficient at the age of 25 years by enzyme assay using fibroblasts. The other two patients were identical twin siblings who presented their first ketoacidotic crisis simultaneously at the age of 3 years 4 months. One of them (GK77b) died during the first crisis and the other (GK77) survived. Even during severe crises, C5-OH and C5:1 were within normal ranges in their blood acylcarnitine profiles and trace amounts of tiglylglycine and small amounts of 2-methyl-3-hydroxybutyrate were detected in their urinary organic acid profiles. They were H144P homozygotes. This H144P mutation has retained the highest residual T2 activity in the transient expression analysis of mutant cDNA thus far, while the S390P mutation did not retain any residual T2 activity. The "mild" H144P mutation may result in subtle profiles in blood acylcarnitine and urinary organic acid analyses. T2-deficient patients with "mild" mutations have severe ketoacidotic crises but their chemical phenotypes may be subtle even during acute crises.

A novel mutation (c.951C>T) in an exonic splicing enhancer results in exon 10 skipping in the human mitochondrial acetoacetyl-CoA thiolase gene

Mitochondrial acetoacetyl-CoA thiolase (T2) deficiency is an inherited disorder affecting isoleucine catabolism and ketone body metabolism. A Japanese female developed a severe ketoacidotic attack at the age of 7 months. Urinary organic acid analysis showed elevated excretion of 2-methyl-3-hydroxybutyrate but not tiglylglycine. She was diagnosed as having T2 deficiency by enzyme assay using fibroblasts. Mutation analysis revealed a compound heterozygote of c.556G>T(D186Y) and c.951C>T(D317D). Since c.951C>T does not cause amino acid change, we performed cDNA analysis and found that exon 10 skipping had occurred in the c.951C>T allele. A computer search using an ESE finder showed that an exonic splicing enhancer sequence, SF2/ASF, was located in CTGA(951)CGC. We hypothesized that the exonic splicing enhancer is necessary for accurate splicing since the first nucleotide of exon 10 is C, which weakens the splice acceptor site of intron 9. We made a mini gene construct including exon 9-truncated intron 9-exon 10-truncated intron 10-exon 11 for a splicing experiment. We also made three mutant constructs which alter the SF2/ASF site (947C>T, 951C>T, 952G>A). An min-gene splicing experiment clearly showed that exon 10 skipping was induced in all three mutant constructs. Moreover, additional substitution of G for C at the first nucleotide of exon 10 resulted in normal splicing in these three mutants. These results confirmed that c.951C>T diminished the effect of the exonic splicing enhancer and caused exon 10 skipping.

A common mutation, R208X, identified in Vietnamese patients with mitochondrial acetoacetyl-CoA thiolase (T2) deficiency

Mitochondrial acetoacetyl-CoA thiolase (T2) deficiency is an inborn error of metabolism affecting isoleucine catabolism and ketone body utilization. This disorder is clinically characterized by intermittent ketoacidotic episodes with no clinical symptoms between episodes. In general, T2 gene mutations are heterogeneous. No common mutations have been identified and more than 70 mutations have been identified in 70 patients with T2 deficiency (including unpublished data). We herein identified a common mutation, R208X, in Vietnamese patients. We identified R208X homozygously in six patients and heterozygously in two patients among eight Vietnamese patients. This R208X mutation was also identified heterozygously in two Dutch patients, however, R208X mutant alleles in the Vietnamese have a different haplotype from that in the Dutch, when analyzed using Msp I and Taq I polymorphisms in the T2 gene. The R208X mutant allele was not so frequent in the Vietnamese since we could not find that mutant allele in 400 healthy Vietnamese controls using the Nla III restriction enzyme assay. DNA diagnosis of T2 deficiency may be applicable to the Vietnamese population.

Genomic features defining exonic variants that modulate splicing

A comparative analysis of SNPs and their exonic and intronic environments identifies the features predictive of splice affecting variants.

Background

Single point mutations at both synonymous and non-synonymous positions within exons can have severe effects on gene function through disruption of splicing. Predicting these mutations in silico purely from the genomic sequence is difficult due to an incomplete understanding of the multiple factors that may be responsible. In addition, little is known about which computational prediction approaches, such as those involving exonic splicing enhancers and exonic splicing silencers, are most informative.

Results

We assessed the features of single-nucleotide genomic variants verified to cause exon skipping and compared them to a large set of coding SNPs common in the human population, which are likely to have no effect on splicing. Our findings implicate a number of features important for their ability to discriminate splice-affecting variants, including the naturally occurring density of exonic splicing enhancers and exonic splicing silencers of the exon and intronic environment, extensive changes in the number of predicted exonic splicing enhancers and exonic splicing silencers, proximity to the splice junctions and evolutionary constraint of the region surrounding the variant. By extending this approach to additional datasets, we also identified relevant features of variants that cause increased exon inclusion and ectopic splice site activation.

