Stabilization of the predominant disease-causing aldolase variant (A149P) with zwitterionic osmolytes.

Hereditary fructose intolerance (HFI) is a disease of carbohydrate metabolism that can result in hyperuricemia, hypoglycemia, liver and kidney failure, coma, and death. Currently, the only treatment for HFI is a strict fructose-free diet. HFI arises from aldolase B deficiency, and the most predominant HFI mutation is an alanine to proline substitution at position 149 (A149P). The resulting aldolase B with the A149P substitution (AP-aldolase) has activity that is <100-fold that of the wild type. The X-ray crystal structure of AP-aldolase at both 4 and 18 °C reveals disordered adjacent loops of the (α/ß)(8) fold centered around the substitution, which leads to a dimeric structure as opposed to the wild-type tetramer. The effects of osmolytes were tested for restoration of structure and function. An initial screen of osmolytes (glycerol, sucrose, polyethylene glycol, 2,4-methylpentanediol, glutamic acid, arginine, glycine, proline, betaine, sarcosine, and trimethylamine N-oxide) reveals that glycine, along with similarly structured compounds, betaine and sarcosine, protects AP-aldolase structure and activity from thermal inactivation. The concentration and functional moieties required for thermal protection show a zwitterion requirement. The effects of osmolytes in restoring structure and function of AP-aldolase are described. Testing of zwitterionic osmolytes of increasing size and decreasing fractional polar surface area suggests that osmolyte-mediated AP-aldolase stabilization occurs neither primarily through excluded volume effects nor through transfer free energy effects. These data suggest that AP-aldolase is stabilized by binding to the native structure, and they provide a foundation for developing stabilizing compounds for potential therapeutics for HFI.

Hereditary fructose intolerance: functional study of two novel ALDOB natural variants and characterization of a partial gene deletion.

Hereditary fructose intolerance (HFI) is an autosomal recessive metabolic disease caused by impaired functioning of human liver aldolase (ALDOB). At least 54 subtle/point mutations and only two large intragenic deletions have been found in the ALDOB gene. Here we report two novel ALDOB variants (p.R46W and p.Y343H) and an intragenic deletion that we found in patients with suspected HFI. The residual catalytic activity of the recombinant p.R46W and p.Y343H variants toward F1P was particularly altered. We also characterized a large intragenic deletion that we found in six unrelated patients. This is the first report of six unrelated patients sharing the same ALDOB deletion, thus indicating a founder effect for this allele in our geographic area. Because this deletion involves ALDOB exon 5, it can mimic worldwide common pathogenic genotypes, that is, homozygous p.A150P and p.A175D. Finally, the identification of only one ALDOB mutation in symptomatic patients suggests that HFI symptoms can, albeit rarely, appear also in heterozygotes. Therefore, an excessive and continuous fructose dietary intake may have deleterious effects even in apparently asymptomatic HFI carriers.

Hereditary fructose intolerance: frequency and spectrum mutations of the aldolase B gene in a large patients cohort from France--identification of eight new mutations.

We investigated the molecular basis of hereditary fructose intolerance (HFI) in 160 patients from 92 families by means of a PCR-based mutation screening strategy, consisting of restriction enzyme digestion and direct sequencing. Sixteen different mutations of the aldolase B (ALDOB) gene were identified in HFI patients. As in previous studies, p.A150P (64%), p.A175D (16%) and p.N335K (5%) were the most common mutated alleles, followed by p.R60X, p.A338V, c.360_363delCAAA (p.N120KfsX30), c.324G>A (p.K108K) and c.625-1G>A. Eight novel mutations were also identified in 10 families with HFI: a one-base deletion (c.146delT (p.V49GfsX27)), a small deletion (c.953del42bp), a small insertion (c.689ins TGCTAA (p.K230MfsX136)), one splice site mutation (c.112+1G>A), one nonsense mutation (c.444G>A (p.W148X)), and three missense mutations (c.170G>C (p.R57P), c.839C>A (p.A280P) and c.932T>C (p.L311P)). Our strategy allows to diagnose 75% of HFI patients using restriction enzymatic analysis and to enlarge the diagnosis to 97% of HFI patients when associated with direct sequencing.

