Copy number variation meta-analysis reveals a novel duplication at 9p24 associated with multiple neurodevelopmental disorders.

BACKGROUND: Neurodevelopmental and neuropsychiatric disorders represent a wide spectrum of heterogeneous yet inter-related disease conditions. The overlapping clinical presentations of these diseases suggest a shared genetic etiology. We aim to identify shared structural variants spanning the spectrum of five neuropsychiatric disorders. METHODS: We investigated copy number variations (CNVs) in five cohorts, including schizophrenia (SCZ), bipolar disease (BD), autism spectrum disorders (ASD), attention deficit hyperactivity disorder (ADHD), and depression, from 7849 cases and 10,799 controls. CNVs were called based on intensity data from genome-wide SNP arrays and CNV frequency was compared between cases and controls in each disease cohort separately. Meta-analysis was performed via a gene-based approach. Quantitative PCR (qPCR) was employed to validate novel significant loci. RESULTS: In our meta-analysis, two genes containing CNVs with exonic overlap reached genome-wide significance threshold of meta P value < 9.4 × 10-6 for deletions and 7.5 × 10-6 for duplications. We observed significant overlap between risk CNV loci across cohorts. In addition, we identified novel significant associations of DOCK8/KANK1 duplications (meta P value = 7.5 × 10-7) across all cohorts, and further validated the CNV region with qPCR. CONCLUSIONS: In the first large scale meta-analysis of CNVs across multiple neurodevelopmental/psychiatric diseases, we uncovered novel significant associations of structural variants in the locus of DOCK8/KANK1 shared by five diseases, suggesting common etiology of these clinically distinct neurodevelopmental conditions.

Genome-wide association study of paliperidone efficacy.

OBJECTIVE: Clinical response to the atypical antipsychotic paliperidone is known to vary among schizophrenic patients. We carried out a genome-wide association study to identify common genetic variants predictive of paliperidone efficacy. METHODS: We leveraged a collection of 1390 samples from individuals of European ancestry enrolled in 12 clinical studies investigating the efficacy of the extended-release tablet paliperidone ER (n1=490) and the once-monthly injection paliperidone palmitate (n2=550 and n3=350). We carried out a genome-wide association study using a general linear model (GLM) analysis on three separate cohorts, followed by meta-analysis and using a mixed linear model analysis on all samples. The variations in response explained by each single nucleotide polymorphism (hSNP) were estimated. RESULTS: No SNP passed genome-wide significance in the GLM-based analyses with suggestive signals from rs56240334 [P=7.97×10 for change in the Clinical Global Impression Scale-Severity (CGI-S); P=8.72×10 for change in the total Positive and Negative Syndrome Scale (PANSS)] in the intron of ADCK1. The mixed linear model-based association P-values for rs56240334 were consistent with the results from GLM-based analyses and the association with change in CGI-S (P=4.26×10) reached genome-wide significance (i.e. P<5×10). We also found suggestive evidence for a polygenic contribution toward paliperidone treatment response with estimates of heritability, hSNP, ranging from 0.31 to 0.43 for change in the total PANSS score, the PANSS positive Marder factor score, and CGI-S. CONCLUSION: Genetic variations in the ADCK1 gene may differentially predict paliperidone efficacy in schizophrenic patients. However, this finding should be replicated in additional samples.

Large-scale candidate gene study to identify genetic risk factors predictive of paliperidone treatment response in patients with schizophrenia.

OBJECTIVE: Clinical response to antipsychotic medications can vary markedly in patients with schizophrenia. Identifying genetic variants associated with treatment response could help optimize patient care and outcome. To this end, we carried out a large-scale candidate gene study to identify genetic risk factors predictive of paliperidone efficacy. PATIENTS AND METHODS: A central nervous system custom chip containing single nucleotide polymorphisms from 1204 candidate genes was utilized to genotype a discovery cohort of 684 schizophrenia patients from four clinical studies of paliperidone extended-release and paliperidone palmitate. Variants predictive of paliperidone efficacy were identified and further tested in four independent replication cohorts of schizophrenic patients (N=2856). RESULTS: We identified an SNP in ERBB4 that may contribute toward differential treatment response to paliperidone. The association trended in the same direction as the discovery cohort in two of the four replication cohorts, but ultimately did not survive multiple testing corrections. The association was not replicated in the other two independent cohorts. We also report several SNPs in well-known schizophrenia candidate genes that show suggestive associations with paliperidone efficacy. CONCLUSION: These preliminary findings suggest that genetic variation in the ERBB4 gene may differentially affect treatment response to paliperidone in individuals with schizophrenia. They implicate the neuregulin 1 (NRG1)-ErbB4 pathway for modulating antipsychotic response. However, these findings were not robustly reproduced in replication cohorts.

