Here we report a case of a secondary AML developing from donor-derived cells in a breast cancer patient who underwent allogeneic stem cell transplantation. Donor cell leukemia is a rare although well-recognized disease entity following SCT that occurs as the result of oncogenic transformation of apparently normal donor hematopoietic cells in the transplant recipient.
Many studies have reported an increased risk of breast cancer patients to develop leukemia after chemotherapy, radiotherapy and G-CSF administration [1, 5, 16, 17]. Because of this, risk estimates on the eventual development of post-treatment AML/MDS have to be cast when deciding a patient's treatment. In these studies, increased risk of AML/MDS has been reported for cases treated with alkylating agents and anthracyclines. Post-treatment secondary leukemias associated with prior administration of alkylating agents (e.g., cyclophosphamide) typically differ from those arising after treatment with DNA topoisomerase II inhibitors, such as anthracyclines. Accordingly, AML's developing after topoisomerase II inhibitors are given typically show an early onset, and display monocytic and myelomonocytic features in association with abnormalities of chromosomes 11 and 21 (especially balanced translocations involving the 11q23 and 21q22 regions). Whereas those arising after treatment with alkylating agents frequently show neutrophil/granulocytic maturation together with abnormalities of chromosome 5 and 7. These changes are seen in the absence of chromosomal translocations and the leukemias emerge much later after therapy .
Exposure to radiotherapy may further increase the risk for AML [19–21]. In turn, Smith et al. found the M4/M5 subtypes to be more frequent in patients receiving intense treatment regimens and concluded that this could be the result of cyclophosphamide-induced promotion of a doxorubicin-associated leukemogenic effect. In the case reported here, the association of chemotherapy and radiotherapy protocols most probably played a role in the development of AML. Secondary AML outcomes are thus believed to arise from genomic instability (i.e., deletions, mutations, translocations) induced by therapy-associated DNA damage [22, 23]. Two different hypotheses remain which could contribute to explain the development of secondary AML: a truly stochastic event, or individual differences on cancer susceptibility . The latter appears to better explain the case reported here. In this regard, previous reports have described polymorphisms conferring sensitivity to chemotherapy, which may contribute to the incidence of secondary AML outcomes [24–26]. Alternatively, it is possible that germline variations in DNA repair genes may also enhance the risk of therapy-induced secondary AML in patients carrying DNA-repair deficient genes . In fact it has also been shown that single nucleotide polymorphisms (SNP) in DNA repair genes may code malfunctioning proteins, in association with an increased predisposition to cancer and the response of leukemia patients following chemotherapy [12, 28, 29]. Here we investigated five alleles of DNA repair genes already ascribed to susceptibility to leukemia (XRCC1, XPA, XPD, XRCC3 and RAD51). As expected, polymorphic alleles found by genotyping in both the patient post-SCT hematopoietic cells and the donor's cells were the same, revealing successful transplant, which could also explain the otherwise observed resistance to treatment and clinical evolution. In turn, pre-SCT patient cells were heterozygous for allelic variants in the XPD and XRCC3 genes that are associated with lower repair capacity, conspicuously bearing on susceptibility to breast cancer and chemotherapy-related leukemia (AML) in this patient. Consistent with our observations, Allan et al. showed that individuals carrying at least one XPD- Lys751Gln allele were more likely to have an adverse prognosis following chemotherapy. The XPD Gln751 polymorphic protein was shown to fail in engaging apoptosis in chemotherapy-damaged cells, thus avoiding elimination of mutated myeloid precursors. In order to search for an association of the poor outcome of the patient's secondary post-SCT leukemia, the same gene set of polymorphisms was screened for in the donor's DNA. Remarkably, donor DNA genotyped homozygous not only for the XPD (Lys751Gln) suboptimal allele, but also for the XRCC3-deficient allele (Thr243Met). Both post-SCT blast cells and donor DNA were coincident in XPD- and XRCC3-deficient homozygosity, supporting a donor origin for the leukemic blasts.
The XRCC3 protein plays a critical role in Homologous Recombination Repair (HRR) accounting for repair of DNA double-strand breaks (DSB). Whenever it is deficient, unsealed or misrepaired breaks can generate oncogenic chromosomal translocations. When found together, allelic variants in Lys751Gln XPD and Thr241Met XRCC3 polymorphisms have been associated with both a worse repair capacity and resistance to apoptosis [11, 30]. Once introduced into the patient's drug-intoxicated bone marrow, disrupted DNA repair and apoptosis pathways in the donor's cells may have accumulated unrepaired damages while escaping apoptosis, thus boosting aggressiveness. Indeed, this is further supported by the observation of a switch from normal to highly abnormal post-SCT of the patient's karyotype and FISH analysis. This supports the theory that in donor origin leukemia, the host environment in which the original malignancy developed could trigger an oncogenic process in donor cells, favored by the immunosuppressive status after transplantation, especially because the donor is still healthy [31, 32].
An additional interesting finding is the maturation observed phenotypically towards the basophilic versus mast cell lineages based on coexpression of CD203c and both CD123, CD117 . Cases of de novo AML with predominant basophilic and/or mastocytic, cell phenotypes are uncommon and they account for only 4-5% of all cases of acute nonlymphocytic leukemia [34, 35]. Although acute basophilic leukemia (ABL) has been long diagnosed as such, current knowledge of this specific AML subtype remains limited .
In conclusion, here we describe a very rare case of donor origin AML from a sibling who harbored germline XPD and XRCC3 homozygous polymorphisms in a breast cancer patient following chemotherapy. The blasts showed SNP profiles of the donor including susceptible alleles for unfavorable genotypes in DNA repair genes. Determination of the donor's DNA repair genotype, capable of stimulating genetic instability in a diseased recipient, could be important for future transplantation procedures and therefore should be investigated further. DNA repair mechanisms are responsible for maintenance of the genome integrity avoiding additional mutations in key cell cycle regulation genes which are responsable for "leukemization". To ensure that DNA repair mechanisms are properly working, more donor cell gene polymorphisms or mutations should be studied. After the interpretation of results, a reference panel of low DNA repair capacity polymorphisms or mutations should be included in the international guidelines for screening donor DNA before SCT.