Hemochromatosis: genetic testing and clinical practice

The availability of a facile treatment for hemochromatosis renders early diagnosis of iron overload syndromes mandatory, and in many instances genetic testing allows identification of individuals at risk of developing clinical disease before pathologic iron storage occurs. Numerous proteins implicated in iron homeostasis have recently come to light, and defects in the cognate genes are associated with iron storage. Although most adult patients with hereditary iron overload are homozygous for the C282Y mutation of the HFE gene, an increasing number with hereditary iron storage have an HFE genotype not characteristic of the disease. Heterozygosity for mutations in the gene encoding ferroportin 1 (FPN1) is probably the second most common genetic cause of hereditary iron storage in adults; here the primarily affected cell is the macrophage. Rare defects, including mutations in the transferrin receptor 2 (TFR2) gene, have also been identified in pedigrees affected with "non-HFE hemochromatosis." Homozygous mutations in the newly identified genes encoding hemojuvelin (HFE2) and hepcidin (HAMP) cause juvenile hemochromatosis. At the same time, heterozygosity for mutations in these genes can modify the clinical expression of iron storage in patients predisposed to iron storage in adult life. Hemochromatosis might thus be considered as a polygenic disease with strong environmental influences on its clinical expression. As our mechanistic understanding of iron pathophysiology improves, our desire to integrate clinical decision making with the results of laboratory tests and molecular analysis of human genes poses increasing challenges.

Clinical evaluation of chemokine and enzymatic biomarkers of Gaucher disease

PURPOSE

Gaucher disease is an exemplary orphan disorder. Enzyme replacement therapy with imiglucerase is effective, but very expensive. To improve the assessment of severity of disease and responses to this costly treatment, we have evaluated several enzymatic biomarkers and a newly-described chemokine.

SUBJECTS AND METHODS

We studied 48 untreated adults with Type I Gaucher disease: 20 patients were studied after the introduction of enzyme replacement. Disease activity was monitored by serial measurement of platelet count, visceral volumes (spleen and liver) by magnetic resonance imaging, serum activities of total acid phosphatase, angiotensin-converting enzyme (ACE) and the lysosomal chitinase, chitotriosidase. Pulmonary and activation-regulated chemokine (PARC/CCL 18) was also determined in serum by ELISA.

RESULTS

Serum PARC concentrations were elevated 10-40-fold in patients with Gaucher disease compared with 67 healthy controls, without overlap (P<0.0001). Unlike chitotriosidase, PARC was detectable in all individuals. Serum PARC was a reliable indicator of splenic (R=0.53, P<0.01) and liver (R=0.65, P<0.01) volume and platelet count (R=0.50, P<0.01). In splenectomized patients and in patients with null alleles of the chitotriosidase gene, serum PARC concentration correlates with visceral volume and other biomarkers of disease activity. Unlike chitotriosidase, serum PARC concentrations showed unbiased covariation with splenic and platelet responsiveness to enzyme replacement.

CONCLUSION

Serum PARC concentrations are correlated with visceral Gaucher disease and with key clinical responses to enzyme complementation. Determination of this chemokine is a facile and universally applicable method that permits objective monitoring of enzyme replacement therapy for patients with Gaucher disease.

Primary iron overload with inappropriate hepcidin expression in V162del ferroportin disease

Ferroportin disease (hemochromatosis type 4) is a recently recognized disorder of human iron metabolism, characterized by iron deposition in macrophages, including Kupffer cells. Mutations in the gene encoding ferroportin 1, a cellular iron exporter, are responsible for this iron storage disease, inherited as an autosomal dominant trait. We present clinical, histopathological, and radiological findings in a family with the most common ferroportin mutation, V162del. In the index case, the disorder is characterized by abundant deposition of hemosiderin in all tissues investigated (mesenteric lymph node, liver, gastric and duodenal mucosa, and also in squamous cell carcinoma of the lung). The radiological findings indicated the presence of excess iron in bone marrow and spleen. Despite a significant burden of iron, no features of chronic liver disease were found in affected members of the family, including individuals aged up to 80 years. Hyperferritinemia greater than 1,000 microg/L was a penetrant biochemical finding before the second decade in life and was associated with significantly increased serum concentrations of pro-hepcidin that correlated positively with urinary hepcidin concentrations. In conclusion, the systemic iron burden in ferroportin disease is not a sufficient cause for chronic liver disease. In patients with most, but not all, ferroportin mutations, retention of iron in macrophages of the liver and other organs may protect against damage to parenchymal cells. Finally, macrophage iron storage in ferroportin disease is associated with elevated serum pro-hepcidin levels.

King George III and porphyria: an elemental hypothesis and investigation

In 1969 it was proposed that the episodic madness suffered by King George III (1738-1820) resulted from an acute hereditary porphyria, variegate porphyria, caused by deficiency of protoporphyrinogen oxidase. The diagnosis was based on the historical archive and a contentious claim that living members of the House of Hanover were affected with the condition. A re-examination of the medical evidence and the appearance of new historical material have suggested that porphyria did indeed exist in the Royal Houses of Europe. We report the analysis of hair obtained from George III. Although no genomic DNA could be obtained, metal analysis revealed high concentrations of arsenic. Since arsenic interferes with haem metabolism, it might have contributed to the King's unusually severe and prolonged bouts of illness. We have identified sources of arsenic in the context of the medication George III received from physicians.

Therapeutic goals in the treatment of Gaucher disease

Gaucher disease, the most common lysosomal storage disorder, is a heterogeneous multisystem condition. Patients with non-neuronopathic (type 1) Gaucher disease may suffer from hepatomegaly, splenomegaly, thrombocytopenia, bleeding tendencies, anemia, hypermetabolism, skeletal pathology, growth retardation, pulmonary disease, and decreased quality of life. Enzyme replacement therapy (ERT) with mannose-terminated glucocerebrosidase reverses or ameliorates many of the manifestations of type 1 Gaucher disease. However, the variable disease pattern and severity, and the uncertain manner of progression, render the decision to initiate ERT difficult. Thus, implementation of treatment and evaluation of the therapeutic response must be tailored to the individual patient. To obtain an evidence-based consensus on contemporary therapeutic goals, an international panel of physicians with extensive clinical experience in Gaucher disease met to review the extant literature on its treatment. The panel adopted an integrated system-based approach to arrive at a comprehensive guide to individualized management. Here we establish goals of treatment in Gaucher disease and propose a comprehensive schedule of monitoring of all relevant aspects to confirm the achievement, maintenance, and continuity of the therapeutic response.

Tartrate-resistant acid phosphatase: a potential target for therapeutic gold

Gold compounds are disease-modifying agents for the treatment of rheumatoid arthritis. They act on the immune system but the mechanism is not fully understood. Gold has been shown to affect antigen processing by T-cells and also reduces expression of cytokines in macrophages. Tartrate-resistant acid phosphatase (TRAP), expressed by osteoclasts, macrophages and dendritic cells is an enzyme with roles in skeletal metabolism and the immune response. TRAP is able to degrade skeletal phosphoproteins including osteopontin, identical to the T-cell cytokine, Eta-1; we thus propose that TRAP regulates the Eta-1 pathway common to the immune system and skeleton. We compared the distribution of osteopontin and TRAP in sections of 18-day-old embryonic mice by immunohistochemistry. Both proteins occurred in the same locations. To determine whether gold compounds exert their effects by modification of TRAP activity, we examined the action of gold chloride and the prodrugs, aurothioglucose and aurothiomalate on the dephosphorylation of osteopontin by TRAP. Aurothioglucose and aurothiomalate had little effect on phosphatase activity; gold chloride was a potent non-competitive inhibitor (Ki < 47 x 10(-9) M). These findings indicate a possible molecular mechanism for the action of therapeutic gold and further implicate TRAP in the control of immunity.