SULT4A1 haplotype: conflicting results on its role as a biomarker of antipsychotic response.

AIM: Based on previous pharmacogenetic findings, we investigated the possible association between SULT4A1-1 haplotype and antipsychotic treatment response. MATERIALS & METHODS: Using Mixed Model Repeated Measures, we tested the relationship between SULT4A1-1 status (+carrier, -noncarrier) and clinical improvement (in Positive and Negative Syndrome Scale total score) among European ancestry patients treated with paliperidone extended release (n=937), paliperidone palmitate (n=990), risperidone (n=507) and olanzapine (n=381) in 12 schizophrenia, two schizoaffective disorder and three bipolar I disorder trials. SULT4A1-1 haplotype was determined using tagging SNP rs763120. RESULTS: There was no significant difference between SULT4A1-1(+) and SULT4A1-1(-) patients for treatment response to paliperidone or olanzapine. SULT4A1-1(-) patients had better treatment response to risperidone in one schizophrenia trial, but not in another schizophrenia trial or bipolar mania trial. CONCLUSION: Across three psychiatric disorders (n=2815 patients), we observed no consistent association between SULT4A1-1 status and atypical antipsychotic effect.

GWAS meta analysis identifies TSNARE1 as a novel Schizophrenia / Bipolar susceptibility locus.

We carried out a GWAS meta-analysis of combined mixed-ancestry schizophrenia, schizoaffective, and bipolar cohorts that resulted in the identification of six genome-wide significant loci, including one novel locus at chr8q24.3, encompassing TSNARE1 (P = 1.28 × 10(-9)). The analysis included a total of 13,394 cases and 34,676 controls. While the function of TSNARE1 remains unknown, bioinformatic predictions based on phylogenetic ancestry indicate it may have a vertebrate-specific function in intracellular protein transport and synaptic vesicle exocytosis.

Prospective-retrospective biomarker analysis for regulatory consideration: white paper from the industry pharmacogenomics working group.

One approach to delivering cost-effective healthcare requires the identification of patients as individuals or subpopulations that are more likely to respond to an appropriate dose and/or schedule of a therapeutic agent, or as subpopulations that are less likely to develop an adverse event (i.e., personalized or stratified medicine). Biomarkers that identify therapeutically relevant variations in human biology are often only uncovered in the later stage of drug development. In this article, the Industry Pharmacogenomics Working Group provides, for regulatory consideration, its perspective on the rationale for the conduct of what is commonly referred to as the prospective-retrospective analysis (PRA) of biomarkers. Reflecting on published proposals and materials presented by the US FDA, a decision tree for generating robust scientific data from samples collected from an already conducted trial to allow PRA is presented. The primary utility of the PRA is to define a process that provides robust scientific evidence for decision-making in situations where it is not necessary, nor practical or ethical to conduct a new prospective clinical study.

Learning from product labels and label changes: how to build pharmacogenomics into drug-development programs.

The 2010 US FDA-Drug Industry Association (DIA) Pharmacogenomics (PGx) Workshop follows a series that began in 2002 bringing together multidisciplinary experts spanning regulatory authorities, medical research, healthcare and industry. This report summarizes the 'Building PGx into Labels' sessions from the workshop, which discussed the critical elements in developing PGx outcomes leading to product labels that inform efficacy and/or safety. Examples were drawn from US prescribing information, which integrated PGx knowledge into medical decisions (e.g., panitumumab, warfarin and clopidogrel). Attendees indicated the need for broader dialog and for guidelines on evidentiary considerations for PGx to be included into product labels. Also discussed was the understanding of appropriate PGx placement on labels; how to encourage adoption by medical communities of label recommendations on PGx tests; and, given the global nature of drug development, worldwide considerations including European Summary of Product Characteristics.

