Clinical relevance of DPYD variants c.1679T>G, c.1236G>A/HapB3, and c.1601G>A as predictors of severe fluoropyrimidine-associated toxicity: a systematic review and meta-analysis of individual patient data

BACKGROUND

The best-known cause of intolerance to fluoropyrimidines is dihydropyrimidine dehydrogenase (DPD) deficiency, which can result from deleterious polymorphisms in the gene encoding DPD (DPYD), including DPYD*2A and c.2846A>T. Three other variants-DPYD c.1679T>G, c.1236G>A/HapB3, and c.1601G>A-have been associated with DPD deficiency, but no definitive evidence for the clinical validity of these variants is available. The primary objective of this systematic review and meta-analysis was to assess the clinical validity of c.1679T>G, c.1236G>A/HapB3, and c.1601G>A as predictors of severe fluoropyrimidine-associated toxicity.

METHODS

We did a systematic review of the literature published before Dec 17, 2014, to identify cohort studies investigating associations between DPYD c.1679T>G, c.1236G>A/HapB3, and c.1601G>A and severe (grade ≥3) fluoropyrimidine-associated toxicity in patients treated with fluoropyrimidines (fluorouracil, capecitabine, or tegafur-uracil as single agents, in combination with other anticancer drugs, or with radiotherapy). Individual patient data were retrieved and analysed in a multivariable analysis to obtain an adjusted relative risk (RR). Effect estimates were pooled by use of a random-effects meta-analysis. The threshold for significance was set at a p value of less than 0·0167 (Bonferroni correction).

FINDINGS

7365 patients from eight studies were included in the meta-analysis. DPYD c.1679T>G was significantly associated with fluoropyrimidine-associated toxicity (adjusted RR 4·40, 95% CI 2·08-9·30, p<0·0001), as was c.1236G>A/HapB3 (1·59, 1·29-1·97, p<0·0001). The association between c.1601G>A and fluoropyrimidine-associated toxicity was not significant (adjusted RR 1·52, 95% CI 0·86-2·70, p=0·15). Analysis of individual types of toxicity showed consistent associations of c.1679T>G and c.1236G>A/HapB3 with gastrointestinal toxicity (adjusted RR 5·72, 95% CI 1·40-23·33, p=0·015; and 2·04, 1·49-2·78, p<0·0001, respectively) and haematological toxicity (adjusted RR 9·76, 95% CI 3·03-31·48, p=0·00014; and 2·07, 1·17-3·68, p=0·013, respectively), but not with hand-foot syndrome. DPYD*2A and c.2846A>T were also significantly associated with severe fluoropyrimidine-associated toxicity (adjusted RR 2·85, 95% CI 1·75-4·62, p<0·0001; and 3·02, 2·22-4·10, p<0·0001, respectively).

INTERPRETATION

DPYD variants c.1679T>G and c.1236G>A/HapB3 are clinically relevant predictors of fluoropyrimidine-associated toxicity. Upfront screening for these variants, in addition to the established variants DPYD*2A and c.2846A>T, is recommended to improve the safety of patients with cancer treated with fluoropyrimidines.

FUNDING

None.

Adverse drug reactions to azathioprine therapy are associated with polymorphism in the gene encoding inosine triphosphate pyrophosphatase (ITPase)

Adverse drug reactions to azathioprine (AZA), the pro-drug of 6-mercaptopurine (6-MP), occur in 15% to 28% of patients and the majority are not explained by thiopurine methyltransferase (TPMT) deficiency. Inosine triphosphate pyrophosphatase (ITPase) deficiency results in the benign accumulation of the inosine nucleotide ITP. 6-MP is activated through a 6-thio-IMP intermediate and, in ITPase deficient patients, potentially toxic 6-thio-ITP is predicted to accumulate. The association between polymorphism in the ITPA gene and adverse drug reactions to AZA therapy was studied in patients treated for inflammatory bowel disease. Sixty-two patients with inflammatory bowel disease suffering adverse drug reactions to AZA therapy were genotyped for ITPA 94C>A and IVS2 + 21A>C polymorphisms, and TPMT*3A, *3C, *2 polymorphisms. Genotype frequencies were compared to a consecutive series of 68 controls treated with AZA for a minimum of 3 months without adverse effect. The ITPA 94C>A deficiency-associated allele was significantly associated with adverse drug reactions [odds ratio (OR) 4.2, 95% confidence interval (CI) 1.6-11.5, P = 0.0034]. Significant associations were found for flu-like symptoms (OR 4.7, 95% CI 1.2-18.1, P = 0.0308), rash (OR 10.3, 95% CI 4.7-62.9, P = 0.0213) and pancreatitis (OR 6.2,CI 1.1-32.6, P = 0.0485). Overall, heterozygous TPMT genotypes did not predict adverse drug reactions but were significantly associated with a subgroup of patients experiencing nausea and vomiting as the predominant adverse reaction to AZA therapy (OR 5.5, 95% CI 1.4-21.3, P = 0.0206). Polymorphism in the ITPA gene predicts AZA intolerance. Alternative immunosuppressive drugs, particularly 6-thioguanine, should be considered for AZA-intolerant patients with ITPase deficiency.