Clinical evaluation of biomarkers in Gaucher disease

Novel or candidate biomarkers require thorough evaluation to establish their utility in a clinical setting. This paper describes an evaluation of several established enzyme markers of Gaucher disease and a newly-described chemokine, pulmonary and activation-regulated chemokine (PARC). The ability of the biomarkers to rank patients with Gaucher disease in order of disease severity and organ bulk, and to reflect changes in key clinical parameters in response to enzyme replacement therapy were evaluated. PARC concentrations were found to be reliably correlated with visceral disease and with key clinical responses to enzyme replacement in an unbiased manner. Unlike chitotriosidase and serum angiotensin-converting enzyme activity, genetic variation in serum PARC did not appear to influence its utility as a biomarker.

CONCLUSION

For each new candidate biomarker of lysosomal storage diseases, a similar clinical evaluation will be required, though the approach will need to be modified according to the clinical features and natural history of each disorder.

Substrate reduction therapy for lysosomal storage diseases

The treatment of disordered lipoprotein metabolism with the statin class of drugs is one of the most striking successes in the field of applied medical science: here the use of selective inhibitors of the first committed step of cholesterol biosynthesis, in a complex and highly regulated pathway, leads to improved outcome from a common lipid storage disease that is a blight on whole populations--atherosclerosis. By the same token, substrate reduction is an emerging therapeutic strategy for the arcane field of the lysosomal storage diseases (LSDs). Reduced biosynthesis of glucosylceramide is postulated to allow correction of the imbalance between formation and breakdown of glycosphingolipids; the therapeutic effect of substrate reduction depends upon the presence of residual hydrolytic activity towards those accumulated glycosphingolipid substrates derived from glucosylceramide. First pioneered in the laboratory by Norman Radin, this approach has now been introduced into the clinic: based on the ability to inhibit uridine diphosphate glucosylceramide transferase, the semi-selective iminosugar, N-butyldeoxynojirimycin, is licensed for the treatment of type 1 Gaucher disease.

CONCLUSION

Inhibition of substrate formation has wide application in the treatment of LSDs. Decreased glucosylceramide biosynthesis has therapeutic potential in glycosphingolipidoses other than Gaucher disease, and offers promise in several neurodegenerative storage disorders that are currently beyond the reach of other procedures. The results of ongoing clinical trials of miglustat in type 3 Gaucher disease, Niemann-Pick disease type C and GM2 gangliosidosis are eagerly awaited.

Biomarkers in lysosomal storage diseases: a review

A biomarker is generally an analyte that indicates the presence or extent of a biological process, which is itself directly linked to the clinical manifestations and outcome of a particular disease. An ideal biomarker provides indirect but ongoing and specific determinations of disease activity. These characteristics emphasize the value of surrogate biomarkers for non-invasive and detailed monitoring to demonstrate the efficacy of orphan drugs in clinical trials. The emergence of novel laboratory methods has facilitated the search for biomarkers in lysosomal storage diseases (LSDs), by allowing the systematic identification of molecules whose expression is altered as a result of the primary storage pathology. In Gaucher disease, for example, a chemokine, CCL18, has been identified as a biomarker for clinical development that reflects disease severity and response to treatment.

CONCLUSION

New methods for the identification of novel biomarkers have the potential to provide mechanistic insights into the molecular pathogenesis of LSDs, including Fabry disease and Gaucher disease.

Monitoring enzyme replacement therapy in Fabry disease--role of urine globotriaosylceramide

Anderson-Fabry disease (referred to as Fabry disease) is an X-linked disorder characterized by a deficiency of the lysosomal enzyme alpha-galactosidase A and the subsequent accumulation in various tissues of globotriaosylceramide (Gb(3)), the main substrate of the defective enzyme. Enzyme replacement therapy (ERT) offers a specific treatment for patients with Fabry disease, though monitoring of treatment is hampered by a lack of surrogate markers of response. In this study, the efficacy of long-term ERT in six Fabry hemizygotes and two symptomatic heterozygotes has been evaluated. Patients were administered recombinant alpha-galactosidase A every 2 weeks for up to a year. The efficacy of ERT was assessed by monitoring symptomatology and renal function. Urinary glycolipid concentration was estimated by a novel tandem mass spectrometric method. Urine glycolipid (Gb(3)) was elevated at baseline and fell impressively on ERT where patients were hemizygotes and in the absence of renal transplantation. In heterozygotes and in a recipient of a renal allograft, elevations and changes in urine glycolipids were less pronounced. In one patient, after several months of ERT, there was a transient increase in Gb(3) concentrations to baseline (pre-ERT) levels, associated with the presence of antibodies to the recombinant alpha-galactosidase A. The marked decline in urine Gb(3) on ERT, and its subsequent increase in association with an inhibitory antibody response, suggest that this analyte deserves further investigation as a potential marker of disease severity and response to treatment.

Treatment with miglustat reverses the lipid-trafficking defect in Niemann–Pick disease type C

Niemann-Pick disease type C (NP-C) is a hereditary neurovisceral lipid storage disorder. Although traditionally considered a primary cholesterol storage disorder, a variety of glycolipids accumulate in NP-C cells, which resemble those from glycosphingolipidosis patients. Substrate reduction therapy (SRT) with miglustat, an inhibitor of glycosphingolipid biosynthesis, is a novel therapy for the glycosphingolipidoses. We report the use of SRT in a patient with NP-C. We show that depletion of glycosphingolipids by miglustat treatment reduces pathological lipid storage, improves endosomal uptake and normalises lipid trafficking in peripheral blood B lymphocytes. The demonstration that treatment with miglustat, which has no direct effect on cholesterol metabolism, corrects the abnormal lipid trafficking seen in B lymphocytes in NP-C indicates that glycosphingolipid accumulation is the primary pathogenetic event in NP-C. These observations support the use of SRT in patients with this devastating neurodegenerative disease.

Twin pairs showing discordance of phenotype in adult Gaucher's disease

BACKGROUND

Non-neuronopathic (type 1) Gaucher's disease, a recessive disorder caused by glucocerebrosidase deficiency, shows marked variability in the severity and extent of clinical expression: many individuals who harbour two mutant alleles remain mildly affected or asymptomatic. Despite much effort, it is not possible accurately to predict disease severity from the genotype, or to identify those patients destined to develop severe disease and meriting early treatment.

AIM

To determine the degree to which variance in Gaucher disease is determined by non-heritable factors.

DESIGN

Case reports of monozygotic and dizygotic twin pairs.

RESULTS

For the monozygotic twin pair, homozygous for the frequent N370S glucocerebrosidase allele, there was no evidence that significant lipid storage was ever initiated in the unaffected twin. In contrast, pathological storage of glucocerebroside has been present in the macrophages of both members of the dizygotic twin pair (compound heterozygotes for the N370S and L444P alleles) from an early age but, by the age of 57 years, only one has developed symptoms.