Ketohexokinase C blockade ameliorates fructose-induced metabolic dysfunction in fructose-sensitive mice.

Increasing evidence suggests a role for excessive intake of fructose in the Western diet as a contributor to the current epidemics of metabolic syndrome and obesity. Hereditary fructose intolerance (HFI) is a difficult and potentially lethal orphan disease associated with impaired fructose metabolism. In HFI, the deficiency of aldolase B results in the accumulation of intracellular phosphorylated fructose, leading to phosphate sequestration and depletion, increased adenosine triphosphate (ATP) turnover, and a plethora of conditions that lead to clinical manifestations such as fatty liver, hyperuricemia, Fanconi syndrome, and severe hypoglycemia. Unfortunately, there is currently no treatment for HFI, and avoiding sugar and fructose has become challenging in our society. In this report, through use of genetically modified mice and pharmacological inhibitors, we demonstrate that the absence or inhibition of ketohexokinase (Khk), an enzyme upstream of aldolase B, is sufficient to prevent hypoglycemia and liver and intestinal injury associated with HFI. Herein we provide evidence for the first time to our knowledge of a potential therapeutic approach for HFI. Mechanistically, our studies suggest that it is the inhibition of the Khk C isoform, not the A isoform, that protects animals from HFI.

Isolation and characterization of a mutant liver aldolase in adult hereditary fructose intolerance. Identification of the enzyme variant by radioassay in tissue biopsy specimens.

Hereditary fructose intolerance (HFI) is a metabolic disorder caused by enzymic deficiency of aldolase B, a genetically distinct cytosolic isoenzyme expressed exclusively in liver, kidney, and intestine. The molecular basis of this enzyme defect has been investigated in three affected individuals from a nonconsanguineous kindred, in whom fructose-l-phosphate aldolase activities in liver or intestinal biopsy samples were reduced to 2-6% of mean control values. To identify a putative enzyme mutant in tissue extracts, aldolase B was purified from human liver by affinity chromatography and monospecific antibodies were prepared from antiserum raised in sheep. Immunodiffusion gels showed a single precipitin line common to pure enzyme and extracts of normal liver and intestine, but no reaction with extracts of brain, muscle, or HFI liver. However, weak positive staining for aldolase in hepatocyte and enterocyte cytosol was demonstrated by indirect immunofluorescence of HFI tissues. This was abolished by pretreatment with pure enzyme protein. Accordingly, a specific radioimmunoassay (detection limit 7.5 ng) was established to quantify immunoreactive aldolase B in human biopsy specimens. Extracts of tissue from affected patients gave 10-25% immunoreactive enzyme in control samples; immunoreactive aldolase in intestinal extracts from four heterozygotes was reduced (to 55%) when compared with seven samples from normal control subjects (P < 0.05). In extracts of HFI tissues, there was a sevenfold reduction in apparent absolute specific activity (1.02 vs. 8.82 U/mg) of immunoreactive fructose-l-phosphate aldolase B, but the apparent specific activity in heterozygotes (7.71 U/mg) was only slightly impaired. Displacement radioimmunotitration of aldolase B in liver supernatants showed a significant (P < 0.005) decrease in antibody avidity for immunoreactive protein in HFI tissue when compared with the pure enzyme or extract of normal control liver. Immunoaffinity chromatography on antialdolase B-Sepharose facilitated isolation and purification of enzyme from liver biopsy specimens. Active aldolase in normal liver, with substrate activity ratios and Michaelis constants identical to biochemically purified human enzyme, could be recovered from antibody columns. Chromatography on monospecific Fab' antialdolase B enabled pure enzyme protein to be retrieved quantitatively from normal control and HFI liver: direct chemical assay showed 1.88 and 1.15 mg aldolase protein/g of tissue, respectively. This confirmed that the catalytic properties of the HFI aldolase were profoundly impaired with specific activities of fructose-l-phosphate cleavage of 7.21 and 0.07 U/mg, respectively. Radioimmunoassay gave estimates of 7.66 and 1.18 U/mg, respectively. Sodium dodecyl sulfate-polyacrylamide electrophoresis indicated that immunopurified aldolase from HFI liver possessed a single subunit size similar to material from control liver extracts: M(r) 39,100 vs. 37,900+/-700 (SD) D, respectively. Electrofocusing under denaturing conditions of aldolase isolated in parallel from control and HFI liver revealed the same complement of subunits and, despite qualitative differences in distribution of bands during degradation, no additional charged species. Fructose phosphate aldolase deficiency in hereditary fructose intolerance is attended by the synthesis of an immunoreactive, but functionally and structurally modified enzyme variant that results from a restricted genetic mutation.