Genetic basis of inosine triphosphate pyrophosphohydrolase deficiency

Inosine triphosphate pyrophosphohydrolase (ITPase) deficiency is a common inherited condition characterized by the abnormal accumulation of inosine triphosphate (ITP) in erythrocytes. The genetic basis and pathological consequences of ITPase deficiency are unknown. We have characterized the genomic structure of the ITPA gene, showing that it has eight exons. Five single nucleotide polymorphisms were identified, three silent (138G-->A, 561G-->A, 708G-->A) and two associated with ITPase deficiency (94C-->A, IVS2+21A-->C). Homozygotes for the 94C-->A missense mutation (Pro32 to Thr) had zero erythrocyte ITPase activity, whereas 94C-->A heterozygotes averaged 22.5% of the control mean, a level of activity consistent with impaired subunit association of a dimeric enzyme. ITPase activity of IVS2+21A-->C homozygotes averaged 60% of the control mean. In order to explore further the relationship between mutations and enzyme activity, we examined the association between genotype and ITPase activity in 100 healthy controls. Ten subjects were heterozygous for 94C-->A (allele frequency: 0.06), 24 were heterozygotes for IVS2+21A-->C (allele frequency: 0.13) and two were compound heterozygous for these mutations. The activities of IVS2+21A-->C heterozygotes and 94C-->A/IVS2+21A-->C compound heterozygotes were 60% and 10%, respectively, of the normal control mean, suggesting that the intron mutation affects enzyme activity. In all cases when ITPase activity was below the normal range, one or both mutations were found. The ITPA genotype did not correspond to any identifiable red cell phenotype. A possible relationship between ITPase deficiency and increased drug toxicity of purine analogue drugs is proposed.

Pyrimidine pathways in health and disease

Genetic defects involving enzymes essential for pyrimidine nucleotide metabolism have provided new insights into the vital physiological functions of these molecules in addition to nucleic acid synthesis. Such aberrations disrupt the haematological, nervous or mitochondrial systems and can cause adverse reactions to analogue therapy. Regulation of pyrimidine pathways is also known to be disrupted in malignancies. Nine genetic defects have now been identified but only one is currently treatable. Diagnosis is aided by the accumulation of specific metabolites. Recently, progress has been made in understanding the molecular mechanisms underlying inborn errors of pyrimidine metabolism, together with the key clinical issues and the implications for the future development of novel drugs and therapeutic strategies.

Atypical familial juvenile hyperuricemic nephropathy associated with a hepatocyte nuclear factor-1beta gene mutation

BACKGROUND

Familial juvenile hyperuricemic nephropathy (FJHN) is a dominantly inherited condition characterized by young-onset hyperuricemia, gout, and renal disease. The etiologic genes are unknown, although a locus on chromosome 16 has been identified in some kindreds. Mutations in the gene encoding hepatocyte nuclear factor (HNF)-1beta have been associated with dominant inheritance of a variety of disorders of renal development, particularly renal cystic disease and early onset diabetes; hyperuricemia has been reported in some kindreds.

METHODS

To assess a possible role for the HNF-1beta gene in some FJHN kindreds we sequenced the HNF-1beta gene in subjects from three unrelated FJHN families with atypical features of renal cysts or abnormalities of renal development. We also compared serum urate levels in subjects with HNF-1beta mutations with populations of controls, type 2 diabetic subjects, and subjects with mild chronic renal failure without HNF-1beta mutations.