Conclusions

We identified a number of features that have statistically significant representation among exonic variants that modulate splicing. These analyses highlight putative mechanisms responsible for splicing outcome and emphasize the role of features important for exon definition. We developed a web-tool, Skippy, to score coding variants for these relevant splice-modulating features.

A novel single-base substitution (c.1124A>G) that activates a 5-base upstream cryptic splice donor site within exon 11 in the human mitochondrial acetoacetyl-CoA thiolase gene

Most mutations related to aberrant splicing occur in conserved splice acceptor and donor sites. Some exonic mutations also affect splicing. We identified and characterized a point mutation (c.1124A>G) in an Australian patient (GK43) with mitochondrial acetoacetyl-CoA thiolase (T2) deficiency. GK43 is a homozygote of c.1124A>G, which activates a cryptic splice donor site 5 bases upstream from c.1124A>G within exon 11, causing aberrant splicing in most transcripts. The aberrant splicing results in c.1120-1163 (44-base) deletion, causing a frameshift in T2 mRNA. A mini-gene splicing experiment confirmed that the c.1124A>G substitution was responsible for this aberrant splicing. This cryptic splice site has a Shapiro and Senapathy score (70.0) in a normal sequence but if mutated, the score (84.3) becomes higher than the one in the authentic splice donor site of intron 11 (81.4). This is an example in which a point mutation activates a cryptic splice donor site motif that is used preferentially over a downstream authentic splice site.

Kinetic and expression analyses of seven novel mutations in mitochondrial acetoacetyl-CoA thiolase (T2): identification of a Km mutant and an analysis of the mutational sites in the structure

Mitochondrial acetoacetyl-CoA thiolase (T2) deficiency is an inborn error of metabolism that affects isoleucine catabolism and ketone body metabolism. We identified 7 novel and 2 previously reported mutations in six T2-deficient patients. Transient expression analysis of wild-type and eight mutant cDNAs was performed at 40, 37 and 30 degrees C. Although no significant residual activity was detected, mutant proteins were detected in the N158D, N158S, R208Q, Y219H and N282H mutants. Accumulation of these mutant proteins was temperature-sensitive with the highest expression levels at lower temperatures. Expression of Q73P and N353K cDNAs yielded neither residual T2 protein nor enzyme activity. An E252del mutant T2 was detected with a relative protein amount and enzyme activity of 30% and 25%, respectively, in comparison to the wild-type at 37 degrees C. The E252del mutant protein was more stable at 30 degrees C expression than 37 degrees C, but was essentially undetectable at 40 degrees C, indicating its temperature-sensitive instability. Kinetic studies revealed a twofold K(m) elevation for substrates coenzyme A and acetoacetyl-CoA in the E252del mutant, while V(max) was comparable to the wild-type. We conclude that the E252del is a temperature-sensitive K(m) mutant. This correlates well with the effect predicted from the T2 tertiary structure analysis, using the crystal structure of the human T2 homotetramer. The probable effect of the other mutations on the T2 tertiary structure was also evaluated.

Identification of Alu-mediated, large deletion-spanning exons 2-4 in a patient with mitochondrial acetoacetyl-CoA thiolase deficiency

Mitochondrial acetoacetyl-CoA thiolase (T2) deficiency is a rare inherited metabolic disorder affecting isoleucine catabolism and ketone body metabolism. So far, more than 39 different mutations have been identified in 60 T2-deficient patients. However, no large deletions have been reported. We herein report the first case of a large T2 gene deletion from intron 1 to intron 4 in a T2-deficient patient (GK41). cDNA analysis revealed that an aberrant cDNA with exons 2-5 skipping was a major transcript, associated with a minor transcript of exons 2-4 skipping with a 94-bp insertion composed of an intron 1 sequence. Genomic analysis indicated an absence of PCR amplification of exons 2-4 and gene deletion was revealed by Southern blot analysis. Cloning and sequencing long range PCR products revealed a 6.4kb deletion. Alu element-mediated unequal homologous recombination between an Alu-Sx in intron 1 and another Alu-Y in intron 4 appears to be responsible for this deletion.

Biased exon/intron distribution of cryptic and de novo 3' splice sites

We compiled sequences of previously published aberrant 3′ splice sites (3′ss) that were generated by mutations in human disease genes. Cryptic 3′ss, defined here as those resulting from a mutation of the 3′YAG consensus, were more frequent in exons than in introns. They clustered in ∼20 nt region adjacent to authentic 3′ss, suggesting that their under-representation in introns is due to a depletion of AG dinucleotides in the polypyrimidine tract (PPT). In contrast, most aberrant 3′ss that were induced by mutations outside the 3′YAG consensus (designated ‘de novo’) were in introns. The activation of intronic de novo 3′ss was largely due to AG-creating mutations in the PPT. In contrast, exonic de novo 3′ss were more often induced by mutations improving the PPT, branchpoint sequence (BPS) or distant auxiliary signals, rather than by direct AG creation. The Shapiro–Senapathy matrix scores had a good prognostic value for cryptic, but not de novo 3′ss. Finally, AG-creating mutations in the PPT that produced aberrant 3′ss upstream of the predicted BPS in vivo shared a similar ‘BPS-new AG’ distance. Reduction of this distance and/or the strength of the new AG PPT in splicing reporter pre-mRNAs improved utilization of authentic 3′ss, suggesting that AG-creating mutations that are located closer to the BPS and are preceded by weaker PPT may result in less severe splicing defects.