Polymorphisms in the multiple drug resistance protein 1 and in P-glycoprotein 1 are associated with time to event outcomes in patients with advanced multiple myeloma treated with bortezomib and pegylated liposomal doxorubicin.

Single nucleotide polymorphisms (SNPs) in the multiple drug resistance protein 1 (MRP1) and P-glycoprotein 1 (MDR1) genes modulate their ability to mediate drug resistance. We therefore sought to retrospectively evaluate their influence on outcomes in relapsed and/or refractory myeloma patients treated with bortezomib or bortezomib with pegylated liposomal doxorubicin (PLD). The MRP1/R723Q polymorphism was found in five subjects among the 279 patient study population, all of whom received PLD + bortezomib. Its presence was associated with a longer time to progression (TTP; median 330 vs. 129 days; p = 0.0008), progression-free survival (PFS; median 338 vs. 129 days; p = 0.0006), and overall survival (p = 0.0045). MDR1/3435(C > T), which was in Hardy-Weinberg equilibrium, showed a trend of association with PFS (p = 0.0578), response rate (p = 0.0782) and TTP (p = 0.0923) in PLD + bortezomib patients, though no correlation was found in the bortezomib arm. In a recessive genetic model, MDR1/3435 T was significantly associated with a better TTP (p = 0.0405) and PFS (p = 0.0186) in PLD + bortezomib patients. These findings suggest a potential role for MRP1 and MDR1 SNPs in modulating the long-term outcome of relapsed and/or refractory myeloma patients treated with PLD + bortezomib. Moreover, they support prospective studies to determine if such data could be used to tailor therapy to the genetic makeup of individual patients.

Effect of the cytochrome P450 2C19 inhibitor omeprazole on the pharmacokinetics and safety profile of bortezomib in patients with advanced solid tumours, non-Hodgkin's lymphoma or multiple myeloma.

BACKGROUND AND OBJECTIVE: Bortezomib, an antineoplastic for the treatment of relapsed multiple myeloma and mantle cell lymphoma, undergoes metabolism through oxidative deboronation by cytochrome P450 (CYP) enzymes, primarily CYP3A4 and CYP2C19. Omeprazole, a proton-pump inhibitor, is primarily metabolized by and demonstrates high affinity for CYP2C19. This study investigated whether coadministration of omeprazole affected the pharmacokinetics, pharmacodynamics and safety profile of bortezomib in patients with advanced cancer. The variability of bortezomib pharmacokinetics with CYP enzyme polymorphism was also investigated. PATIENTS AND METHODS: This open-label, crossover, pharmacokinetic drug-drug interaction study was conducted at seven institutions in the US and Europe between January 2005 and August 2006. Patients who had advanced solid tumours, non-Hodgkin's lymphoma or multiple myeloma, were aged >/=18 years, weighed >/=50 kg and had a life expectancy of >/=3 months were eligible. Patients received bortezomib 1.3 mg/m2 on days 1, 4, 8 and 11 for two 21-day cycles, plus omeprazole 40 mg in the morning of days 6-10 and in the evening of day 8 in either cycle 1 (sequence 1) or cycle 2 (sequence 2). On day 21 of cycle 2, patients benefiting from therapy could continue to receive bortezomib for six additional cycles. Blood samples for pharmacokinetic/pharmacodynamic evaluation were collected prior to and at various timepoints after bortezomib administration on day 8 of cycles 1 and 2. Blood samples for pharmacogenomics were also collected. Pharmacokinetic parameters were calculated by noncompartmental analysis of plasma concentration-time data for bortezomib administration on day 8 of cycles 1 and 2, using WinNonlin version 4.0.1.a software. The pharmacodynamic profile was assessed using a whole-blood 20S proteasome inhibition assay. RESULTS: Twenty-seven patients (median age 64 years) were enrolled, 12 in sequence 1 and 15 in sequence 2, including eight and nine pharmacokinetic-evaluable patients, respectively. Bortezomib pharmacokinetic parameters were similar when bortezomib was administered alone or with omeprazole (maximum plasma concentration 120 vs 123 ng/mL; area under the plasma concentration-time curve from 0 to 72 hours 129 vs 135 ng . h/mL). The pharmacodynamic parameters were also similar (maximum effect 85.8% vs 93.7%; area under the percent inhibition-time curve over 72 hours 4052 vs 3910 % x h); the differences were not statistically significant. Pharmacogenomic analysis revealed no meaningful relationships between CYP enzyme polymorphisms and pharmacokinetic/pharmacodynamic parameters. Toxicities were generally similar between patients in sequence 1 and sequence 2, and between cycle 1 and cycle 2 in both treatment sequences. Among 26 evaluable patients, 13 (50%) were assessed as benefiting from bortezomib at the end of cycle 2 and continued to receive treatment. CONCLUSION: No impact on the pharmacokinetics, pharmacodynamics and safety profile of bortezomib was seen with coadministration of omeprazole. Concomitant administration of bortezomib and omeprazole is unlikely to cause clinically significant drug-drug interactions and is unlikely to have an impact on the efficacy or safety of bortezomib.

