Donor Cell-Derived Acute Myeloid Leukaemia with 3q26.2 Involvement/ MECOM Rearrangement ‐ A Case Report and Literature Review

Donor cell-derived leukaemia (DCL) is an uncommon complication of allogeneic hematopoietic stem cell transplantation (HSCT). DCL might represent up to 5% of the post-HSCT disease relapses, but case numbers reported in the literature might underestimate the frequency. The leukemogenesis chromosome 20q deletion. We also reviewed the literature of previously described DCL cases, and we discussed the risk factors that might be important to the onset of DCL.


Introduction
Donor cell-derived leukaemia (DCL) is a rare complication of allogenic hematopoietic stem cell transplantation (HSCT). DCL might represent up to 5% of the post-transplant relapsed leukaemia cases [1][2][3][4]. To date, less than 200 DCL cases have been reported since its first recognition in 1971 [1][2][3][4][5][6][7][8]. However, the actual number of DCL cases might be higher due to the inconsistent use of chimeric testing on bone marrow transplant patients [9,10]. Many factors have been proposed as being associated with the initiation of DCL: donor genetic profile, genetic susceptibility of recipients and/or donors, microenvironment of the host, and therapeutic interventions -such as chemotherapy and some targeted therapies [1,2,5,11,12]. Although most of the donors are assumed to be genetically normal, there have been cases reported in which the donor had certain constitutional and/or somatic genetic abnormalities overlooked by current pre-transplant screening. In most of these cases, the abnormalities were investigated and detected only after recipients were identified with chromosomal aberration(s) and/or even developed DCL [1][2][3]8]. There also have been case reports of donors remain free of genetic abnormalities, whereas their recipients developed DCL with newly emerging genetic aberrations [3,8].
The molecular genetic events among all reported DCL cases are heterogeneous. Here we report a case of donor cell-derived acute myeloid leukaemia (AML) with 3q26.2 involvement/MECOM rearrangement. We further explore the underlying mechanisms of DCL initiation and progression along with a testing strategy.

Case Report
The patient was a 65-year-old woman immigrant to the United States who was diagnosed initially with AML in her native country 5 years ago. Her initial bone marrow aspiration and biopsy showed 30% blasts. Conventional cytogenetic analysis demonstrated a female complex karyotype (no detailed information available) and FISH analysis showed 5q-, 7q-, -7, and +22 as well as loss of one copy of TP53; Sanger sequencing revealed a TP53 G443 frameshift mutation with a variant allele frequency (VAF) of 30% at that time. The patient was first treated with cytarabine plus daunorubicin and achieved complete remission. A post-chemotherapy bone marrow aspiration and biopsy specimen showed 1% blast and molecular analysis was negative for TP53 mutation. Conventional cytogenetic analysis and FISH tests were not repeated after therapy. The patient subsequently underwent allogeneic HSCT twice (9 months apart) from the same HLA-matched male sibling. Her first HSCT was complicated with cytomegalovirus (CMV) viremia. It is necessary to point out that according to our clinical notes and the discharge summary from her primary hospital the reason and the conditioning details for the second HSCT remain unknown. However, her second HSCT was complicated by graft-versus-host disease (GVHD) mainly in skin and gut. She experienced engraft chimerism and relapse of AML 16 months after the second transplant. She then received azacitidine for 6 courses and achieved a second remission. Seven month later, her disease relapsed again and bone marrow evaluation showed 8% blasts and an acquired PTPN11 mutation. She was then administered low-dose cytarabine with venetoclax (initially 800 mg, then reduced to 600 mg) for 3 cycles. Due to her persistent disease, she was then referred to our institution.