RESULTS

A splice-site mutation in intron 2, designated IVS2+1G>T, showed complete co-segregation with FJHN in one family with diabetes. Serum urate levels were significantly higher in the HNF-1beta subjects compared with the normal control subjects (384 micromol/L vs. 264 micromol/L, P = 0.002) and the type 2 diabetic subjects (397 micromol/L vs. 271 micromol/L, P = 0.01). Comparison of serum urate levels in the HNF-1beta subjects with gender-matched subjects with renal impairment of other causes did not reach significance (402 micromol/L vs. 352 micromol/L, P = 0.2).

CONCLUSION

Hyperuricemia and young-onset gout are consistent features of the phenotype associated with HNF-1beta mutations, but the mechanism is uncertain. Families with HNF-1beta mutations may fit diagnostic criteria for FJHN. Identification of HNF-1beta patients by recognizing the features of diabetes and disorders of renal development is important in resolving the genetic heterogeneity in FJHN.

Optimising outcome on thiopurines in inflammatory bowel disease by co-prescription of allopurinol

BACKGROUND AND AIMS

Azathioprine and mercaptopurine remain first line immunomodulatory treatments for inflammatory bowel disease. Toxicity and non-response are significant issues. Co-prescription of allopurinol with reduced-dose (25-33%) azathioprine or mercaptopurine may overcome these problems. We present the outcome of co-prescription in a large single-centre cohort.

METHOD

Patients on thiopurine/allopurinol co-prescription were identified. Indication for and outcome on combination treatment were established. Blood parameters and metabolite results were compared on single agent and combination treatment. Toxicity associated with combination treatment was sought.

RESULTS

110 patients on combination treatment were identified. Clinical remission was achieved in 60/79 (76%) of patients in whom the effect of thiopurine could be studied in isolation. 20/25 patients with hepatotoxicity tolerated combination treatment and normalised their liver function tests. 24/28 patients with atypical side effects tolerated co-therapy. 13/20 non-responders responded to combination treatment. In patients started on combination treatment as first line therapy, 15/23 achieved clinical remission. Thioguanine nucleotides were significantly higher and methylated metabolites significantly lower on combination therapy. Mean cell volume was higher and total white cell and neutrophil counts lower on combination treatment. 13 adverse events occurred, including 6 specific to co-therapy (3 rash, 2 abnormal liver function tests, 1 dosing error). All were minor and self-limiting.

CONCLUSION

This is the largest published experience of the use of allopurinol to optimise outcomes on thiopurine treatment. Combination therapy permitted successful treatment of a significant number of patients who would otherwise have been labelled as thiopurine failures. A few self-limiting side effects were encountered.

Polymorphisms in Folate, Pyrimidine, and Purine Metabolism Are Associated with Efficacy and Toxicity of Methotrexate in Psoriasis

Methotrexate is the gold standard therapy for moderate to severe psoriasis, but there is marked interpersonal variation in its efficacy and toxicity. We hypothesized that in psoriasis patients, specific common polymorphisms in folate, pyrimidine, and purine metabolic enzymes are associated with methotrexate efficacy and/or toxicity. DNA from 203 retrospectively recruited psoriasis patients treated with methotrexate was collected and genotyped by restriction endonuclease digestion or length polymorphism assays. The reduced folate carrier (RFC) 80A allele and the thymidylate synthase (TS) 3'-untranslated region (3'-UTR) 6 bp deletion were associated with methotrexate-induced toxicity (P=0.025 and P=0.025, respectively). RFC 80A and 5-aminoimidazole-4-carboxamide ribonucleotide transformylase (ATIC) 347G were associated with methotrexate discontinuation (P=0.048 and P=0.038). The TS 5'-UTR 28 bp 3R polymorphism correlated with poor clinical outcome (P=0.029), however, this was not the case when patients with palmoplantar pustular psoriasis were not included in the analysis. Stronger associations between specific polymorphisms and methotrexate-induced toxicity and discontinuation were found in a subanalysis of patients on methotrexate not receiving folic acid supplementation. We have demonstrated preliminary evidence that specific polymorphisms of enzymes involved in folate, pyrimidine, and purine metabolism could be useful in predicting clinical response to methotrexate in patients with psoriasis.