Robust physical methods that enrich genomic regions identical by descent for linkage studies: confirmation of a locus for osteogenesis imperfecta.

BACKGROUND: The monogenic disease osteogenesis imperfecta (OI) is due to single mutations in either of the collagen genes ColA1 or ColA2, but within the same family a given mutation is accompanied by a wide range of disease severity. Although this phenotypic variability implies the existence of modifier gene variants, genome wide scanning of DNA from OI patients has not been reported. Promising genome wide marker-independent physical methods for identifying disease-related loci have lacked robustness for widespread applicability. Therefore we sought to improve these methods and demonstrate their performance to identify known and novel loci relevant to OI. RESULTS: We have improved methods for enriching regions of identity-by-descent (IBD) shared between related, afflicted individuals. The extent of enrichment exceeds 10- to 50-fold for some loci. The efficiency of the new process is shown by confirmation of the identification of the Col1A2 locus in osteogenesis imperfecta patients from Amish families. Moreover the analysis revealed additional candidate linkage loci that may harbour modifier genes for OI; a locus on chromosome 1q includes COX-2, a gene implicated in osteogenesis. CONCLUSION: Technology for physical enrichment of IBD loci is now robust and applicable for finding genes for monogenic diseases and genes for complex diseases. The data support the further investigation of genetic loci other than collagen gene loci to identify genes affecting the clinical expression of osteogenesis imperfecta. The discrimination of IBD mapping will be enhanced when the IBD enrichment procedure is coupled with deep resequencing.

Developing the evidence base for applying pharmacogenomics: proceeds from DIA Workshop IV--Breakout Session 1.

The 4th Drug Information Association Workshop in a series of workshops on Pharmacogenomics: 'Biomarkers and Pharmacogenomics in Drug Development and Regulatory Decision Making' took place on December 10-12, 2007 in Bethesda, MD, USA. A number of breakout sessions were conducted to focus on different aspects of the development of biomarkers. Breakout Session 1 considered the evidence base for the development of pharmacogenomic markers from both safety and efficacy perspectives, with a view to understanding the challenges in the design of appropriate clinical studies during drug development. Case studies based on data generated during all stages of drug development were used to stimulate discussion and refine considerations for what constitutes a sufficient evidence base to support the effective use of novel, pharmacogenomic biomarkers. The discussions were open and lively, and some broad principles were drawn from the discussion. In this article, we summarize the case studies presented and develop key discussion points that arose out of the meeting. These case studies help to illustrate the current state of the art in the application of pharmacogenomics in drug development and the challenges being faced in the development of pharmacogenomics from interesting, exploratory associations into predictive biomarkers with clinical utility. We hope that this will serve as a stimulus to consideration of the critical issues facing the implementation of pharmacogenomics into drug development.

PhRMA white paper on ADME pharmacogenomics.

Pharmacogenomic (PGx) research on the absorption, distribution, metabolism, and excretion (ADME) properties of drugs has begun to have impact for both drug development and utilization. To provide a cross-industry perspective on the utility of ADME PGx, the Pharmaceutical Research and Manufacturers of America (PhRMA) conducted a survey of major pharmaceutical companies on their PGx practices and applications during 2003-2005. This white paper summarizes and interprets the results of the survey, highlights the contributions and applications of PGx by industrial scientists as reflected by original research publications, and discusses changes in drug labels that improve drug utilization by inclusion of PGx information. In addition, the paper includes a brief review on the clinically relevant genetic variants of drug-metabolizing enzymes and transporters most relevant to the pharmaceutical industry.