DISCUSSION

Non-heritable factors influence Gaucher disease expression in genetically predisposed individuals. Understanding the interactions between heritable and non-heritable factors will be critical for an analysis of pathogenesis, and the treatment of individuals predisposed to Gaucher disease.

Spectrum of hemojuvelin gene mutations in 1q-linked juvenile hemochromatosis

Juvenile or type 2 hemochromatosis (JH) is transmitted as a recessive trait that leads to severe iron overload and organ damage typically before age 30 years. Linkage to a locus on chromosome 1q has been found in most patients with JH. The recently identified causal gene encodes hemojuvelin, a protein with a proposed crucial role in iron metabolism. A second, rare type of JH, with clinical expression identical to the 1q-linked form, is due to inactivation of hepcidin, the key regulator of iron homeostasis. Here we report the spectrum of mutations of the hemojuvelin gene (HJV) in 34 patients who did not show hepcidin mutations. This represents the largest cohort of patients with JH collected worldwide. We identified 17 different (16 novel) mutations of HJV, both at the homozygous and at the compound heterozygous state. Mutations either generate premature termination codons or were missense substitutions, affecting highly conserved residues, relevant to the protein structure and/or function.

Sustained therapeutic effects of oral miglustat (Zavesca, N-butyldeoxynojirimycin, OGT 918) in type I Gaucher disease

It has been shown that treatment with miglustat (Zavesca, N-butyldeoxynojirimycin, OGT 918) improves key clinical features of type I Gaucher disease after 1 year of treatment. This study reports longer-term efficacy and safety data. Patients who had completed 12 months of treatment with open-label miglustat (100-300 mg three times daily) were enrolled to continue with therapy in an extension study. Data are presented up to month 36. Liver and spleen volumes measured by CT or MRI were scheduled every 6 months. Biochemical and haematological parameters, including chitotriosidase activity (a sensitive marker of Gaucher disease activity) were monitored every 3 months. Safety data were also collected every 3 months. Eighteen of 22 eligible patients at four centres entered the extension phase and 14 of these completed 36 months of treatment with miglustat. After 36 months, there were statistically significant improvements in all major efficacy endpoints. Liver and spleen organ volumes were reduced by 18% and 30%, respectively. In patients whose haemoglobin value had been below 11.5 g/dl at baseline, mean haemoglobin increased progressively from baseline by 0.55 g/dl at month 12 (NS), 1.28 g/dl at month 24 (p =0.007), and 1.30 g/dl at month 36 (p =0.013). The mean platelet count at month 36 increased from baseline by 22 x 10(9)/L. No new cases of peripheral neuropathy occurred since previously reported. Diarrhoea and weight loss, which were frequently reported during the initial 12-month study, decreased in magnitude and prevalence during the second and third years. Patients treated with miglustat for 3 years show significant improvements in organ volumes and haematological parameters. In conclusion, miglustat was increasingly effective over time and showed acceptable tolerability in patients who continued with treatment for 3 years.

Marked elevation of the chemokine CCL18/PARC in Gaucher disease: a novel surrogate marker for assessing therapeutic intervention

Gaucher disease is characterized by storage of glucosylceramide in lysosomes of tissue macrophages as the result of an autosomal recessively inherited deficiency in glucocerebrosidase. Progressive accumulation of these glycolipid-laden Gaucher cells causes a variety of debilitating symptoms. The disease can be effectively treated by costly intravenous infusions with recombinant glucocerebrosidase. Chitotriosidase is massively secreted by Gaucher cells and its plasma levels are used to monitor efficacy of enzyme therapy. Broad-scale application is hampered by the common genetic defect in this surrogate marker. We report that in plasma of symptomatic patients with Gaucher disease the chemokine CCL18 is on average 29-fold elevated, without overlap between patient and control values (median control plasma level is 33 ng/mL, range, 10-72 ng/mL; median Gaucher plasma level is 948 ng/mL, range, 237-2285 ng/mL). Plasma CCL18 concentrations decrease during therapy, comparably to chitotriosidase levels. Immunohistochemistry demonstrates that Gaucher cells are the prominent source of CCL18. Plasma CCL18 levels can serve as alternative surrogate marker for storage cells in patients with Gaucher disease and monitoring of plasma CCL18 levels proves to be useful in determination of therapeutic efficacy, especially in patients who are deficient in chitotriosidase activity. The potential physiologic consequences of chronically elevated CCL18 in patients with Gaucher disease are discussed.

Screening hepcidin for mutations in juvenile hemochromatosis: identification of a new mutation (C70R)

Juvenile or type 2 hemochromatosis (JH) is a genetic disease caused by increased intestinal iron absorption that leads to early massive iron overload. The main form of the disease is caused by mutations in a still unknown gene on chromosome 1q. Recently, we recognized a second type of JH with clinical features identical to the 1q-linked form, caused by mutations in the gene encoding hepcidin (HEPC). Hepcidin is a hepatic antimicrobial-like peptide whose role in iron homeostasis was first defined in animal models; deficiency of hepcidin in mice leads to iron overload, whereas its hepatic overexpression in transgenic animals causes iron deficiency. To define the prevalence of HEPC mutations in JH we screened the HEPC gene for mutation in 21 unrelated JH subjects. We identified a new mutation (C70R), which affects 1 of the 8 conserved cysteines that form the disulfide bonds and are critical for the stability of the polypeptide.

Tartrate-resistant acid phosphatase knockout mice

TRACP is a lysosomal enzyme found in diverse tissues, where it is expressed in dendritic cells as well as osteoclasts and macrophages. To investigate the function of TRACP in vivo, we have generated mice in which the gene-encoding TRACP has been selectively disrupted by targeted homologous recombination in murine embryonic stem cells. Homozygous TRACP "knockout" mice have progressive foreshortening and deformity of the long bones and axial skeleton suggesting a role for TRACP in endochondral ossification. There is increased mineralization reflecting a mild osteopetrosis caused by reduced osteoclast modeling activity. These knockout mice also display an impairment of macrophage function with abnormal immunomodulatory cytokine responses. Superoxide formation and nitrite production were enhanced in stimulated macrophages lacking TRACP as was the secretion of the proinflammatory cytokines TNF-alpha, interleukin (IL)-1beta, and IL-12. TRACP knockout mice showed delayed clearance of the microbial pathogen Staphylococcus aureus after sublethal intraperitoneal inoculation. The macrophages lacking TRACP showed an increase in tartrate-sensitive lysosomal acid phosphatase activity (LAP). The TRACP knockout mice were bred with mice lacking LAP. Mice lacking both TRACP and LAP had even shorter bones than the TRACP single knockouts. Osteopontin, identical to the T-cell cytokine eta-1, accumulated adjacent to actively resorbing osteoclasts suggesting that both phosphatases are important for processing this protein. We propose that TRACP may be an important regulator of osteopontin/eta-1 activity common to both the immune system and skeleton.