Mechanism of allopurinol induced TPMT inhibition

Up to 1/5 of patients with wildtype thiopurine-S-methyltransferase (TPMT) activity prescribed azathioprine (AZA) or mercaptopurine (MP) demonstrate a skewed drug metabolism in which MP is preferentially methylated to yield methylmercaptopurine (MeMP). This is known as thiopurine hypermethylation and is associated with drug toxicity and treatment non-response. Co-prescription of allopurinol with low dose AZA/MP (25-33%) circumvents this phenotype and leads to a dramatic reduction in methylated metabolites; however, the biochemical mechanism remains unclear. Using intact and lysate red cell models we propose a novel pathway of allopurinol mediated TPMT inhibition, through the production of thioxanthine (TX, 2-hydroxymercaptopurine). In red blood cells pre-incubated with 250 μM MP for 2h prior to the addition of 250 μM TX or an equivalent volume of Earle's balanced salt solution, there was a significant reduction in the concentration of MeMP detected at 4h and 6h in cells exposed to TX (4 h, 1.68, p=0.0005, t-test). TX acts as a direct TPMT inhibitor with an apparent Ki of 0.329 mM. In addition we have confirmed that the mechanism is relevant to in vivo metabolism by demonstrating raised urinary TX levels in patients receiving combination therapy. We conclude that the formation of TX in patients receiving combination therapy with AZA/MP and allopurinol, likely explains the significant reduction of methylated metabolites due to direct TPMT inhibition.

Assessment of thiopurine methyltransferase enzyme activity is superior to genotype in predicting myelosuppression following azathioprine therapy in patients with inflammatory bowel disease

BACKGROUND

Myelosuppression occurs in 2-7% of inflammatory bowel disease (IBD) patients treated with azathioprine, and can be associated with reduced activity of thiopurine methyltransferase (TPMT) in some patients. It has been proposed that pretreatment assessment of TPMT status reduces the incidence of toxicity and is cost-effective.

AIMS

To determine if screening for TPMT status predicts side-effects to azathioprine in patients with IBD and to ascertain whether screening by TPMT enzyme activity or genotype is superior.

METHODS

Sequential IBD patients were identified and azathioprine tolerance recorded. Blood was collected for measurement of TPMT activity and TPMT*3C, TPMT*3A and TPMT*2 genotypes.

RESULTS

Of 130 patients, 25% stopped azathioprine because of toxicity. Four patients experienced severe myelosuppression (WCC < 2). Eleven of 17 patients with reduced TPMT activity were heterozygotes, including one patient with marked TPMT deficiency who experienced severe myelosuppression. There was no association between intermediate TPMT deficiency and any side-effect.

CONCLUSIONS

Moderate reduction of TPMT activity in heterozygotes was not associated with toxicity, but very low TPMT activity caused severe myelosuppression in one patient. This would have been predicted by measuring TPMT activity but not by genotyping. Measurement of TPMT activity may therefore be superior to genotype in predicting severe myelosuppression.

Pharmacogenetic association with adverse drug reactions to azathioprine immunosuppressive therapy following liver transplantation

Azathioprine (AZA) is a thiopurine prodrug commonly used in triple-immunosuppressive therapy following liver transplantation. Approximately 1 in 10 patients suffers side effects in response to the drug, the most problematic being bone marrow toxicity. There is evidence that polymorphisms in the genes encoding thiopurine methyltransferase (TPMT) and inosine triphosphate pyrophosphatase (ITPase) predict adverse drug reactions to AZA therapy. Furthermore, common genetic polymorphisms in the gene encoding methylenetetrahydrofolate reductase (MTHFR) may have an indirect impact on thiopurine drug methylation by influencing levels of the methyl donor S-adenosylmethionine (SAM). The aim of this study was to determine whether polymorphisms in these candidate pharmacogenetic loci predict adverse drug reactions to AZA immunosuppressive therapy in liver transplant patients. A series of 65 liver transplant recipients were recruited to the study from the Liver Transplant Out-Patient clinic at The Royal Infirmary of Edinburgh. Clinical response to AZA was retrospectively correlated against TPMT activity, TPMT*2, *3A, and *3A genotypes, inosine triphosphatase (ITPA) 94C>A and IVS2+21A>C genotypes, and MTHFR 677C>T and 1298A>C genotypes. Variant TPMT, ITPA, and MTHFR genotypes were not significantly associated with adverse drug reactions to AZA, including bone marrow suppression. However, the 2 patients who suffered nodular regenerative hyperplasia (NRH) were both heterozygous for the TPMT*3A mutation. In conclusion, our findings suggest that TPMT, ITPA, and MTHFR genotypes do not predict adverse drug reactions, including bone marrow suppression, in liver transplant patients. However, the possible association between NRH and a heterozygous TPMT genotype should be investigated further.

