Identification of the Genotype – Phenotype Correlation in the Inosine Monophosphate Dehydrogenase Enzyme
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Mycophenolate mofetil (MMF) is widely used to minimise acute rejection following solid organ transplantation as it inhibits inosine monophosphate dehydrogenase (IMPDH) and thereby reduces lymphocyte activation. The effects of MMF and azathioprine on renal allograft outcome were examined by analysis of the national transplant database held at National Health Service (NHS) Blood and Transplant, Stoke Gifford, Bristol, UK. In a paired kidney analysis, MMF treated patients had a 3 year death censored graft survival of 91% (n=217) contrasts to 97% (n=231) in azathioprine treated patients (p=0.07) with an increased acute rejection rate in the first year after transplantation (44 v 31%, n=105 v 74, p<0.01). In a further study, 13% (n=71) of patients were found to be taking less than 1 g of MMF which was associated with a 3-fold increased risk of graft failure and inferior graft function up to 36 months. One strategy to improve graft outcome would entail targeting MMF dose according to pre-transplant IMPDH activity, which is known to display wide variability between patients, in order to maximize efficacy and minimize toxicity. Therefore, it was decided to measure pre-transplant IMPDH activity and to investigate associations with renal allograft outcome and MMF dose tolerated after transplantation. IMPDH activity was measured by detection of generated XMP by a validated HPLC method in the peripheral mononuclear cells of 55 patients waiting for renal transplantation and was found to exhibit a 4-fold variation of IMPDH activity. Black males had significantly increased IMPDH activity contrasts to Black females (p=0.01). Within the first year of transplantation, 71% (n=12) patients required a reduction in MMF dose. There was no association between pre-transplant IMPDH activity and MMF dose achieved at 1 year or MMF associated side effects or eGFR up to 36 months.It was proposed that the inter-individual variability of IMPDH activity may be associated with genetic polymorphisms and therefore sequencing of the exons of IMPDH I and II was undertaken. Two novel single nucleotide polymorphisms (SNPs), Leu244Leu and Ala285Thr, were identified in the IMPDH I gene. Patients with these variants did not exhibit differential IMPDH activity. Genotyping for established intronic SNPs was undertaken in our patient cohort as well as a random sample of 1040 recipients from the Collaborative Transplant Study DNA bank based at the University of Heidelberg, Germany. The presence of these SNPs did not increase the risk of rejection or affect graft function or MMF dose tolerated at 1 year after transplantation and there was no association between pre-transplant IMPDH activity, 5 year graft and patient survival and genotype. In our study, MMF treatment did not result in improved renal allograft outcomes in comparison to azathioprine therapy. Furthermore, we suggest that measurement of pre-transplant IMPDH activity or genotyping of the IMPDH enzymes is unlikely to assist in optimizing MMF dose and renal allograft outcome.
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