Molecular characterization of the first missense mutation in the fibrinogen Aalpha-chain gene identified in a compound heterozygous afibrinogenemic patient

Congenital afibrinogenemia is a rare coagulopathy characterized by extremely low levels of functional and immunoreactive fibrinogen in plasma, associated with a hemorrhagic phenotype of variable severity. It is transmitted as an autosomal recessive trait and is invariantly associated with mutations affecting 1 of the 3 fibrinogen genes (FGA, FGB, and FGG, coding for Aalpha, Bbeta, and gamma chain, respectively). Most genetic defects causing afibrinogenemia are truncating mutations, whereas only few missense mutations (6) have been identified so far, all located in FGB. In this study, the mutational screening of an afibrinogenemic Italian male identified the first missense mutation (Met51Arg) in FGA leading to afibrinogenemia. The patient was a compound heterozygote for a previously described frameshift mutation (1215delT) in the same gene. Met51Arg involves a residue located at the very beginning of the coiled-coil domain, in a region demonstrated to play a pivotal role in hexamer formation. In-vitro expression experiments showed that Met51Arg strongly reduces secretion of hexameric fibrinogen, whereas traces of not completely assembled trimeric intermediate were found in conditioned media. Western blot analysis on the proband's plasma confirmed the presence in vivo of the trimeric fibrinogen, supporting the hypothesis that Met51Arg prevents the final step of fibrinogen assembly.     

Congenital afibrinogenemia: intracellular retention of fibrinogen due to a novel W437G mutation in the fibrinogen Bbeta-chain gene

Congenital afibrinogenemia is a rare autosomal recessive coagulation disorder characterised by hemorrhagic manifestations of variable entity and by severe plasma fibrinogen deficiency. Among the 31 afibrinogenemia-causing mutations so far reported, only 2 are missense mutations and both are located in the fibrinogen Bbeta-chain gene. Direct sequencing of the fibrinogen gene cluster in two afibrinogenemic Iranian siblings revealed a novel homozygous T>G transversion in exon 8 (nucleotide position 8025) of the fibrinogen Bbeta-chain gene. The resulting W437G missense mutation involves a highly conserved amino acid residue, located in the C-terminal globular D domain. The role of the W437G amino acid substitution on fibrinogen synthesis, folding, and secretion was assessed by in vitro expression experiments in COS-1 cells, followed by qualitative and quantitative analyses of intracellular and secreted mutant fibrinogen. Results of both pulse-chase experiments and enzyme-linked immunosorbent assays demonstrated intracellular retention of the mutant W437G fibrinogen and marked reduction of its secretion. These data, besides elucidating the pathogenetic role of the W437G mutation in afibrinogenemia, underline the importance of the Bbeta-chain D domain in fibrinogen folding and secretion.     

Two Novel Missense Mutations in the Connexin 26 Gene in Turkish Patients with Nonsyndromic Hearing Loss

Most nonsyndromic hearing losses are caused by mutations in the GJB2 gene, and studies have revealed that the forms and frequencies of these mutations are largely dependent on ethnic origin. In the present study, we aimed to characterize the mutation profiles of 151 patients with hearing loss in Turkey. The entire coding region of the GJB2 was directly sequenced in all patients. We found 35 (23.2%) individuals carrying GJB2 mutations. Seven different mutations were identified, five of which were previously known (35delG, delE120, R184P, M163V, L90P), the remaining two being novel variants (M34V, L205V). The most common mutation was 35delG followed by delE120. The 35delG mutation was homozygous in 22 cases (14.5%) and heterozygous in 4 cases (2.6%). Compound heterozygosity for 35delG was also observed. The delE120 mutation was found in three patients in homozygous form. A homozygous L90P and heterozygous mutations M163V and M34V were found in single cases.     