Structure of the thermolabile mutant aldolase B, A149P: molecular basis of hereditary fructose intolerance.

Hereditary fructose intolerance (HFI) is a potentially lethal inborn error in metabolism caused by mutations in the aldolase B gene, which is critical for gluconeogenesis and fructose metabolism. The most common mutation, which accounts for 53% of HFI alleles identified worldwide, results in substitution of Pro for Ala at position 149. Structural and functional investigations of human aldolase B with the A149P substitution (AP-aldolase) have shown that the mutation leads to losses in thermal stability, quaternary structure, and activity. X-ray crystallography is used to reveal the structural basis of these perturbations. Crystals of AP-aldolase are grown at two temperatures (4 degrees C and 18 degrees C), and the structure solved to 3.0 angstroms resolution, using the wild-type structure as the phasing model. The structures reveal that the single residue substitution, A149P, causes molecular disorder around the site of mutation (residues 148-159), which is propagated to three adjacent beta-strand and loop regions (residues 110-129, 189-199, 235-242). Disorder in the 110-129-loop region, which comprises one subunit-subunit interface, provides an explanation for the disrupted quaternary structure and thermal instability. Greater structural perturbation, particularly at a Glu189-Arg148 salt bridge in the active-site architecture, is observed in the structure determined at 18 degrees C, which could explain the temperature-dependent loss in activity. The disorder revealed in these structures is far greater than that predicted by homology modeling and underscores the difficulties in predicting perturbations of protein structure and function by homology modeling alone. The AP-aldolase structure reveals the molecular basis of a hereditary disease and represents one of only a few structures known for mutant proteins at the root of the thousands of other inherited disorders.

The spectrum of aldolase B (ALDOB) mutations and the prevalence of hereditary fructose intolerance in Central Europe.

We investigated the molecular basis of hereditary fructose intolerance (HFI) in 80 patients from 72 families by means of a PCR-based mutation screening strategy, consisting of heteroduplex analysis, restriction enzyme digest, DNA single strand electrophoresis, and direct sequencing. For a subset of patients mutation screening with DHPLC was established which turned out to be as fast and as sensitive as the more conventional methods. Fifteen different mutations of the aldolase B (ALDOB) gene were identified in HFI patients. As in smaller previous studies, p.A150P (65%), p.A175D (11%) and p.N335K (8%) were the most common mutated alleles, followed by c.360_363delCAAA, p.R60X, p.Y204X, and c.865delC. Eight novel mutations were identified in eight families with HFI: a small indel mutation (c.1044_1049delTTCTGGinsACACT), two small deletions (c.345_372del28; c.841_842delAC), two splice site mutations (c.113-1G>A, c.799+2T>A), one nonsense mutation (c.612T>G (p.Y204X)), and two missense mutations (c.532T>C (p.C178R), c.851T>C (p.L284P)). By mutation screening for the three most common ALDOB mutations by DHPLC in 2,000 randomly selected newborns we detected 21 heterozygotes. Based on these data and after correction for less common and private ALDOB mutations, HFI prevalence in central Europe is estimated to be 1:26,100 (95% confidence interval 1: 12,600-79,000).