Mitochondrial acetoacetyl-CoA thiolase (T2) deficiency: T2-deficient patients with "mild" mutation(s) were previously misinterpreted as normal by the coupled assay with tiglyl-CoA

Mitochondrial acetoacetyl-CoA thiolase (T2) deficiency is an inborn error of metabolism that affects the catabolism of isoleucine and ketone bodies. This disorder is characterized by intermittent ketoacidotic episodes. Recently, we diagnosed T2 deficiency in two patients (GK45 and GK47) by the absence of potassium ion-activated acetoacetyl-CoA thiolase activity, whereas these patients were previously misinterpreted as normal by a coupled assay with tiglyl-CoA as a substrate. This method has been widely used for the enzymatic diagnosis of the T2 deficiency in the United States and Europe. We hypothesized that some residual T2 activity showed normal results in the assay. To prove this hypothesis, we analyzed these two patients together with three typical T2-deficient patients (GK46, GK49, and GK50) at the DNA level. Expression analysis of mutant cDNAs clearly showed that GK45 and GK47 had "mild" mutations (A132G, D339-V340insD) that retained some residual T2 activity, at least one of two mutant alleles, whereas the other three patients had null mutations (c.52-53insC, G152A, H397D, and IVS8+1g>t) in either allele. These results raise the possibility that T2-deficient patients with mild mutations have been misinterpreted as normal by the coupled assay with tiglyl-CoA.

Single base substitutions at the initiator codon in the mitochondrial acetoacetyl-CoA thiolase (ACAT1/T2) gene result in production of varying amounts of wild-type T2 polypeptide

Initiator codon mutations are relatively uncommon and less well characterized compared to other types of mutations. We identified a novel initiator codon mutation (c.2T>C) heterozygously in a Japanese patient (Patient GK30) with mitochondrial acetoacetyl-CoA thiolase (T2) gene deficiency (ACAT1 deficiency); c.149delC was on the other allele. We examined translation efficiencies of nine mutant T2 cDNAs harboring one-base substitutions at the initiator methionine codon using in vivo transient expression analysis. We found that all the mutants produced wild-type T2 polypeptide, to various degrees (wild type (100%) > c.1A>C (66%) > c.2T>C, c.3G>C, c.3G>T (22%) > c3G>A, c.1A>G (11%) > c.2T>A, c.2T>G, c.1A>T (7.4%)). T2 mRNA expression levels in Patient GK08 (a homozygote of c.2T>A) and Patient GK30 fibroblasts, respectively, were almost the same as in control fibroblasts, when examined using semiquantitative PCR. This means that initiator codon mutations did not affect T2 mRNA levels. We propose that all one-base substitutions at the initiator methionine codon in the T2 gene could be mutations, which retain some residual T2 activity.

Characterization of six mutations in five Spanish patients with mitochondrial acetoacetyl-CoA thiolase deficiency: effects of amino acid substitutions on tertiary structure

Mitochondrial acetoacetyl-CoA thiolase (T2) deficiency is an inborn error of ketone body and isoleucine metabolism. We identified and characterized 6 mutations, DelE85, K124R, A127V, Q145E, G152A, and E345V in 5 Spanish T2-deficient patients. Transient expression of mutant cDNAs was done at 37 and at 30 degrees C. Expression of the Q145E mutant cDNA resulted in about 12.5% normal amount at 37 degrees C and it retained 15% residual T2, indicating that specific activity of Q145E mutant protein was almost normal. This mutation reduced the heat stability of T2 activity. Although no significant residual activity was detected in either the G152A and A127V substitution, mutant proteins were detected, at 12.5% the normal amount at 37 degrees C and one-half normal at 30 degrees C for A127V, and 25 % only at 30 degrees C for G152A. Mutant proteins with Q145E, G152A, or A127V accumulated at 30 degrees C expression were stable for 48 h at 37 degrees C after cycloheximide treatment. Expression of DelE85, K124R, and E345V cDNAs gave neither residual T2 protein nor T2 activity. We constructed an improved tertiary structural model of T2 based on the X-ray crystal structure of acetoacetyl-CoA thiolase of Zoogloea ramigera. On the basis of this model, K124, A127, and G152 are located near the active site, mutations of which might affect catalytic function whereas Q145E, De185E, and E345V are distant from the active site with mutants being expected to destabilize the tertiary structure, especially during protein folding and dimerization.