Typical type 2 diabetes mellitus and HFE gene mutations: a population-based case - control study

Diabetes mellitus is a recognized consequence of hereditary haemochromatosis. Whether the common HFE mutations, that associate with this condition and pre-dispose to increases in serum iron indices, are over-represented in diabetic populations remains controversial. We present data from the largest case-control study of the C282Y and H63D HFE allele frequencies in typical type 2 diabetes mellitus, as defined by an age of onset greater than 30 years and no requirement for insulin in the first year post-diagnosis. We also present a meta-analysis of all similar studies to date. We see no evidence for over-representation of iron loading HFE alleles in type 2 diabetes mellitus, suggesting that screening for HFE mutations in this population is of no value.

Introduction

In an era dominated by molecular biology, genomics and proteomics it is perhaps not surprising that a relatively small number of scientists worldwide have focused their research on an obscure family of glycoconjugates known as the glycosphingolipids. At first glance, these molecules have very little to commend them; they are complex and variable, not abundant in relation to other cellular lipids, are the product of multi–gene pathways making them hard to manipulate, are not trivial to purify or characterize and the technologies to analyse them have traditionally been insensitive and cumbersome. To compound the problem, we do not fully understand the role they play in cells let alone how they contribute to homeostasis in the intact organism. Theories abound but, until recently, hard evidence has been lacking. For decades they have therefore been a minority interest overlooked by the majority of investigators in both the biological and biomedical sciences.

Future perspectives for glycolipid research in medicine

Medical interest in glycolipids has been mainly directed to the rare and complex glycosphingolipid storage disorders that are principally caused by unitary deficiencies of lysosomal acid hydrolases. However, glycolipids are critical components of cell membranes and occur within newly described membrane domains known as lipid rafts. Glycolipids are components of important antigen systems and membrane receptors; they participate in intracellular signalling mechanisms and may be presented to the immune system in the context of the novel CD1 molecules present on T lymphocytes. A knowledge of their mechanism of action in the control of cell growth and survival as well as developmental pathways is likely to shed light on the pathogenesis of the glycosphingolipid storage disorders as well as the role of lipid second messengers in controlling cell mobility and in the mobilization of intracellular calcium stores (a biological role widely postulated particularly for the lysosphingolipid metabolite sphingosine 1-phosphate). Other sphingolipid metabolites such as ceramide 1-phosphate may be involved in apoptotic responses and in phagocytosis and synaptic vesicle formation. The extraordinary pharmaceutical success of enzymatic complementation for Gaucher's disease using macrophage-targeted human glucocerebrosidase has focused further commercial interest in other glycolipid storage diseases: the cost of targeted enzyme therapy and its failure to restore lysosomal enzymatic deficiencies in the brain has also stimulated interest in the concept of substrate reduction therapy using diffusible inhibitory molecules. Successful clinical trials of the iminosugar N-butyldeoxynojirimycin in type 1 Gaucher's disease prove the principle of substrate reduction therapy and have attracted attention to this therapeutic method. They will also foster important further experiments into the use of glycolipid synthesis inhibitors for the severe neuronopathic glycosphingolipidoses, for which no definitive treatment is otherwise available. Future glycolipid research in medicine will be directed to experiments that shed light on the role of sphingolipids in signalling pathways, and in the comprehensive characterization and their secretory products in relation to the molecular pathogenesis of the storage disorders; experiments of use to improve the efficiency of complementing enzymatic delivery to the lysosomal compartment of storage cells are also needed. Further systematic screening for inhibitory compounds with specific actions in the pathways of glycosphingolipid biosynthesis will undoubtedly lead to clinical trials in the neuronopathic storage disorders and to wider applications in the fields of immunity and cancer biology.

Co-localization of the mammalian hemochromatosis gene product (HFE) and a newly identified transferrin receptor (TfR2) in intestinal tissue and cells

Mutations in the HFE gene and a newly identified second transferrin receptor gene, TfR2, cause hemochromatosis. The cognate proteins, HFE and TfR2, are therefore of key importance in human iron homeostasis. HFE is expressed in small intestinal crypt cells where transferrin-iron entry may determine subsequent iron absorption by mature enterocytes, but the physiological function of TfR2 is unknown. Using specific peptide antisera, we examined the duodenal localization of HFE and TfR2 in humans and mice, with and without HFE deficiency, by confocal microscopy. We also investigated potential interactions of these proteins in human intestinal cells in situ. Duodenal expression of HFE and TfR2 (but not TfR1) in wild-type mice and humans was restricted to crypt cells, in which they co-localized. HFE deficiency disrupted this interaction, altering the cellular distribution of TfR2 in human crypts. In human Caco-2 cells, HFE and TfR2 co-localized to a distinct CD63-negative vesicular compartment showing marked signal enhancement on exposure to iron-saturated transferrin ligand, indicating that HFE preferentially interacts with TfR2 in a specialized early endosomal transport pathway for transferrin-iron. This interaction occurs specifically in small intestinal crypt cells that differentiate to become iron-absorbing enterocytes. Our immunohistochemical findings provide evidence for a novel mechanism for the regulation of iron balance in mammals.

The role of the iminosugar N-butyldeoxynojirimycin (miglustat) in the management of type I (non-neuronopathic) Gaucher disease: a position statement

N-Butyldeoxynojirimycin (NB-DNJ, miglustat 'Zavesca') is an orally active iminosugar which inhibits the biosynthesis of macromolecular substrates that accumulate pathologically in glycosphingolipidoses. Clinical trials of NB-DNJ in patients with Gaucher's disease demonstrate the therapeutic potential of such substrate inhibitors in the glycolipid storage disorders. However, macrophage-targetted enzyme replacement using intravenous mannose-terminated human glucocerebrosidase (imiglucerase, Cerezyme) is highly effective in ameliorating many of the manifestations of Gaucher's disease and is a treatment in widespread use. Given that imiglucerase and miglustat are now both licensed for the treatment of Gaucher's disease, there is a need to review their therapeutic status. Here the treatment of type 1 (non-neuronopathic) Gaucher disease is evaluated with particular reference to the emerging role of oral N-butyldeoxynojirimycin (miglustat) as a substrate-reducing agent. This position statement represents the consensus viewpoint of an independent international advisory council to the European Working Group on Gaucher Disease.

Hemochromatosis--neonatal and young subjects

Juvenile hemochromatosis (JH) is an autosomal recessive disease causing iron overload before age 30 in both sexes. JH is characterised by hypogonadism, growth retardation and cardiomyopathy. Linkage of JH to chromosome lq is established in pedigrees throughout Europe. Studies of 29 patients in 20 families of diverse ethnic origin confirm early-onset iron overload. Neonatal hemochromatosis (NH) is a syndrome of unknown origin characterized by congenital cirrhosis or fulminant hepatitis with hepatic and extra-hepatic iron deposits. We assessed 40 infants from 27 families and identified 3 patterns of disease transmission. In 12 of the 27 there was >1 affected infant and in 5 families all infants were affected by NH. In 19 families unaffected children were also born. In 4 families there was bacterial or viral maternal infection associated with NH. In two families, antibodies to DNA or ribonuclear proteins were identified. In 12 families, unaffected children were born to the same parents in the absence of maternal antibodies or infection and without indications of maternal transmission. Consanguinity was observed in 1 family with 4 affected offspring (1 stillbirth + 3 neonatal deaths). Sequence analysis of HFE, beta2M, and both human heme oxygenase genes failed to identify any causal mutations in nuclear NH families but our study points to the existence of a cohort of patients likely to suffer from an autosomal recessive trait. A genome wide scanning study is underway to identify the putative locus.