Loss-of-function mutations in filaggrin gene associate with psoriasis vulgaris in Chinese population

Loss-of-function mutations in filaggrin gene (FLG; OMIM #135940) have been reported to cause the semi-dominant keratinizing disorders such as ichthyosis vulgaris (IV; OMIM #146700) and atopic dermatitis (AD; OMIM #605803). Recent linkage analysis and immunohistochemical studies suggest the possible contribution of FLG to psoriatic susceptibility. However, no susceptibility variant in FLG gene associated with psoriasis (OMIM #177900) has been identified. In this study, we identified a non-sense mutation of FLG (p.K4022X) in a Chinese psoriasis/IV coexisting family. The homozygous p.K4022X mutation was detected in a psoriasis patient, whereas the heterozygous p.K4022X mutation was identified in two IV patients and four apparently normal family members. We also genotyped p.K4022X variant in 441 sporadic Chinese psoriasis patients and found homozygous mutation in two patients, while no homozygous variant was found in 500 control individuals. After sequencing the entire coding region of FLG gene in 441 psoriasis patients, we identified another five mutations (p.R826X, p.W2583X, c.7945delA, c.3321delA and p.Q2417X). Although all six FLG mutations as a whole was not significantly associated with psoriasis (P = 0.105), mutation p.K4022X was significantly associated with psoriasis (P < 0.05). Our data thus indicates an association of FLG with psoriasis in Chinese population.     

Combined enzymatic complex I and III deficiency associated with mutations in the nuclear encoded NDUFS4 gene

Combined OXPHOS-system enzyme deficiencies are observed in approximately 25% of all OXPHOS-system disturbances. Of these, combined complex I and III deficiency is relatively scarce. So far, only mtDNA and thymidine phosphorylase (TP) mutations have been associated with combined OXPHOS-system disturbances. In this report we show, for the first time, that a nuclear gene mutation in a structural, nuclear encoded complex I gene is associated with combined complex I and III deficiency. After our initial report we describe mutations in the NDUFS4 gene of complex I in two additional patients. The first mutation is a deletion of G at position 289 or 290. Amino acid 96 changes from a tryptophan to a stop codon. The mutation was found homozygous in the patient; both parents are heterozygous for the mutation. The second mutation is a transition from C to T at cDNA position 316. Codon is changed from CGA (arginine) to TGA (stop). The patient is homozygous for the mutation; both parents are heterozygous. Both mutations in the NDUFS4 gene led to a premature stop in Leigh-like patients with an early lethal phenotype. We hypothesise that the structural integrity of the OXPHOS system, in mammal supermolecular structures, may be responsible for the observed biochemical features.     

Association of fibrinogen and plasmin inhibitor,but not coagulation factor XIII gene polymorphisms with coronary artery disease

BackgroundIn the final phase of clot formation, fibrinogen constitutes frame, whereas factor XIII (FXIII) active form is responsible for the covalent cross-linking of fibrin fibres and plasmin inhibitor (PI), thus contributing to clot stability. It could be expected that any change of coagulation factors'' structure affects the clot formation and modulates the atherothrombotic risk. The aim was to determine the frequency of four single nucleotide polymorphisms: (i) A > G in codon 312 of the fibrinogen α-chain gene (rs6050, Thr312AlaFGA), (ii) C > T at position 10034 of the 3 - untranslated region in the fibrinogen γ-chain gene (rs2066865, 10034C > T FGG), (iii) C > T in codon 564 of the FXIII-A subunit gene (rs5982, Pro564LeuFXIII-A), and (iv) C > T in codon 6 of the plasmin inhibitor gene (rs2070863, Arg6TrpPI) in Croatian patients and their association with coronary artery disease (CAD).MethodsWe performed the unrelated case-control association study on the consecutive sample of patients 18 years old, who had undergone coronary angiography for investigation of chest pain and suspected CAD. The cases were patients with confirmed CAD (N=201), and the controls were the subjects with no CAD (N=119). Samples were genotyped using PCR-RFLP analysis.ResultsObserved frequencies of the rare alleles of Thr312Ala FGA, 10034C > T FGG, Leu564Pro FXIII-A and Arg6Trp PI polymorphisms were 21%, 17%, 14%, 20%, respectively. Patients with 10034C > T FGG CC genotype had 3.5 times (95% CI 1.02-12.03) higher adjusted odds for CAD than patients with 10034C > T FGG TT genotype. Patients with Arg6Trp PI CC genotype had 3.86 times (95% CI 1.23-12.12) higher odds for CAD than patients with Arg6Trp PI TT genotype. It seems that those genotype-related higher odds are also male-gender related. No difference was observed regarding any other investigated polymorphism.ConclusionsOur finding suggests that 10034C > T FGG and Arg6Trp PI are associated with CAD.     