Case-control study of the association between select HLA genes and anti-erythropoietin antibody-positive pure red-cell aplasia.

AIMS: Antibody (Ab)-positive pure red-cell aplasia (PRCA) is a very rare but serious adverse event associated with recombinant human erythropoietin treatment (4.1 reports per 100,000 patient-years) in which patients produce antibodies to recombinant and endogenous erythropoietin, halting red blood cell production. In a previous case series, four Thai subjects with chronic kidney disease and Ab-positive PRCA were reported to have the HLA-DRB1*9 allele. To confirm a possible association of HLA-DRB1*9 and Ab-positive PRCA, we performed a pharmacogenomic analysis using subjects from an earlier case-control study of risk factors associated with Ab-positive PRCA, which had been performed using subjects from Europe or Canada. The primary goal of the analysis was to test the association between HLA-DRB1*9 and Ab-positive PRCA. A secondary goal was to perform an exploratory analysis in order to identify additional HLA alleles potentially associated with Ab-positive PRCA. PATIENTS & METHODS: Subjects were taken from a case-control study of Ab-positive PRCA in chronic kidney disease patients treated in Europe or Canada. Ab-positive PRCA cases (n=24) were matched to controls (n=81) by timing of treatment exposure and, when possible, by location. RESULTS: The allele frequency of HLA-DRB1*9 was 12.5% in cases vs 1.2% in controls (p=0.002). The frequency of the HLA-DRB1*9/other genotype was 25.0% in cases vs 2.5% in controls (p=0.004; OR: 10.8 [95% CI: 2.2-53.7]). Within the exploratory analysis, six additional HLA alleles (HLA-A*25, HLA-B*53, HLA-C*12, HLA-DQB1*3, HLA-DQB1*6 and HLA-DRB1*4) were also found to be associated with Ab-positive PRCA. CONCLUSION: This study confirmed that HLA-DRB1*9 occurs at a significantly higher frequency in Ab-positive PRCA cases than in controls; however, within this sample set, carrying the *9 allele was neither necessary nor sufficient to cause Ab-positive PRCA.

Returning genetic research results to individuals: points-to-consider.

This paper is intended to stimulate debate amongst stakeholders in the international research community on the topic of returning individual genetic research results to study participants. Pharmacogenetics and disease genetics studies are becoming increasingly prevalent, leading to a growing body of information on genetic associations for drug responsiveness and disease susceptibility with the potential to improve health care. Much of these data are presently characterized as exploratory (non-validated or hypothesis-generating). There is, however, a trend for research participants to be permitted access to their personal data if they so choose. Researchers, sponsors, patient advocacy groups, ethics committees and regulatory authorities are consequently confronting the issue of whether, and how, study participants might receive their individual results. Noted international ethico-legal guidelines and public policy positions in Europe and the United States are reviewed for background. The authors offer 'Points-to-Consider' regarding returning results in the context of drug development trials based on their knowledge and experience. Theses considerations include: the clinical relevance of data, laboratory qualifications, informed consent procedures, confidentiality of medical information and the competency of persons providing results to participants. The discussion is framed as a benefit-to-risk assessment to balance the potential positive versus negative consequences to participants, while maintaining the integrity and feasibility of conducting genetic research studies.

Genetic variations of KCNQ1, KCNH2, SCN5A, KCNE1, and KCNE2 in drug-induced long QT syndrome patients.

Administration of specific drugs may occasionally induce acquired long QT syndrome (aLQTS), a disorder that predisposes to ventricular arrhythmias, typically of the torsade de pointes (TdP) type, and sudden cardiac death. "Forme fruste" mutations in congenital LQTS (cLQTS) genes have been reported repeatedly as the underlying cause of aLQTS, and are therefore considered as an important risk factor. We evaluated the impact of genetic susceptibility for aLQTS through mutations in cLQTS genes. Five cLQTS genes ( KCNH2, KCNQ1, SCN5A, KCNE1, KCNE2) were thoroughly screened for genetic variations in 32 drug-induced aLQTS patients with confirmed TdP and 32 healthy individuals. Missense forme frust mutations were identified in four aLQTS patients: D85N in KCNE1 (two cases), T8A in KCNE2, and P347S in KCNH2. Three other missense variations were found both in patients and controls, and are thus unlikely to significantly influence aLQTS susceptibility. In addition, 13 silent and six intronic variations were detected, four of which were found in a single aLQTS patient but not in the controls. We conclude that missense mutations in the examined cLQTS genes explain only a minority of aLQTS cases.