The genetic consequences of our sweet tooth

First reported in 1956, hereditary fructose intolerance (HFI) illustrates vividly how interactions between genes and nutrients can influence taste preferences; the disease also reflects the ascendancy of sucrose and fructose as energy sources and as the world's principal sweeteners. However, HFI is not the only genetic ill to have emerged from our obsession with sugar: the slave trade, which had such a key part in the development of the sugar industry, also included major genetic consequences in its haunting legacy.

Overlapping functions of lysosomal acid phosphatase (LAP) and tartrate-resistant acid phosphatase (Acp5) revealed by doubly deficient mice

To date, two lysosomal acid phosphatases are known to be expressed in cells of the monocyte/phagocyte lineage: the ubiquitously expressed lysosomal acid phosphatase (LAP) and the tartrate-resistant acid phosphatase-type 5 (Acp5). Deficiency of either acid phosphatase results in relatively mild phenotypes, suggesting that these enzymes may be capable of mutual complementation. This prompted us to generate LAP/Acp5 doubly deficient mice. LAP/Acp5 doubly deficient mice are viable and fertile but display marked alterations in soft and mineralised tissues. They are characterised by a progressive hepatosplenomegaly, gait disturbances and exaggerated foreshortening of long bones. Histologically, these animals are distinguished by an excessive lysosomal storage in macrophages of the liver, spleen, bone marrow, kidney and by altered growth plates. Microscopic analyses showed an accumulation of osteopontin adjacent to actively resorbing osteoclasts of Acp5- and LAP/Acp5-deficient mice. In osteoclasts of phosphatase-deficient mice, vacuoles were frequently found which contained fine filamentous material. The vacuoles in Acp5- and LAP/Acp5 doubly-deficient osteoclasts also contained crystallite-like features, as well as osteopontin, suggesting that Acp5 is important for processing of this protein. This is further supported by biochemical analyses that demonstrate strongly reduced dephosphorylation of osteopontin incubated with LAP/Acp5-deficient bone extracts. Fibroblasts derived from LAP/Acp5 deficient embryos were still able to dephosphorylate mannose 6-phosphate residues of endocytosed arylsulfatase A. We conclude that for several substrates LAP and Acp5 can substitute for each other and that these acid phosphatases are essential for processing of non-collagenous proteins, including osteopontin, by osteoclasts.

Classification and genetic features of neonatal haemochromatosis: a study of 27 affected pedigrees and molecular analysis of genes implicated in iron metabolism

Neonatal haemochromatosis (NH) is a severe and newly recognised syndrome of uncertain aetiology, characterised by congenital cirrhosis or fulminant hepatitis and widespread tissue iron deposition. NH occurs in the context of maternal disease including viral infection, as a complication of metabolic disease in the fetus, and sporadically or recurrently, without overt cause, in sibs. Although an underlying genetic basis for NH has been suspected, no test is available for predictive analysis in at risk pregnancies. As a first step towards an understanding of the putative genetic basis for neonatal haemochromatosis, we have conducted a systematic study of the mode of transmission of this disorder in a total of 40 infants born to 27 families. We have moreover carried out a molecular analysis of candidate genes (beta(2)-microglobulin, HFE, and haem oxygenases 1 and 2) implicated in iron metabolism. No pathogenic mutations in these genes were identified that segregate consistently with the disease phenotype in multiplex pedigrees. However, excluding four pedigrees with clear evidence of maternal infection associated with NH, a pedigree showing transmission of maternal antinuclear factor and ribonucleoprotein antibodies to the affected infants, and two families with possible matrilineal inheritance of disease in maternal half sibs, a large subgroup of the affected pedigrees point to the inheritance of an autosomal recessive trait. This included 14 pedigrees with affected and unaffected infants and a single pedigree where all four affected infants were the sole offspring of consanguineous but otherwise healthy parents. We thus report three distinct patterns of disease transmission in neonatal haemochromatosis. In the differentiation of a large subgroup showing transmission of disease in a manner suggesting autosomal recessive inheritance, we also provide the basis for further genome wide studies to define chromosomal determinants of iron storage disease in the newborn.

Tartrate-resistant acid phosphatase (Acp 5): identification in diverse human tissues and dendritic cells

Histochemical demonstration of tartrate-resistant acid phosphatase (TRAP) is used for the specific identification of osteoclasts. The enzyme, which we have shown to be critical for normal bone development in mice, is also characteristic of monohistiocytes, including alveolar macrophages, and is associated with diverse pathological conditions such as Gaucher's disease and hairy cell leukemia. TRAP activity is enhanced in serum when bone resorption is increased, and the activity is used routinely to monitor treatment responses in Gaucher's disease. We have lately shown widespread expression of the enzyme in murine tissues with particular reference to the skin, thymus, gut epithelia, and isolated dendritic cells, suggesting a possible role in immunity. To further clarify the significance of TRAP in human physiology, we have examined its distribution in non-skeletal human tissues and in CD34+ -derived human dendritic cells. TRAP mRNA determined by Northern blotting analysis was expressed abundantly in spleen, liver, colon, lung, small intestine, kidney, stomach, testis, placenta, lymph node, thymus, peripheral blood leukocyte, bone marrow, and fetal liver. Expression of TRAP protein was investigated by immunohistochemistry, with which the enzyme was identified in multiple tissues. Histochemical staining detected enzymatically active protein in spleen, lung, skin, colon, stomach, and ileum. Active TRAP was identified in CD34+ -derived immature dendritic cells and co-localized to intracellular CD63 positive organelles. When these cells were matured by induction with LPS, the TRAP activity increased fivefold and remained within the cell during the phase associated with CD63 surface expression. Our findings demonstrate widespread expression of TRAP in human tissues. Its abundant expression in epithelia and dendritic cells suggests a potential role in antigen processing and in immune responses.

Intestinal iron uptake determined by divalent metal transporter is enhanced in HFE-deficient mice with hemochromatosis

BACKGROUND & AIMS

Overexpression of duodenal divalent metal transporter (DMT1) messenger RNA occurs in hemochromatosis and HFE-knockout mice, suggesting that DMT1 mediates enhanced absorption of iron; however, increased expression of functional DMT1 protein has yet to be substantiated. We examined the role of DMT1 and the mucosal iron uptake defect in HFE-knockout mice.

METHODS

Unidirectional iron uptake of 59Fe by small intestinal mucosa in vitro was compared between matched pairs of HFE-knockout and wild-type mice. DMT1-specific antibodies were used to block iron transport and to quantify duodenal protein expression.

RESULTS

Ferrous iron uptake at 3.5-450 micromol/L was greatly enhanced in HFE-knockouts compared with wild-type, the apparent V(max) for Fe2+ transport being doubled (P < 0.01). Supplied as Fe3+, uptake was only enhanced in HFE-knockouts at < or =18 micromol/L, when the iron was almost completely converted to Fe2+ by mucosal ferrireductases. DMT1 antibody reduced the apparent Vmax for mucosal Fe2+ transport in HFE-knockouts to below wild-type control values (P < 0.02); immunoreactive mucosal DMT1 protein was increased nearly 2-fold in HFE-knockouts (P < 0.01).