PKU mutations R408Q and F299C in Norway: Haplotype associations,geographic distributions and phenotype characteristics

Summary Details are given concerning the phenylketonuria (PKU) mutations R408Q and F299C. Both mutations were identified among 47 PKU patients, derived from the Norwegian PKU registry. A novel PKU mutation (R408Q) was identified, by single-strand conformation polymorphism analysis, on six out of eight mutant haplotype 12 chromosomes and on none of the other PKU chromosomes. The F299C mutation occurred exclusively on mutant haplotype 8, and was the only mutation associated with this haplotype (on six chromosomes). One patient homozygous for each mutation was found. The patient homozygous for F299C manifested severe PKU, whereas the R408Q homozygote exhibited a mild PKU variant. Pedigree analysis of these families has not, so far, revealed consanguinity. Information on the place of birth of the relevant grandparents of the PKU patients with these mutations suggests that each of these mutations in Norway has originated from a common gene source.     

Novel mutations of endothelin-B receptor gene in Pakistani patients with Waardenburg syndrome

Mutations in EDNRB gene have been reported to cause Waardenburg-Shah syndrome (WS4) in humans. We investigated 17 patients with WS4 for identification of mutations in EDNRB gene using PCR and direct sequencing technique. Four genomic mutations were detected in four patients; a G to C transversion in codon 335 (S335C) in exon 5 and a transition of T to C in codon (S361L) in exon 5, a transition of A to G in codon 277 (L277L) in exon 4, a non coding transversion of T to A at −30 nucleotide position of exon 5. None of these mutations were found in controls. One of the patients harbored two novel mutations (S335C, S361L) in exon 5 and one in Intronic region (−30exon5 A>G). All of the mutations were homozygous and novel except the mutation observed in exon 4. In this study, we have identified 3 novel mutations in EDNRB gene associated with WS4 in Pakistani patients.     

Association of deep venous thrombosis with prothrombotic gene polymorphism identified in lung cancer cases

Venous thrombosis is a significant cause of morbidity and mortality in patients with malignancies. We aimed to investigate the association between prothrombotic gene polymorphisms detected in lung cancer cases and deep venous thrombosis (DVT). Totally 66 patients with an established diagnosis of lung cancer, of which 33 developed DVT, were enrolled. Multiplex PCR technique and reverse hybridization strip assay were performed on DNA extracted from peripheral blood, in order to analyze prothrombin G20210A, factor V G1691A, methylenetetrahydrofolate reductase (MTHFR) C677T and A1298C, angiotensin converting enzyme (ACE), plasminogen activator inhibitor-1 (PAI-1), and glycoprotein IIIa (Gp IIIa) gene mutations. Among prothrombotic gene polymorphisms investigated in this study, the commonest ones were PAI-1 4G/5G (56% heterozygous, 39% homozygous) and ACE gene mutations (58% heterozygous, 17% homozygous). The presence of homozygous MTHFR A1298C mutation was significantly associated with DVT (P = 0.020). Comparing the lung cancer patients with and without DVT, only MTHFR A1298C gene polymorphism differed significantly (P = 0.040). We determined a higher rate of prothrombotic gene mutations in lung cancer patients who developed DVT. However, statistical significance was achieved only for MTHFR A1298C gene mutation. Therefore, nongenetic factors for disturbance of hemostatic metabolism should also be considered in lung cancer patients.     

Cytochrome b558-negative, autosomal recessive chronic granulomatous disease: two new mutations in the cytochrome b558 light chain of the NADPH oxidase (p22-phox).

Chronic granulomatous disease (CGD) is characterized by the failure of activated phagocytes to generate superoxide. Defects in at least four different genes lead to CGD. Patients with the X-linked form of CGD have mutations in the gene for the beta-subunit of cytochrome b558 (gp91-phox). Patients with a rare autosomal recessive form of CGD have mutations in the gene for the alpha-subunit of this cytochrome (p22-phox). Usually, this leads to the absence of cytochrome b558 in the phagocytes (A22(0) CGD). We studied the molecular defect in five European patients from three unrelated families with this type of CGD. P22-phox mRNA was reverse-transcribed, and the coding region was amplified by PCR in one fragment and sequenced. Three patients from one family, with parents that were first cousins, were homozygous for a single base substitution (G-297-->A) resulting in a nonconservative amino acid change (Arg-90-->Gln). This mutation was previously found in a compound heterozygote A22(0) CGD patient. Another patient, also from first-cousin parents, was homozygous for an A-309-->G mutation in the open reading frame that predicts a nonconservative amino acid replacement (His-94-->Arg). The fifth patient was also born from a first-cousin marriage and was shown to be homozygous for the absence of exon 4 from the cDNA. In this patient, a G-->A substitution was found at position 1 of intron 4 in the genomic DNA. Therefore, the absence of exon 4 in the cDNA of this patient is due to a splicing error.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mutations in the pterin-4α-carbinolamine dehydratase (PCBD) gene cause a benign form of hyperphenylalaninemia