Mutation analysis in congenital Long QT Syndrome--a case with missense mutations in KCNQ1 and SCN5A.

Long QT Syndrome (LQTS) is a cardiac disease characterized by a prolonged QT interval on a surface electrocardiogram (ECG) and by clinical symptoms such as seizures, syncope, and cardiac sudden death. At present, causal mutations of LQTS have been identified in five cardiac ion-channel genes. Because a causal mutation is usually unique to a specific family and can be located in any region of any of these five genes, a mutation analysis effort may require screening of the complete coding regions of each of these genes. The causative nature of a detected mutation can then be determined either by family history or by functional studies, such as the electrophysiological signature of the mutation. Here we describe a mutation analysis of an LQTS patient who carries two heterozygous missense mutations in two different LQTS genes. The first mutation identified, A572D in SCN5A, was not linked with clinical LQTS features in the two other mutation carriers in the family; neither was it identified in 90 healthy controls. Therefore, this mutation most likely has either a mild effect on cardiac ion-channel function or represents a very rare polymorphism. The second mutation, V254M in KCNQ1, co-segregated with higher QT intervals and symptoms in other family members, and was previously reported in another LQTS family. Because the clinical LQTS symptoms are most pronounced in the proband, a combined effect of both mutations cannot be excluded, although no functional data are available to support such an hypothesis. We conclude that, for newly presented LQTS cases, a mutation analysis strategy should routinely screen the complete coding region of all LQTS genes, followed by an evaluation of the identified mutation(s) in conjunction with family or functional data.

A novel mutation in the chloride channel gene, CLCNKB, as a cause of Gitelman and Bartter syndromes.

BACKGROUND: Gitelman syndrome (GS) and Bartter syndrome (BS) are hereditary hypokalemic tubulopathies with distinct phenotypic features. GS has been considered a genetically homogeneous disorder caused by mutation in the gene encoding the NaCl cotransporter (TSC) of the distal convoluted tubule. In contrast, BS is caused by mutations in the genes encoding either the Na-K-2Cl cotransporter (NKCC2), the K+ channel (ROMK) or the Cl- channel (ClC-Kb) of the thick ascending limb. The purpose of this study was to examine the clinical, biochemical and genetic characteristics of a very large inbred Bedouin kindred in Northern Israel with hereditary hypokalemic tubulopathy. METHODS: Twelve family members affected with hypokalemic tubulopathy, as well as 26 close relatives were clinically and biochemically evaluated. All study participants underwent genetic linkage analysis. Mutation analysis was performed in affected individuals. RESULTS: Evaluation of affected family members (age range 3 to 36 years) revealed phenotypic features of both GS and classic Bartter syndrome (CBS). Features typical of GS included late age of presentation (>15 years) in 7 patients (58%), normal growth in 9 (75%), hypomagnesemia (SMg <0.7mmol/L) in 5 (42%), hypermagnesiuria (FEMg>5%) in 6 (50%) and hypocalciuria (urinary calcium/creatinine mmol/mmol <0.15) in 5 (42%). Features typical of CBS included early age of presentation (<1 year) in 3 (25%), polyuria/dehydration in 4 (33%), growth retardation in 3 (25%), hypercalciuria (urinary calcium/creatinine mmol/mmoverline>0.55) in 4 (33%) and nephrolithiasis in 1 (8%). Linkage analysis in affected patients excluded the TSC gene, SLC12A3, as the mutated gene, but demonstrated linkage to the Cl- channel gene, CLCNKB, on chromosome 1p36. Mutation analysis by direct sequencing revealed a novel homozygous missense mutation, arginine 438 to histidine (R438H), in exon 13 of CLCNKB in all patients. A restriction fragment length polymorphism (RFLP) analysis has been developed to aid in genotyping of family members. CONCLUSIONS: Our findings demonstrate intrafamilial heterogeneity, namely the presence of GS and CBS phenotypes, in a kindred with the CLCNKB R438H mutation. We conclude that GS can be caused by a mutation in a gene other than SLC12A3. The exact role of the CLCNKB R438H mutation in the pathogenesis of the electrolyte and mineral abnormalities in GS and CBS remains to be established.