Genetic basis of inosine triphosphate pyrophosphohydrolase deficiency in the Japanese population

Inosine triphosphate pyrophosphohydrolase (ITPase) is an enzyme that catalyzes the conversion of inosine triphosphate (ITP) to inosine monophosphate and pyrophosphate. In Caucasian populations it is reported that the frequency of cases showing decreased ITPase activity is 5%. The structure of ITPA gene along with five single nucleotide polymorphisms has been reported in Caucasians. We examined ITPase activity and frequency of two polymorphisms (94C>A and IVS2+21A>C) in 100 Japanese individuals. Among these individuals, we observed that three cases with zero activity were homozygote for 94C>A, and were accompanied by abnormal accumulation of ITP in erythrocytes. The cases included in the low ITPase activity group were heterozygote for 94C>A polymorphism. The activity of the heterozygote cases was approximately 27% of the mean value of the wild type. The allele frequency of the 94C>A polymorphism was 0.155, which was 2.6 times higher than that of the Caucasians (0.06). The IVS2+21A>C was not detected in Japanese cases, although it occurred with a frequency of 0.130 in Caucasians. Furthermore, we identified a novel mutation IVS2+68T>G in intron 2 in the case with the lowest enzyme activity in the 94C>A wild type. Since the frequency of ITPA 94C>A polymorphism is higher in the Japanese population than that in Caucasians, it is more important to examine ITPA 94C>A polymorphism in the Japanese population to prevent thiopurine drug toxicity. Pretherapeutic screening of individuals for ITPA polymorphisms should be considered for safer and more tolerable treatment with thiopurine drugs.

Novel pharmacogenetic markers for treatment outcome in azathioprine-treated inflammatory bowel disease

BACKGROUND

Azathioprine (AZA) pharmacogenetics are complex and much studied. Genetic polymorphism in TPMT is known to influence treatment outcome. Xanthine oxidase/dehydrogenase (XDH) and aldehyde oxidase (AO) compete with TPMT to inactivate AZA.

AIM

To assess whether genetic polymorphism in AOX1, XDH and MOCOS (the product of which activates the essential cofactor for AO and XDH) is associated with AZA treatment outcome in IBD.

METHODS

Real-time PCR was conducted for a panel of single nucleotide polymorphism (SNPs) in AOX1, XDH and MOCOS using TaqMan SNP genotyping assays in a prospective cohort of 192 patients receiving AZA for IBD.

RESULTS

Single nucleotide polymorphism AOX1 c.3404A > G (Asn1135Ser, rs55754655) predicted lack of AZA response (P = 0.035, OR 2.54, 95%CI 1.06-6.13) and when combined with TPMT activity, this information allowed stratification of a patient's chance of AZA response, ranging from 86% in patients where both markers were favourable to 33% where they were unfavourable (P < 0.0001). We also demonstrated a weak protective effect against adverse drug reactions (ADRs) from SNPs XDH c.837C > T (P = 0.048, OR 0.23, 95% CI 0.05-1.05) and MOCOS c.2107A > C, (P = 0.058 in recessive model, OR 0.64, 95%CI 0.36-1.15), which was stronger where they coincided (P = 0.019).

CONCLUSION

These findings have important implications for clinical practice and our understanding of AZA metabolism.

Thiopurines in Inflammatory Bowel Disease: New Findings and Perspectives

Thiopurines, available as azathioprine, mercaptopurine, and thioguanine, are immunomodulating agents primarily used to maintain corticosteroid-free remission in patients with inflammatory bowel disease. To provide a state-of-the-art overview of thiopurine treatment in inflammatory bowel disease, this clinical review critically summarises the available literature, as assessed by several experts in the field of thiopurine treatment and research in inflammatory bowel disease.

Review article: malignancy on thiopurine treatment with special reference to inflammatory bowel disease

BACKGROUND

Immunosuppression is a risk factor for carcinogenesis. Thiopurines specifically contribute to this. As thiopurines are used more aggressively in the treatment of IBD, it is likely that we will see more thiopurine-related malignancy.

AIM

To review the literature, exploring how immunosuppression, thiopurines specifically, might cause cancer and which malignancies occur in practice, placing specific emphasis on IBD cohorts.

METHODS

Search terms included 'malignancy' 'cancer' 'azathioprine' 'mercaptopurine' 'tioguanine (thioguanine)' 'thiopurine' and 'inflammatory bowel disease' 'Crohn's disease' 'ulcerative colitis'. We also searched for specific cancers (lymphoma, colorectal cancer, skin cancer, cervical cancer) and reviewed the reference lists of the articles detected.

