Alloanatigenic Reactions after Hematopoietic Cell Transplantation Induce Genomic Alterations in Epithelial Cells as Shown in Human Studies and in an In Vitro Model.

Abstract

Previous studies from our group demonstrated frequent genomic alterations measured by microsatellite instability (MSI) in non-neoplastic epithelial tissues of patients who underwent allogeneic hematopoietic cell transplantation (HCT) (Blood 2006; 107:3389–3396). These genomic alterations were found only after allogeneic but not after autologous HCT, and therefore we hypothesized that an “allogeneic” effect is substantially involved in the mutation process. We extended our previous analyses by examining 210 bucall swabs obtained from 70 patients between day (d+) 26 and d+3514 after allogeneic HCT for the presence of MSI. MSI analysis was performed by PCR and denaturing capillary electrophoresis at three tetranucleotide (THO-1, SEE33, D14S120) and three mononucleotide microsatellite (ZP3, BAT26, SRY) loci. MSI was found in the buccal smears of 38% allografted patients (median time of occurence 322 days). In a prospective trial, in which patients were followed from time before transplantation until d+365, 5 out of 14 (35%) patients exhibited MSI after transplantation although all of them showed stable microsatellites before transplantation. We are currently pefroming statistical analyses in order to identify which clinical factors influence the presence of MSI and we will present the data in the meeting. To test the hypothesis that an “alloantigenic” effect is responsible for th induction of MSI, we developed a model system in which keratinocyte (HaCaT) cells were transfected with a palsmid vector which carries a G418 (neo) selectable marker and a microsatellite repeat (CA) that places the sequence for Hygromycin Resistance (HygR) out of frame for protein translation. In this reporter system, DNA slippage mutations can restore the HygR reading frame and become detectable by hygromycin treatment as hygromycin resistant (HygR+) colonies. Pools of stably transfected (neo+) HaCaT cells were treated with supernatant (SN) of major histocompatibility complex nonmatched mixed lymphocyte cultures (MLC) and assayed for HygR+ colonies 48h later. Cells transfected with a control, in-frame hygromycin B gene construct (p12) were used as positive controls. Using this system, we found that HaCaT cells aquire hygromycin resistance after treatment with supernatatant from MLC. Treatment of cells with hydrogen hyperoxid which has been shown in a E. Coli system to induce MSI (PNAS 1998, 95:12468–12473) generated HygR+ colonies at a >80% lower frequency than the SN-MLC treatment. Control p12 transfected cells were grown with high efficiency in the presence of hygromycin B. In summary, our in vivo data confirm our previous results and provide evidence of genomic alterations after allogeneic HCT and our in vitro data are compatible with the hypothesis that an “alloantigenic” factor is the driving force in producing detectable MSI in the allografted patients. Elucidating the ultimate mechanisms underlying the genomic instability following allogeneic HCT may prove to be of major therapeutic value.