Lack of plasma membrane targeting of a G172D mutant thiamine transporter derived from Rogers syndrome family.

BACKGROUND: Rogers syndrome, also known as thiamine responsive megaloblastic anemia (TRMA), is an autosomal recessive disorder resulting in megaloblastic anemia, diabetes mellitus and sensorineural deafness. The gene associated with Rogers syndrome encodes for a plasma membrane thiamine transporter, THTR-1, a member of the solute carrier family that includes its homologue THTR-2 and the reduced folate carrier. MATERIALS AND METHODS: Using transient expression of wild-type and a missense mutant THTR-1 protein, derived from a TRMA family, in different cell lines and immunodetection analysis, we determined the expression, posttranslational modification, and subcellular localization of the wild-type and G172D mutant THTR-1. The transport activity of the transfected THTR-1 proteins was measured using a [(3) H] thiamine uptake assay. RESULTS: The mutant THTR-1 protein was undetectable in transfected cells grown at 37 degrees C but was readily expressed in transfected cells cultured at 28 degrees C, thereby allowing for further biochemical and functional analysis. In contrast to its fully glycosylated wild-type mature protein, the mutant THTR-1 protein underwent only the initial stage of N-linked glycosylation. The failure to undergo a complete glycosylation resulted in the lack of plasma membrane targeting and confinement of the mutant THTR-1 to the Golgi and endoplasmic reticulum (ER) compartment. Consistently, either treatment with tunicamycin or substitution of the THTR-1 consensus N-glycosylation acceptor asparagine 63 with glutamine, abolished its glycosylation and plasma membrane targeting. CONCLUSIONS: Taken collectively, these results suggest that the G172D mutation presumably misfolded THTR-1 protein that fails to undergo a complete glycosylation, is retained in the Golgi-ER compartment and thereby cannot be targeted to the plasma membrane. Finally, transfection studies revealed that the mutant G172D THTR-1 failed to transport thiamine. This is the first molecular and functional characterization of a missense mutant THTR-1 derived from a family with Rogers syndrome.

A novel mutation (T65P) in the PAS domain of the human potassium channel HERG results in the long QT syndrome by trafficking deficiency.

The congenital long QT syndrome is a cardiac disease characterized by an increased susceptibility to ventricular arrhythmias. The clinical hallmark is a prolongation of the QT interval, which reflects a delay in repolarization caused by mutations in cardiac ion channel genes. Mutations in the HERG (human ether-à-go-go-related gene KCNH2 can cause a reduction in I(Kr), one of the currents responsible for cardiac repolarization. We describe the identification and characterization of a novel missense mutation T65P in the PAS (Per-Arnt-Sim) domain of HERG, resulting in defective trafficking of the protein to the cell membrane. Defective folding of the mutant protein could be restored by decreased cell incubation temperature and pharmacologically by cisapride and E-4031. When trafficking was restored by growing cells at 27 degrees C, the kinetics of the mutated channel resembled that of wild-type channels although the rate of activation, deactivation, and recovery from inactivation were accelerated. No positive evidence for the formation of heterotetramers was obtained by co-expression of wild-type with mutant subunits at 37 degrees C. As a consequence the clinical symptoms may be explained rather by haploinsufficiency than by dominant negative effects. This study is the first to relate a PAS domain mutation in HERG to a trafficking deficiency at body temperature, apart from effects on channel deactivation.

A novel mutation in the deoxyguanosine kinase gene causing depletion of mitochondrial DNA.

Recently, a homozygous single-nucleotide deletion in exon 2 of the deoxyguanosine kinase gene (DGUOK) was identified as the disease-causing mutation in 3 apparently unrelated Israeli-Druze families with depleted hepatocerebral mitochondrial DNA. We have discovered a novel homozygous nonsense mutation in exon 3 of DGUOK (313C-->T) from a patient born to nonconsanguineous German parents. This finding shows that mutations in DGUOK causing mitochondrial DNA depletion are not confined to a single ethnic group.