RESULTS

Immunosuppression is associated with an increased risk of cancer. Thiopurines are associated with specific additional risks. In IBD cohorts, very few thiopurine-related malignancies have been reported. However, studies suggest a relative risk of 4-5 for lymphoma. This still translates into a low actual risk, (one extra lymphoma in every 300-1400 years of thiopurine treatment).

CONCLUSIONS

Whilst we must be aware of this risk and counsel our patients appropriately, thiopurines remain a mainstay of IBD therapy. We present practical advice aimed at minimizing our patients' risk of developing malignancy, whilst optimizing the benefits that thiopurines can provide.

Mutation in the ITPA gene predicts intolerance to azathioprine

Inosine triphosphate pyrophosphatase (ITPase) deficiency occurs with polymorphic frequencies in Caucasians and results in the benign accumulation of the inosine nucleotide ITP. In 62 patients treated with azathioprine for inflammatory bowel disease, the ITPA 94C>A deficiency-associated allele was significantly associated with adverse drug reactions (OR 4.2, 95% CI 1.6-11.5, p = 0.0034). Significant associations were found for flu-like symptoms (OR 4.7, 95% CI 1.2-18.1, p = 0.0308), rash (OR 10.3, 95% CI 4.7-62.9, p = 0.0213) and pancreatitis (OR 6.2, CI 1.1-32.6, p = 0.0485). Polymorphism in the ITPA gene thus predicts AZA intolerance. Alternative immunosuppressive drugs, particularly 6-thioguanine, should be considered for AZA-intolerant patients with ITPase deficiency.

Genetic basis of hemolytic anemia caused by pyrimidine 5' nucleotidase deficiency

Pyrimidine 5' nucleotidase (P5'N-1) deficiency is an autosomal recessive condition causing hemolytic anemia characterized by marked basophilic stippling and the accumulation of high concentrations of pyrimidine nucleotides within the erythrocyte. It is implicated in the anemia of lead poisoning and is possibly associated with learning difficulties. Recently, a protein with P5'N-1 activity was analyzed and a provisional complementary DNA (cDNA) sequence published. This sequence was used to study 3 families with P5'N-1 deficiency. This approach generated a genomic DNA sequence that was used to search GenBank and identify the gene for P5'N-1. It is found on chromosome 7, consists of 10 exons with alternative splicing of exon 2, and produces proteins 286 and 297 amino acids long. Three homozygous mutations were identified in this gene in 4 subjects with P5'N-1 deficiency: codon 98 GAT-->GTT, Asp-->Val (linked to a silent polymorphism codon 92, TAC-->TAT), codon 177, CAA-->TAA, Gln-->termination, and IVS9-1, G-->T. The latter mutation results in the loss of exon 9 (201 bp) from the cDNA. None of these mutations was found in 100 normal controls. The DNA analysis was complicated by P5'N-1 pseudogenes found on chromosomes 4 and 7. This study is the first description of the structure and location of the P5'N-1 gene, and 3 mutations have been identified in affected patients from separate kindreds. (Blood. 2001;97:3327-3332)

Optimizing the use of thiopurines in inflammatory bowel disease

Immunomodulator drugs, of which thiopurines can be considered the backbone, are widely used in the treatment of inflammatory bowel disease. They have been shown to be highly effective and safe; however, a significant proportion of patients are deemed to have a poor response or suffer adverse reactions. Knowing how to monitor and optimize thiopurine therapy in these scenarios is crucial to effective management. We discuss the metabolism of thiopurines, the use of enzyme/metabolite testing to guide treatment, as well as strategies to circumvent toxicity and side effects, such as allopurinol coprescription. The indications, use in pregnancy, safety profile and duration of thiopurine therapy are also discussed.

Acute kidney injury in two children caused by renal hypouricaemia type 2

BACKGROUND

Renal hypouricaemia is a heterogeneous inherited disorder characterized by impaired tubular uric acid transport with severe complications, such as acute kidney injury and nephrolithiasis. Type 1 is caused by a loss-of-function mutation in the SLC22A12 gene (OMIM #220150), while type 2 is caused by defects in the SLC2A9 gene (OMIM #612076).