Hypertransaminasemia in childhood as a marker of genetic liver disorders.

BACKGROUND: The widespread use of routine biochemical assays has led to increased incidental findings of hypertransaminasemia. We aimed to evaluate the prevalence of different causes of raised aminotransferase levels in children referred to a university department of pediatrics. METHODS: We investigated 425 consecutive children (age range, 1-18 years) with isolated hypertransaminasemia. All patients had raised aminotransferase levels on at least two occasions in the last month before observation. Cases due to major hepatotropic viruses were excluded. RESULTS: During the first 6 months of observation, 259 children showed normalized liver enzymes. Among the remaining 166 patients with hypertransaminasemia lasting for more than 6 months, 75 had obesity-related liver disease; 51, genetic disorders; 7, autoimmune hepatitis; 5, cholelithiasis; 3, choledochal cyst; and 3, celiac disease. Among the 51 children with genetic disorders, 18 had Wilson disease; 14, muscular dystrophy; 4, alpha-1-antitrypsin deficiency; 4, Alagille syndrome; 4, hereditary fructose intolerance; 3, glycogen storage disease (glycogenosis IX); 2, ornithine transcarbamylase deficiency; and 2, Shwachman's syndrome. In 22 children, the hypertransaminasemia persisted for more than 6 months in the absence of a known cause. CONCLUSIONS: Genetic disease accounted for 12% of cases of isolated hypertransaminasemia observed in a tertiary pediatric department. A high level of suspicion is desirable for an early diagnosis of these disorders, which may present with isolated hypertransaminasemia and absence of typical clinical signs.

Six novel alleles identified in Italian hereditary fructose intolerance patients enlarge the mutation spectrum of the aldolase B gene.

Hereditary fructose intolerance (HFI) is a recessively inherited disorder of carbohydrate metabolism caused by impaired functioning of human liver aldolase (B isoform; ALDOB). To-date, 29 enzyme-impairing mutations have been identified in the aldolase B gene. Here we report six novel HFI single nucleotide changes identified by sequence analysis in the aldolase B gene. Three of these are missense mutations (g.6846T>C, g.10236G>T, g.10258T>C), one is a nonsense mutation (g.8187C>T) and two affect splicing sites (g.8180G>C and g.10196A>G). We have expressed in bacterial cells the recombinant proteins corresponding to the g.6846T>C (p.I74T), g.10236G>T (p.V222F), and g.10258T>C (p.L229P) natural mutants to study their effect on aldolase B function and structure. All the new variants were insoluble; molecular graphics data suggest this is due to impaired folding.

Unusual cerebral manifestations in hereditary fructose intolerance.

Five children with hereditary fructose intolerance developed symptoms of neurological impairment. In three of them, neurological involvement was related to the acute hepatic toxicity of fructose (hypoglycemia, abnormal coagulation, cardiovascular collapse); in the other two, such a relationship could not be demonstrated. Neurological impairment is not classic in hereditary fructose intolerance, but its occurrence in the acute phase of the disease is possible and does not constitute an argument against the diagnosis.

Patients with hereditary fructose intolerance have normal erythrocyte aldolase activity.

Erythrocyte fructose 1,6-bisphosphate aldolase (EC 4.1.2.13) activity was measured in eight children and adults with hereditary fructose intolerance and found to be normal when compared with eleven healthy controls. Therefore, hereditary fructose intolerance cannot be diagnosed by assaying red blood cell aldolase.

Isoelectrofocusing of aldolase B from normal human livers and from livers with hereditary fructose intolerance.