CONCLUSIONS

Disruption of the HFE gene up-regulates functional DMT1 transporters and enhances uptake of ferrous iron by this mechanism; DMT1 also mediates increased uptake after reduction of ferric iron presented at physiological concentrations.

Inhibition of substrate synthesis as a strategy for glycolipid lysosomal storage disease therapy

The glycosphingolipid (GSL) lysosomal storage diseases are caused by mutations in the genes encoding the glycohydrolases that catabolize GSLs within lysosomes. In these diseases the substrate for the defective enzyme accumulates in the lysosome and the stored GSL leads to cellular dysfunction and disease. The diseases frequently have a progressive neurodegenerative course. The therapeutic options for treating these diseases are relatively limited, and for the majority there are no effective therapies. The problem is further compounded by difficulties in delivering therapeutic agents to the brain. Most research effort to date has focused on strategies for augmenting enzyme levels to compensate for the underlying defect. These include bone marrow transplantation (BMT), enzyme replacement and gene therapy. An alternative strategy that we have been exploring is substrate deprivation. This approach aims to balance the rate of GSL synthesis with the impaired rate of GSL breakdown. The imino sugar N-butyldeoxynojirimycin (NB-DNJ) inhibits the first step in GSL biosynthesis and has been used to evaluate this approach. Studies in an asymptomatic mouse model of Tay-Sachs disease have shown that substrate deprivation prevents GSL storage in the CNS. In a severe neurodegenerative mouse model of Sandhoff disease, substrate deprivation delayed the onset of symptoms and disease progression and significantly increased life expectancy. Combining NB-DNJ and BMT was found to be synergistic in the Sandhoff mouse model. A clinical trial in type I Gaucher disease has been undertaken and has shown beneficial effects. Efficacy was demonstrated on the basis of significant decreases in liver and spleen volumes, gradual but significant improvement in haematological parameters and disease activity markers, together with diminished GSL biosynthesis and storage as determined by independent biochemical assays. Further trials in type I Gaucher disease are in progress; studies are planned in patients with GSL storage in the CNS.

Mice lacking tartrate-resistant acid phosphatase (Acp 5) have disordered macrophage inflammatory responses and reduced clearance of the pathogen, Staphylococcus aureus

Tartrate-resistant acid phosphatase (TRAP) is a lysosomal di-iron protein of mononuclear phagocytes and osteoclasts. Hitherto, no role for the enzyme in immunity has been identified; however, knockout mice lacking TRAP have a skeletal phenotype caused by an intrinsic osteoclast defect. To investigate a putative function for TRAP in macrophages (Mphi), we investigated proinflammatory responses and systemic microbial clearance in knockout mice compared with age- and gender-matched congenic wild-type mice. Phorbol 12-myristate 13-acetate (PMA)-stimulated and interferon-gamma (IFN-gamma)-induced superoxide formation was enhanced in peritoneal Mphi lacking TRAP; nitrite production in response to stimulation with lipopolysaccharide (LPS) and IFN-gamma was also increased. In addition, secretion of the proinflammatory cytokines, tumour necrosis factor-alpha (TNF-alpha), interleukin (IL)-1beta and IL-12, was significantly greater in TRAP-deficient Mphi when stimulated with LPS, with or without addition of either TNF-alpha or IFN-gamma. The activity of tartrate-sensitive (lysosomal) acid phosphatase was increased in Mphi from the knockout mice but activities of the lysosomal hydrolases N-acetyl beta-glucosaminidase and acid beta-glucuronidase were unchanged, indicating selective activation of compensatory acid phosphatase activity. Evidence of impaired Mphi function in vivo was obtained in TRAP knockout mice, which showed delayed clearance of the microbial pathogen, Staphylococcus aureus, after sublethal intraperitoneal inoculation. After microbial challenge, peritoneal exudates obtained from TRAP knockout mice had a reduced population of Mphi. As peritoneal Mphi and neutrophils lacking TRAP were able to phagocytose and kill S. aureus normally in vitro, TRAP may directly or indirectly influence recruitment of Mphi to sites of microbial invasion. Our study shows that TRAP participates in the inflammatory response of the Mphi and influences effector signalling pathways in innate immunity.

Gaucher disease: understanding the molecular pathogenesis of sphingolipidoses

Gaucher disease is a typical lysosomal storage disease, resulting from an inborn deficiency of glucocerebrosidase. This leads to the accumulation of glycolipids in macrophages, particularly those in the liver, bone marrow, spleen and lung. In addition, disease of the nervous system can arise as a result of the accumulation of endogenous glycosphingolipid metabolites in brain tissue. About 150 mutations of the glucocerebrosidase gene have been identified in patients with Gaucher disease, some of which are predictive of phenotype. However, even patients and siblings with the same mutation, including monozygotic twins, may exhibit marked variability in disease expression and severity, illustrating our lack of understanding of the phenotype-genotype relationship in the sphingolipidoses. Massive organomegaly, particularly of the spleen, is a frequent feature of the disease. Although the liver and spleen may increase greatly in size, the amount of pathological lipid stored in the affected macrophages (Gaucher cells) accounts for less than 2% of the additional tissue mass. It is therefore clear that an inflammatory response occurs in affected individuals and that the clinical phenotype is due to an effect of macrophage storage beyond the physical presence of the Gaucher cells. Factors released by Gaucher cells, including pro-inflammatory cytokines and perhaps cathepsins, provide a mechanistic link between lysosomal storage and the diverse clinical manifestations of Gaucher disease. Emerging proteomic technology and gene expression profiling should not only improve our understanding of pathogenesis but also offer the prospect of identifying novel biomarkers that can be used as surrogate measures of disease activity and responses to treatment.

Pathologic gene expression in Gaucher disease: up-regulation of cysteine proteinases including osteoclastic cathepsin K

Deficiency of lysosomal acid beta-glucosidase induces glycolipid storage in the macrophages of Gaucher disease but the pathways of multisystem tissue injury and destruction are unknown. To investigate the cognate molecular pathology of this inflammatory disorder, genes that were differentially expressed in spleen samples from a patient with Gaucher disease (Gaucher spleen) were isolated. Of 64 complementary DNA (cDNA) fragments sequenced from an enriched Gaucher cDNA library, 5 encode lysosomal proteins (cathepsins B, K, and S, alpha-fucosidase, and acid lipase), 10 encode other known proteins, and 2 represent novel sequences from human macrophage cell lines. Transcript abundance of the cathepsins, novel genes, pulmonary and activation-regulated chemokine (PARC), and NMB, a putative tumor suppressor gene, was greatly increased. Immunoblotting showed increased mature forms of all 3 cathepsins found in samples of Gaucher spleens. Immunofluorescence microscopy showed strong cathepsin B and K reactions in sinusoidal endothelium and Gaucher cells. The respective means, plus or minus SD, of cathepsin B, K, and S activities were 183 +/- 35, 97 +/- 39, and 91 +/- 45 nmol/min/mg protein in 4 Gaucher spleens, and 26 +/- 4, 10.5 +/- 2, and 4.0 +/- 2.1 nmol/min/mg protein in 3 control spleens. Plasma cathepsin B, K, and S activities were also elevated in Gaucher disease plasma (P <.001), but compared with control plasma samples, neither cathepsin B nor K activities were significantly elevated in 8 patients with nonglycosphingolipid lysosomal storage diseases or in 9 patients with other glycosphingolipidoses, which suggests disease specificity. All 3 cathepsin activities were increased 2-fold to 3-fold in Gaucher sera compared with control sera. In all 6 patients treated by enzyme replacement for 16-22 months, serum cathepsin activities decreased significantly (P <.01). Longitudinal studies confirmed the progressive reduction of proteinase activities during imiglucerase therapy but in 3 Gaucher patients with mild disease not so treated, serum cathepsin activities remained constant or increased during follow-up. Enhanced expression of cysteine proteinases may promote tissue destruction. Moreover, the first identification of aberrant cathepsin K expression in hematopoietic tissue other than osteoclasts implicates this protease in the breakdown of the matrix that characterizes lytic bone lesions in Gaucher disease. (Blood. 2000;96:1969-1978)