CASE-DIAGNOSIS/TREATMENT

The cases of two children, a 12- and a 14-year-old boy with acute kidney injury (proband 1: urea 9.4 mmol/l, creatinine 226 μmol/l; proband 2: urea 11.7 mmol/l, creatinine 202 μmol/l) are described. Both are offspring of nonconsanguineous couples in the UK. The concentrations of serum uric acid were consistently below the normal range (0.03 and 0.04 mmol/l) and expressed as an increase in the fractional excretion of uric acid (46 and 93 %).

CONCLUSIONS

A sequencing analysis of the coding region of uric acid transporters SLC22A12 and SLC2A9 was performed. Analysis of genomic DNA revealed two unpublished missense transitions, p.G216R and p.N333S in the SLC2A9 gene. No sequence variants in SLC22A12 were found. Our findings suggest that homozygous and/or compound heterozygous loss-of-function mutations p.G216R and p.N333S cause renal hypouricaemia via loss of uric acid absorption and do lead to acute kidney injury.

Genetic determinants of the thiopurine methyltransferase intermediate activity phenotype in British Asians and Caucasians

OBJECTIVE

Polymorphisms in the TPMT gene open reading frame (ORF) are associated with reduced TPMT activity. Variable number tandem repeats (VNTR*3 to VNTR*9) in the promoter region of the gene consisting of combinations of Type A, B and C repeat units, may modulate TPMT activity. Here we present the allele frequencies of genetic modifiers of TPMT activity in a British Asian population, as well as the concordance between intermediate TPMT activity and ORF and VNTR genotypes in a predominantly Caucasian population.

METHODS

VNTR type and ORF mutations were determined in two selected TPMT activity ranges, intermediate activity (4-8 U, 108 patients), normal (12-15 U, 53 patients) and in 85 British Asians.

RESULTS

In British Asians, TPMT*3C was the prevalent mutant allele (four heterozygotes). One patient was heterozygous for TPMT*3A. Overall VNTR frequencies did not differ from Caucasians. Three new VNTR alleles were designated VNTR*6c, VNTR*6d, and VNTR*7c. Forty-one percent of patients with intermediate activity were heterozygous for a TPMT ORF mutation (3A, 2B, 1C). Marked linkage disequilibrium was noted between VNTR*6b - TPMT*3A (D' = 1), VNTR*4b - TPMT*3C (D' = 0.67) and VNTR*6a - TPMT*1 (D' = 1) alleles. As a result, significant differences (P < 0.05) in the distribution of Type A, B or the total number of repeats summed for both alleles, were found between the ORF heterozygous intermediate activity group and the wild-type intermediate or normal activity groups. No significant difference was found between the two wild-type groups.

CONCLUSION

Our results suggest that TPMT gene VNTRs do not significantly modulate enzyme activity.

A novel nucleotide found in human erythrocytes, 4-pyridone-3-carboxamide-1-beta-D-ribonucleoside triphosphate

We report the identification of a hitherto unknown nucleotide that is present in micromolar concentrations in the erythrocytes of healthy subjects and accumulates at levels comparable with the ATP concentration in erythrocytes of patients with chronic renal failure. The unknown nucleotide was isolated and identified by liquid chromatography with UV and tandem mass detection, (1)H nuclear magnetic resonance and infrared spectroscopy as 4-pyridone-3-carboxamide-1-beta-D-ribonucleoside triphosphate (4PYTP), a structure indicating association with metabolism of the oxidized nicotinamide compounds. Subsequently, we demonstrated formation of 4PYTP in intact human erythrocytes during incubation with the chemically synthesized nucleoside precursor 4-pyridone-3-carboxamide-1-beta-D-ribonucleoside (4PYR). We noted preferential accumulation of monophosphate of 4PYR (4PYMP) over 4PYTP as well as a decrease in erythrocyte ATP concentration during incubation with 4PYR. Both the 4PYR phosphorylation and ATP depletion were blocked by an inhibitor of adenosine kinase. Plasma concentration of 4PYR was detectable but very low (0.013 +/- 0.006 microm) in contrast with the high daily urine excretion of this compound (26.7 +/- 18.2 micromol/24 h) in healthy subjects, indicating much greater renal clearance than other nicotinamide metabolites, nucleosides, or creatinine. We also noted a 40-fold increase in 4PYR plasma concentration in patients with chronic renal failure (0.563 +/- 0.321 microm). We suggest that 4PYTP formation in the erythrocytes is a hitherto unknown process aimed at sequestering potentially toxic 4PYR in a form that could be safely transported and subsequently released and excreted during passage of erythrocytes through the kidney.