Four patients with primapterinuria, postulated to be due to pterin-4α-carbinolamine dehydratase (PCD) deficiency, were diagnosed by biochemical and DNA analysis. All four patients presented in the neonatal period with hyperphenylalaninemia, and elevated neopterin and decreased biopterin levels in the urine. These symptoms are common to 6-pyruvoyltetrahydropterin synthase deficiency and thus there is a danger of misdiagnosis. In addition, all four patients had elevated urinary excretion of primapterin (7-biopterin), the only persistent biochemical abnormality. Analysis of fibroblast DNA from the patients identified the following mutations in the PCBD gene: one patient homozygous for the missense mutation E96K and one homozygous for the nonsense mutation Q97X, both in exon 4; one compound heterozygote with the mutations E96K and Q97X; and one patient with two different homozygous mutations: E26X in exon 2 and R87Q in exon 4. In two families, the parents were investigated and found to be obligate heterozygotes for particular mutations. One sibling was found to be unaffected. These results further substantiate the idea that primapterinuria is associated with mutations in the PCBD gene. Received: 4 March 1998 / Accepted: 17 April 1998     

Genetic hemochromatosis, a Celtic disease: is it now time for population screening?

In populations of northern European ancestry, hereditary hemochromatosis (HH) is tightly linked to mutations within the hemochromatosis gene (HFE gene). Over 93% of Irish HH patients are homozygous for the HFE gene C282Y mutation, providing a reliable diagnostic marker of the disease in this population. However, the prevalence of the C282Y mutation and that of the second HFE gene mutation, H63D, have yet to be determined within the Irish population. The objective of this study was to identify the true prevalence of the genetic form of HH in the Irish population. DNA was extracted from 1002 randomly selected newborn screening cards and analyzed for the C282Y and H63D mutations within the HFE gene. Complete results were obtained from 800 cards. Mutations were identified in 364 (46%) neonates. Eight (1%) neonates were homozygous for C282Y and 8 (1%) were homozygous for H63D. One hundred and fifty-five (19%) neonates were C282Y heterozygous and 226 (28%) were H63D heterozygous. Of these, 33 (4%) carried one copy of both C282Y and H63D mutations, i.e., compound heterozygous. Allele frequencies for C282Y and H63D were 11% and 15%, respectively. The high C282Y allele frequency in the Irish population together with its close linkage to HH indicate that C282Y genotyping is the preferred screening strategy for this disease in Ireland.     

Spectrum of mutations in the Batten disease gene,CLN3.

Batten disease (juvenile-onset neuronal ceroid lipofuscinosis [JNCL]) is an autosomal recessive condition characterized by accumulation of lipopigments (lipofuscin and ceroid) in neurons and other cell types. The Batten disease gene, CLN3, was recently isolated, and four disease-causing mutations were identified, including a 1.02-kb deletion that is present in the majority of patients (The International Batten Disease Consortium 1995). One hundred eighty-eight unrelated patients with JNCL were screened in this study to determine how many disease chromosomes carried the 1.02-kb deletion and how many carried other mutations in CLN3. One hundred thirty-nine patients (74%) were found to have the 1.02-kb deletion on both chromosomes, whereas 49 patients (41 heterozygous for the 1.02-kb deletion) had mutations other than the 1.02-kb deletion. SSCP analysis and direct sequencing were used to screen for new mutations in these individuals. Nineteen novel mutations were found: six missense mutations, five nonsense mutations, three small deletions, three small insertions, one intronic mutation, and one splice-site mutation. This report brings the total number of disease-associated mutations in CLN3 to 23. All patients homozygous for mutations predicted to give rise to truncated proteins were found to have classical JNCL. However, a proportion of the patients (n = 4) who were compound heterozygotes for a missense mutation and the 1.02-kb deletion were found to display an atypical phenotype that was dominated by visual failure rather than by severe neurodegeneration. All missense mutations were found to affect residues conserved between the human protein and homologues in diverse species.     