Localization of iron transport and regulatory proteins in human cells

The recent discovery of HFE, the MHC-Class-I-like gene mutated in up to 90% of patients with hereditary haemochromatosis, and the gene encoding the Nramp2/divalent metal transporter-1 (DMT-1) implicated in ferrous iron transport holds promise for a greater understanding of human iron metabolism. Since the HFE protein can be crystallized as a ternary complex with the transferrin receptor and iron-saturated transferrin, and DMT-1 expression is up-regulated in hereditary haemochromatosis, these proteins are likely to interact in a common pathway for human iron homeostasis. To investigate the cellular interactions between the cognate proteins encoded by these genes, we generated a panel of rabbit and avian antisera from human HFE and DMT-1 derived peptides. The antibodies were characterized by ELISA reactions and Western immunoblotting. Immunohistochemical staining showed that DMT-1 protein localized to the brush border of human duodenum where it is predicted to serve as the principal transporter of ferrous iron from the intestinal lumen. In the human cell lines, Caco-2 (small intestinal phenotype upon differentiation) and K562 (erythroleukaemic) HFE, in the presence of iron-saturated transferrin, co-localized with transferrin receptors in an early endosome compartment using confocal immunofluorescence microscopy. This interaction may be critical in small-intestinal crypt cells which express HFE, where it may function to modulate their intrinsic iron status thereby programming iron absorption by DMT-1 in the mature enterocyte. In undifferentiated Caco-2 cells, DMT-1 localized to a discrete late endosome compartment distinct from that occupied by HFE where, in addition to brush-border iron uptake, it may function to regulate the availability of iron delivery to intracellular iron pools. Disruption of the HFE gene as a result of mutations associated with hereditary haemochromatosis may thus impair homeostatic mechanisms controlling iron absorption within the small-intestine epithelium by a direct interaction with transferrin receptors and by subsequent alteration of DMT-1 expression. Identification of the molecular interactions of HFE with DMT-1 and other key components of the iron transport pathway has implications for a mechanistic understanding of the pathophysiology of human iron storage diseases as well as the regulation of normal iron balance.

Massive hepatic fibrosis in Gaucher's disease: clinico-pathological and radiological features

Hepatomegaly is frequent in patients with type 1 Gaucher's disease and is associated with infiltration of the liver with pathological macrophages. Most patients suffer no significant clinical consequences, but a few develop portal hypertension which may progress to parenchymal liver failure. We describe four patients with Gaucher's disease who have developed portal hypertension. We have reviewed their clinical histories and all available histological and radiological material. All had severe Gaucher's disease with multi-organ involvement, and had undergone splenectomy in childhood. Histologically, this advanced liver disease was characterized by a picture of extreme and advanced confluent fibrosis occupying the central region of the liver. This massive fibrosis is associated with characteristic radiological appearances. The liver histology in these cases is highly unusual and virtually unknown in other conditions. Our studies indicate that without specific treatment the liver disease is progressive and rapidly fatal. However, institution of enzyme replacement therapy with imiglucerase may have beneficial effects even when the condition is far advanced.

Osteoclastic tartrate-resistant acid phosphatase (Acp 5): its localization to dendritic cells and diverse murine tissues

Tartrate-resistant acid phosphatase (TRAP) is a histochemical marker of the osteoclast. It is also characteristic of monohistiocytes, particularly alveolar macrophages, and is associated with diverse pathological conditions, including hairy cell leukemia and AIDS encephalopathy. To study the biology of this enzyme, we investigated its expression and activity in mouse tissues. Confocal fluorescence studies showed that TRAP is localized to the lysosomal compartment of macrophages. In adult mice, high activities of the enzyme were demonstrated in bone, spleen, liver, thymus, and colon, with lower amounts in lung, stomach, skin, brain, and kidney. Trace amounts were detected in testis, muscle, and heart. Expression of TRAP mRNA was investigated in tissue sections by in situ hybridization and protein expression was monitored by histochemical staining or immunohistochemically. TRAP is widely expressed in many tissues, where it is associated with cells principally originating from the bone marrow, including those of osteoclast/macrophage lineage. The cellular distribution of TRAP mRNA and enzyme antigen in the tissues corresponds closely to that of cells staining with an antibody directed to the CD80 (B7) antigen. Therefore, to confirm its putative localization in dendritic cells, isolated bone marrow dendritic cells were matured in culture. These co-stained strongly for TRAP protein and the CD80 antigen. These studies demonstrate that TRAP is a lysosomal enzyme that is found in diverse murine tissues, where it is expressed in dendritic cells as well as osteoclasts and macrophages, as previously shown. (J Histochem Cytochem 48:219-227, 2000)

Expression, purification, and characterization of natural mutants of human aldolase B. Role of quaternary structure in catalysis

Fructaldolases (EC 4.1.2.13) are ancient enzymes of glycolysis that catalyze the reversible cleavage of phosphofructose esters into cognate triose (phosphates). Three vertebrate isozymes of Class I aldolase have arisen by gene duplication and display distinct activity profiles with fructose 1,6-bisphosphate and with fructose 1-phosphate. We describe the biochemical and biophysical characterization of seven natural human aldolase B variants, identified in patients suffering from hereditary fructose intolerance and expressed as recombinant proteins in E. coli, from which they were purified to homogeneity. The mutant aldolases were all missense variants and could be classified into two principal groups: catalytic mutants, with retained tetrameric structure but altered kinetic properties (W147R, R303W, and A337V), and structural mutants, in which the homotetramers readily dissociate into subunits with greatly impaired enzymatic activity (A149P, A174D, L256P, and N334K). Investigation of these two classes of mutant enzyme suggests that the integrity of the quaternary structure of aldolase B is critical for maintaining its full catalytic function.

Metachromatic leucodystrophy: a newly identified mutation in arylsulphatase A, D281Y, found as a compound heterozygote with I179L in an adult onset case

The majority of mutations identified in patients with Metachromatic leucodystrophy are unique to individual families. We report here a new mutation in the arylsulphatase A gene (D281Y) identified in a patient with late-onset Metachromatic leucodystrophy. This mutation was inherited in cis with the common pseudo-deficiency allele and in trans with the previously described I179S (250100.0008) mutation which complicated the enzymatic diagnosis of this condition. Sequence comparison shows D281 to be highly conserved amongst the arylsulphatases. The clinical features of this patient which are predominantly of a slowly progressive psychiatric and intellectual deterioration rather than rapid neurological impairment are typical of I179S compound heterozygotes.