Thioguanine in inflammatory bowel disease: Long-term efficacy and safety

BACKGROUND

Thioguanine (TG) is efficacious in inflammatory bowel disease (IBD), but its toxicity, particularly nodular regenerative hyperplasia (NRH) of the liver, has limited its use. We assessed the long-term clinical outcomes and safety of TG in patients whom were intolerant or refractory to conventional immunomodulators.

METHODS

This is a retrospective, single-centre study of IBD patients treated with TG from 2001-2013. Response was defined as clinical remission (Harvey-Bradshaw Index < 5 for Crohn's disease (CD), Simple Clinical Colitis Activity Index < 4 for ulcerative colitis (UC)) without corticosteroids or, if receiving anti-tumour-necrosis-factor (anti-TNF) therapy, absence of dose escalation. We recorded TG failure, withdrawal and adverse events. Patients were monitored with biochemistry, liver biopsy and/or magnetic resonance imaging (MRI).

RESULTS

54 patients (47 CD and 7 UC) whom received TG (mean dose: 27 mg/d (range: 20-40 mg/d)) as monotherapy (n = 36) or concomitantly with anti-TNF (n = 18) for a median inter-quartile range of 16 (5-37) months (126 patient-years of follow-up). 32 (59%) patients responded to TG at 6 months and 23 (43%) at 12 months. Pancreatitis did not recur amongst the 19 patients with prior thiopurine-induced pancreatitis. 16 (30%) patients ceased TG due to intolerance or toxicity (four serious); NRH was not observed. 6-thioguanine nucleotide concentrations did not correlate with efficacy nor with toxicity.

CONCLUSIONS

TG was efficacious and well tolerated in one out of two patients who had previously failed conventional immunomodulators. NRH did not occur.

Origin and characteristics of an unusual pyridine nucleotide accumulating in erythrocytes: positive correlation with degree of renal failure

BACKGROUND

Our aim was to identify an unusual nucleotide accumulating, with precursors, in erythrocytes in uraemia and to establish its relationship to the candidate uraemic toxin, N1-methyl-2-pyridone-5-carboxamide (M2Py).

METHODS

This nucleotide, plasma creatinine and M2Py were measured in four categories of patients: mild chronic renal failure (CRF), end-stage renal failure (ESRF), haemodialysis (HD) and continuous ambulatory peritoneal dialysis (CAPD); also in healthy controls and patients after successful kidney transplantation (Post-Tx).

RESULTS

The nucleotide was identified using liquid chromatography and mass spectrometry as 2-pyridone-5-carboxamide ribonucleoside triphosphate (2PyTP). In the patient groups, mean 2PyTP concentrations are lowest in CRF (21.8 micromol/l), increasing with degree of renal failure through ESRF (55.1 micromol/l) and HD patients (70.9 micromol/l) and being highest in CAPD patients (216.7 micromol/l). Following successful transplantation, 2PyTP concentrations fall to the control range. Plasma concentrations of M2Py correlate with 2PyTP in the erythrocytes; both are undetectable in patients lacking molybdenum cofactor, needed to oxidise N-methylnicotinamide.

CONCLUSION

Our evidence indicates that M2Py accumulates in parallel with the nucleotide 2PyTP in renal failure: either may be a uraemic toxin, since both increase with the degree of renal failure. Elevated concentrations of 2PyTP in CAPD patients reflect a longer half-life for erythrocytes than in HD patients.

Allele frequency of inosine triphosphate pyrophosphatase gene polymorphisms in a Japanese population

The enzyme inosine triphosphate pyrophosphatase (ITPase) catalyses the pyrophosphohydrolysis of ITP to IMP. ITPase deficiency is a clinically benign autosomal recessive condition characterised by the abnormal accumulation of ITP in erythrocytes. A deficiency of ITPase may predict adverse reactions to therapy with the thiopurine drug 6-mercaptopurine and its prodrug azathioprine. In this study, we examine the frequencies of ITPA polymorphisms in 100 healthy Japanese individuals. The allele frequency of the 94C > A variant in the Japanese sample was 0.135 (Caucasian allele frequency 0.06). The IV2 + 21A > C polymorphism was not found in Japanese (Caucasian allele frequency 0.130). Allele frequencies of the 138G > A, 561G > A and 708G > A polymorphisms were 0.57, 0.18 and 0.06 respectively in the Japanese population, and with the exception of the 138G > A polymorphism, similar to allele frequencies in Caucasians.