一个中国遗传性血色病家系致病基因的突变分析

遗传性血色病(Hereditary hemochromatosis,HHC)是一种罕见的常染色体隐性遗传病。本课题组招募了一个HHC的近亲婚配家系,包括一名患HHC的先证者以及同一代的4名不患HHC的成员。通过对该HHC先证者进行全外显子组测序,在目前已知的与遗传性血色病相关的5个基因(HAMP、HJV、TFR2、FPN和HFE)中,发现在铁调素调节蛋白(Hemojuvelin,HJV)的编码基因HJV上存在两个纯合突变(c.G18C和c.GC962_963AA)。其中,前者能够引起HJV蛋白发生p.Q6H的改变,但该突变的危害性较小,可能与血色病的发病无关;后者能够引起HJV蛋白发生p.C321X的改变,从而翻译出缺失糖基磷脂肌醇锚定结构域的截短型HJV蛋白。除了HJV基因上的纯合突变外,该先证者还携带了其他12个纯合突变,但这些突变的危害性均不强且其所在基因的功能与铁代谢无关。本实验室内部测序数据显示,在一般中国人群中不存在p.C321X突变,提示HJV基因上的p.C321X纯合突变可能是该HHC患者的致病性突变。与此相一致的是,4名不患HHC的家系成员中该位点为野生型纯合子或杂合子,均非p.C321X纯合子。文章首次报道了HJV p.C321X纯合突变可导致HHC,该结果将有助于遗传性血色病的基因诊断和产前咨询。     

Comparison of amplification refractory mutation system and polymerase chain reaction-restriction fragment length polymorphism techniques used for the investigation of MEFV gene exon 10 point mutations in familial Mediterranean fever patients living in Cukurova region (Turkey)

Familial Mediterranean fever (FMF) is an autosomal recessive inherited disease characterized by recurrent fever, serositis and arthritis. The disease is highly prevalent in Mediterranean basin populations. Recently, the gene responsible for FMF (MEFV) was cloned and at least 40 MEFV gene mutations have been identified. The most frequently observed mutations in the MEFV gene are M694V, M694I, M680I, and V726A. These occur within exon 10 of the gene, and account for 85% of the known MEFV alleles. In this study, the reliability and economical aspects of amplification refractory mutation system (ARMS) and polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) techniques were compared for analyzing the frequencies of the major point mutations of 90 unrelated patients with FMF from the Cukurova region in Turkey. Both techniques yielded similar results: The ratio of independent alleles of 90 patients carrying one of the tested mutations was 81.1%; patients consisted of 12 different genotypes. In 64 of 90 patients (71.1%) mutations were observed in both alleles. Thirty-six patients (40%) were homozygous for the same mutation, 28 (31.1%) were heterozygous for different mutations. Eighteen patients (20%) were heterozygous for one allele with one of the four mutations but the other allele was unknown. In 8 patients (8.8%) no mutation could be detected. The most frequently observed mutation was M694V (51.66%), followed by M680I (17.22%), V726A (10.55%), and M694I (1.66%). In conclusion ARMS and PCR-RFLP techniques were equally reliable to detect the mutations in Turkish FMF patients. However, the ARMS technique was found to be more rapid and economical than the PCR-RFLP techniques.     

Hypercholesterolemia and atherosclerosis in low density lipoprotein receptor mutant rats

To establish low density lipoprotein receptor (LDLR) mutant rats as a hypercholesterolemia and atherosclerosis model, we screened the rat LDLR gene for mutations using an N-ethyl-N-nitrosourea mutagenesis archive of rat gene data, and identified five mutations in its introns and one missense mutation (478T>A) in exon 4. The C160S mutation was located in the ligand binding domain of LDLR and was revealed to be equivalent to mutations (C160Y/G) identified in human familial hypercholesterolemia (FH) patients. The wild type, heterozygous, and homozygous mutant rats were fed a normal chow diet or a high fat high cholesterol (HFHC) diet from the age of 10 weeks for 16 weeks. The LDLR homozygous mutants fed the normal chow diet showed higher levels of plasma total cholesterol and LDL cholesterol than the wild type rats. When fed the HFHC diet, the homozygous mutant rats exhibited severe hyperlipidemia and significant lipid deposition from the aortic arch to the abdominal aorta as well as in the aortic valves. Furthermore, the female homozygous mutants also developed xanthomatosis in their paws. In conclusion, we suggest that LDLR mutant rats are a useful novel animal model of hypercholesterolemia and atherosclerosis.     