Inherited disorders of iron storage and transport

Diverse hereditary disorders associated with iron accumulation cause widespread organ damage. New insights into cellular pathways of iron transport have emerged from the identification of molecules implicated in heritable defects of iron metabolism. Unravelling the genetic basis of rare variants of haemochromatosis should provide vital functional information to further our mechanistic understanding of iron homeostasis.

Alteration of substrate specificity by a naturally-occurring aldolase B mutation (Ala337-->Val) in fructose intolerance

A molecular analysis of human aldolase B genes in two newborn infants and a 4-year-old child with hereditary fructose intolerance, the offspring of a consanguineous union, has identified the novel mutation Ala337-->Val in homozygous form. This mutation was also detected independently in two other affected individuals who were compound heterozygotes for the prevalent aldolase B allele, Ala149-->Pro, indicating that the mutation causes aldolase B deficiency. To test for the effect of the mutation, catalytically active wild-type human aldolase B and the Val337 variant enzyme were expressed in Escherichia coli. The specific activities of the wild-type recombinant enzyme were 4.8 units/mg and 4.5 units/mg towards fructose 1,6-bisphosphate (FBP) and fructose 1-phosphate (F-1-P) as substrates with Michaelis constants of 4 microM and 2.4 mM respectively. The specific activities of purified tetrameric Val337 aldolase B, which affects an invariant residue in the C-terminal region, were 4.2 units/mg and 2.6 units/mg towards FBP and F-1-P as substrates respectively; the corresponding Michaelis constants were 22 microM and 24 mM. The FBP-to-F-1-P substrate activity ratios were 0.98 and 1.63 for wild-type and Val337 variant enzymes respectively. The Val337 mutant aldolase had an increased susceptibility to proteolytic cleavage in E. coli and rapidly lost activity on storage. Comparative CD determinations showed that the Val337 protein had a distinct thermal denaturation profile with markedly decreased enthalpy, indicating that the mutant protein is partly unfolded. The undegraded mutant had preferentially decreased affinity and activity towards its specific F-1-P substrate and maintained appreciable activity towards FBP. In contrast, fluorescence studies of the mutant showed an increased binding affinity for products of the aldolase reaction, indicating a role for the C-terminus in mediating product release. These findings in a rare but widespread naturally occurring mutant implicate the C-terminus in the activity of human aldolase B towards its specific substrates and demonstrate its role in maintaining the overall stability of the enzyme tetramer.

Juvenile hemochromatosis locus maps to chromosome 1q

Juvenile hemochromatosis (JH) is an autosomal recessive disorder that leads to severe iron loading in the 2d to 3d decade of life. Affected members in families with JH do not show linkage to chromosome 6p and do not have mutations in the HFE gene that lead to the common hereditary hemochromatosis. In this study we performed a genomewide search to map the JH locus in nine families: six consanguineous and three with multiple affected patients. This strategy allowed us to identify the JH locus on the long arm of chromosome 1. A maximum LOD score of 5.75 at a recombination fraction of 0 was detected with marker D1S498, and a LOD score of 5. 16 at a recombination fraction of 0 was detected for marker D1S2344. Homozygosity mapping in consanguineous families defined the limits of the candidate region in an approximately 4-cM interval between markers D1S442 and D1S2347. Analysis of genes mapped in this interval excluded obvious candidates. The JH locus does not correspond to the chromosomal localization of any known gene involved in iron metabolism. These findings provide a means to recognize, at an early age, patients in affected families. They also provide a starting point for the identification of the affected gene by positional cloning.

Mice deficient in the urea-cycle enzyme, carbamoyl phosphate synthetase I, die during the early neonatal period from hyperammonemia

Ammonia liberated during amino acid catabolism in mammals is highly neurotoxic and is detoxified by the five enzymes of the urea cycle that are expressed within the liver. Inborn errors of each of the urea cycle enzymes occur in humans. Carbamoyl phosphate synthetase I (CPSase I; EC 6.3.4.16) is located within the inner mitochondrial matrix and catalyzes the initial rate-limiting step of the urea cycle. Unless treated, complete deficiency of CPSase I, a rare autosomal recessive disease, causes death in newborn infants. Survivors are often mentally retarded and suffer frequent hyperammonemic crises during intercurrent illness or other catabolic stresses. Biochemically, CPSase I deficiency is characterized by high levels of blood ammonia, glutamine, and alanine, with low or absent citrulline and arginine levels. As a first step toward the development of gene therapy directed to the hepatocyte, we have generated a CPSase I-deficient mouse by gene targeting. Mice with homozygous disruption of CPSase I (CPSase [-/-] mice) die within 36 hours of birth with overwhelming hyperammonemia, and without significant liver pathology. This animal is a good model of human CPSase I deficiency.

Hereditary juvenile haemochromatosis: a genetically heterogeneous life-threatening iron-storage disease

Juvenile haemochromatosis is a rare inborn error of iron metabolism with clinical manifestations before 30 years of age. Unlike adult haemochromatosis which principally affects men, juvenile haemochromatosis affects the sexes equally; it causes early endocrine failure, dilated cardiomyopathy and joint disease. We report four patients (two of each sex) from three pedigrees affected by juvenile haemochromatosis with a mean onset at 22 years (range 14-30). All had endocrine deficiency with postpubertal gonadal failure secondary to pituitary disease; two suffered near-fatal cardiomyopathy with heart failure. Mean time to diagnosis from the first clinical signs of disease was 9.8 years (range 0.5-20) but general health and parameters of iron storage responded favourably to iron-depletion therapy. A 24-year-old man listed for heart transplantation because of cardiomyopathy [left ventricular (LV) ejection fraction 16%] responded to intravenous iron chelation with desferrioxamine combined with phlebotomy (ejection fraction 31%). A 27-year-old woman with subacute biventricular heart failure refractory to medication required orthotopic cardiac transplantation before the diagnosis was established (LV ejection fraction 25%). Genetic studies showed that these two patients with cardiomyopathy from unrelated families were heterozygous for the HFE 845G-->A (C282Y) mutation and wild-type at the H63D locus: complete sequencing of the intron-exon boundaries and entire coding sequence of the HFE gene failed to identify additional lesions. Two siblings in a pedigree without cardiomyopathy were wild-type at the HFE C282Y locus; although the brother harboured a single copy of the 187C-->G (H63D) allele, segregation analysis showed that in neither sibling was the iron-storage disease linked to MHC Class I markers on chromosome 6p. Juvenile haemochromatosis is thus a genetically heterogenous disorder distinct from the common adult variant.

Haemochromatosis: an inherited metal and toxicity syndrome

A newly-identified major histocompatibility Class I-like gene, HFE (originally HLA-H) located approximately 3.5 Mb telomeric to the Class I cluster on chromosome 6p 21.3 harbours mutations in haemochromatosis. Two of these, Cys282Tyr (C282Y) and His63Asp (H63D, a minor determinant) have diagnostic utility as approximately 90% of adults are homozygous or compound heterozygotes for these alleles. The pathophysiological role of HFE is unclear: it is expressed as a surface molecule on many cells and the C282Y mutation disrupts interactions with beta 2-microglobulin, thus preventing surface expression. Lately, there has been experimental evidence that HFE protein interacts with the transferrin-receptor, affecting receptor turnover or its affinity for ligand.