By isoelectrofocusing in thin-layer acrylamide-ampholine gel, normal human aldolase B has been resolved into 5 bands. Moreover we were able to specifically stain (after isoelectrofocusing) the mutated aldolase B in livers with hereditary fructose intolerance, and to show that only the 3 most anodic bands are seen. Some different hypotheses are discussed to account for the microheterogeneity of the normal aldolase B, and for the different isoelectrofocusing pattern found in livers with hereditary fructose intolerance.

Case report: heterogeneity of aldolase B in hereditary fructose intolerance.

Hereditary fructose intolerance (HFI) is a recessive genetic disorder with an estimated disease frequency of 1 in 20,000 and a carrier frequency of 1 in 70. Affected individuals are unable to assimilate fructose from fruit sugars and may develop severe hypoglycemia, metabolic problems, and death if misdiagnosed. Those who survive childhood learn to avoid sweets, effectively preventing further symptoms and complications. The disease is caused by a genetically defective hepatic enzyme, aldolase B. Traditionally, diagnosis has been made by intravenous fructose challenge or by liver biopsy, both difficult and risky invasive tests. Identification of mutations of the aldolase B gene by analysis of DNA from blood leukocytes is now possible, allowing for potential noninvasive diagnosis of subjects at risk in the future. The authors demonstrate heterozygosity for an aldolase B gene mutation in a patient with HFI.

[Clinical heterogeneity in fructose intolerance]. / Eterogeneità clinica nell'intolleranza al fruttosio.

We observed eight infants with hereditary fructose intolerance which had been diagnosed by the fructose tolerance test and an aldolase assay on biopsied liver. None of these had been diagnosed before their admission to our department. The most frequent symptoms were vomiting and failure to thrive. All the patients had hepatomegaly. Laboratory findings were indicative of disturbed hepatic function. Hypoglycemia was found in only 3 out of 8 patients. The course was lethal in 2 patients; the 6 survivors are doing well following a fructose-free diet. The importance of practising paediatricians having the detailed nutritional history of the patient and precise knowledge of infant food formulae is stressed. The danger of using fructose continuing solutions for infusion therapy is pointed out. We also report a case of F-1,6-diphosphatase deficiency.

Fatty liver disease and hypertransaminasemia hiding the association of clinically silent Duchenne muscular dystrophy and hereditary fructose intolerance.

We report a case with the association of well self-compensated hereditary fructose intolerance and still poorly symptomatic Duchenne type muscular dystrophy. This case illustrates the problems of a correct diagnosis in sub-clinical patients presenting with "cryptogenic" hypertransaminasemia.

Plasma lysosomal enzyme activities in congenital disorders of glycosylation, galactosemia and fructosemia.

BACKGROUND: Variable increases in the plasma activity of different lysosomal enzymes have been reported in patients with congenital disorders of glycosylation (CDG). In particular, elevated plasma aspartylglucosaminidase activity (AGA) has been found in the majority of CDG type I patients. We report on the plasma activity of AGA and other lysosomal enzymes in patients with different types of primary and secondary CDG defects. METHODS: AGA, alpha-mannosidase, beta-mannosidase and beta-hexosaminidase activities were assayed in the plasma of patients with CDGI (4CDGIa, 4CDGIx) and CDGIIx (5, all with a combined N- and O-glycosylation defect), classical galactosemia (GALT) (n=3) and hereditary fructose intolerance (HFI) (n=2). RESULTS: Increased AGA and beta-hexosaminidase activities were found in all and 7/8 of the GDGI patients respectively. All enzymic activities were normal in the CDGIIx patients. Elevated AGA and beta-hexosaminidase activity was also seen in GALT and HFI patients before treatment, when transferrin isoelectric focusing (TfIEF) patterns were also abnormal. CONCLUSIONS: Increased AGA plasma activity, although a consistent finding in CDGI patients, is not specific to this group of disorders since it is also observed in untreated cases of GALT and HFI. Furthermore, plasma AGA activity cannot serve as a marker for CDGII disorders. In conjunction with TfIEF it could be used in the follow up of GALT and HFI patients.