Hennekam syndrome can be caused by FAT4 mutations and be allelic to Van Maldergem syndrome

The Hennekam lymphangiectasia–lymphedema syndrome is a genetically heterogeneous disorder. It can be caused by mutations in CCBE1 which are found in approximately 25 % of cases. We used homozygosity mapping and whole-exome sequencing in the original HS family with multiple affected individuals in whom no CCBE1 mutation had been detected, and identified a homozygous mutation in the FAT4 gene. Subsequent targeted mutation analysis of FAT4 in a cohort of 24 CCBE1 mutation-negative Hennekam syndrome patients identified homozygous or compound heterozygous mutations in four additional families. Mutations in FAT4 have been previously associated with Van Maldergem syndrome. Detailed clinical comparison between van Maldergem syndrome and Hennekam syndrome patients shows that there is a substantial overlap in phenotype, especially in facial appearance. We conclude that Hennekam syndrome can be caused by mutations in FAT4 and be allelic to Van Maldergem syndrome.     

Novel mutations in the transmembrane natriuretic peptide receptor <Emphasis Type="Italic">NPR-B</Emphasis> gene in four Indian families with acromesomelic dysplasia,type Maroteaux

Acromesomelic dysplasia, type Maroteaux is a disorder characterized by disproportionate short stature predominantly affecting the middle and distal segments of the upper and lower limbs. It is an autosomal recessive disorder due to mutation in NPR2 gene which impairs skeletal growth. To screen the mutations in the gene NPR2, all of its coding exons and splice junction sites were PCR amplified from genomic DNA of affected individuals of four families and sequenced. Four homozygous mutations in four different families were identified. These include three novel mutations including a deletion frameshift mutation (p.Cys586Ter), one nonsense mutation (p.Arg479Ter), one missense mutation (p.Val187Asp) and one reported missense mutation (p.Tyr338Cys). The study describes phenotypes of Indian patients and expands the mutation spectrum of the disorder.     

Identification and functional analysis of two novel mutations in the multidrug resistance protein 2 gene in Israeli patients with Dubin-Johnson syndrome

Dubin-Johnson syndrome (DJS) is an inherited disorder characterized by conjugated hyperbilirubinemia and is caused by a deficiency of the multidrug resistance protein 2 (MRP2) located in the apical membrane of hepatocytes. The aim of this study was to identify the mutations in two previously characterized clusters of patients with Dubin-Johnson syndrome among Iranian and Moroccan Jews and determine the consequence of the mutations on MRP2 expression and function by expression studies. All 32 exons and adjacent regions of the MRP2 gene were screened by polymerase chain reaction and DNA sequencing. Two novel mutations were identified in exon 25. One mutation, 3517A-->T, predicting a I1173F substitution, was found in 22 homozygous Iranian Jewish DJS patients from 13 unrelated families and a second mutation, 3449G-->A, predicting a R1150H substitution, was found in 5 homozygous Moroccan Jewish DJS patients from 4 unrelated families. Use of four intragenic dimorphisms and haplotype analyses disclosed a specific founder effect for each mutation. The mutations were introduced into an MRP2 expression vector by site-directed mutagenesis, transfected into HEK-293 cells, and analyzed by a fluorescence transport assay, immunoblot, and immunocytochemistry. Continuous measurement of probenecid-sensitive carboxyfluorescein efflux revealed that both mutations impaired the transport activity of MRP2. Immunoblot analysis and immunocytochemistry showed that MRP2 (R1150H) matured properly and localized at the plasma membrane of transfected cells. In contrast, expression of MRP2 (I1173F) was low and mislocated to the endoplasmic reticulum of the transfected cells. These findings provide an explanation for the DJS phenotype in these two patient groups. Furthermore, the close localization of the two mutations identify this region of MRP2 as important for both activity and processing of the protein.