Discussion
To the best of our knowledge, this is the first DCL case with confirmed 3q26.2 involvement/MECOM rearrangement reported in the literature [1][2][3][4]. Aldoss and Song reported a donor cell AML case with chromosome 3 involvement in 2017 [18]. The karyotype of that case was reported as 46,XX,t(3;13)(p10;q10),inv(11)(p15q23) [13]/46,XX [2] and originated from donor cells, but without 3q26.2 involvement/MECOM rearrangement. The patient achieved complete remission and switched to a normal donor karyotype "46,XX [19]" after chemotherapy. Dias et al [8] reported a case with inv(3)(p26q25) after a sex-matched HSCT. Further intensive investigation revealed that the donor carried this constitutional chromosomal aberration. The recipient in their report remained a complete remission after HSCT and did not develop DCL, but his karyotype switched from 46,XY (recipient type) to 46,XY,inv(3)(p26q25) (donor type) during post-transplant follow-up interval of at least 18 months. Interestingly, the inv(3) was applied as an efficient biomarker for distinguishing donor cells from recipient cells in this circumstance.
It should be noted that del20q occurred simultaneously with 3q26.2 involvement/MECOM rearrangement after HSCT in this patient. Del20q is a recurrent cytogenetic abnormality observed rarely in the healthy aging population (approximately 0.1%) [23], and more often in patients with myeloid malignancies including AML (1-2%), MDS (5-7%), and in myeloproliferative diseases (MPNs) (up to 10%) [11,15,16]. Yoon et al [24] reported one case with a donor-derived del20q clone. Although the size of the clone expanded during a follow-up of 18 months in that case, neither disease relapse nor DCL or other donor cell-derived malignancy occurred. Isolated del20q is usually considered as an intermediate prognostic factor in myeloid malignancies [11,14,15]. Hence, it seems most likely that 3q26.2 involvement/MECOM rearrangement, rather than del20q, had the most impact on the poor prognosis of donor cell-derived AML in the patient we report.
HSCT is considered as the sole curative option for certain types of AML [25], but post-transplant disease relapse occurs. At the same time, relapsed disease may occasionally differ from the original disease at various levels, such as clinical presentation, subtypes of diseases (e.g., myeloid versus lymphoid malignancies due to lineage switch), and/or even different cell of origin (recipient cells versus donor cells). It has been reported that DCL may represent up to 5% of all post-transplant "relapsed" leukaemia cases, and it is critical to distinguish DCL from the truly relapsed diseases, especially if a second allogeneic HSCT is being considered as the next therapy option. In this case, a post-transplant "relapsed AML" would have been diagnosed if based only on the pathologic and immunophenotypic findings in the bone marrow and peripheral blood. However, the newly identified chromosomal abnormalities including 3q26.2 involvement/MECOM rearrangement were different from the initial disease, at least implying an emergence of different clone(s) of leukemic cells, and the male sex identified by chromosome and XY FISH analyses confirmed origin from donor cells. Mutations of BCOR, FLT3, PIGA and PTPN11 detected after transplant by NGS-based testing were mostly of donor cells, while her previously reported TP53 G443 frameshift mutation was no longer detected.
The factors underlying leukemogenesis of DCL in this case remain unknown. It is believed that multiple factors from recipient, donor as well as therapeutic interventions may play independent and/or synergetic roles in the pathogenesis of donor cell AML ( Figure 4) [1][2][3]10]. The microenvironment after myeloablative conditioning in the recipient may be carcinogenic to donor cells, and the later can be transformed more easily, especially if any occult pre-leukaemic clones exist, which is normally suppressed by normally functioning immune system of the donor [12]. On the other hand, the recipient's compromised T-cell immunity and microenvironment may have facilitated the growth and expansion of the pre-leukaemic clones to a full-blown AML with distinct cytogenetic and molecular genetic characteristics [4,7]. Our patient had CMV viremia that occurred after the first HSCT, and GVHD and relapsed AML after the second HSCT from the same donor, indicating a dramatically compromised post-transplant immune system and/or delayed immune reconstitution in the recipient. According to current clinical practice standards, bone marrow donors are evaluated for general health, but not for any apparent and/or occult comorbid conditions such as constitutional chromosomal aberration(s) and/or other cancer-predisposing genetic conditions. Some donors have been reported to carry constitutional chromosomal aberration(s), such as karyotype of 47,XXY or 47,XXY, inversions involving chromosome(s) 3, 9, 10 and 12, trisomy 8, Robertsonian translocation(s), and/or other balanced reciprocal translocation(s) [8,26]. These chromosomal aberrations are mostly detected after transplant, and the same and/or evolved chromosomal aberrations were detected in the recipients. It remains uncertain whether one or some of these constitutional chromosomal aberration(s) are directly associated with pathogenesis of DCL, but this always triggers the debate about whether bone marrow donors should be more intensively screened, including testing for known tumour-susceptible genetic conditions such as apparent chromosomal aberrations. Clonal haematopoiesis of indeterminate potential (CHIP) is a common occult condition characterized as expansion of age-acquired somatic mutations in hematopoietic lineages that are associated with tumorigenesis, persistence of measurable residual diseases, disease relapse as well as the development of DCL after HSCT in hematologic malignancies. It is believed that CHIP is a novel risk factor for DCL [27], especially if donor and/or recipient are aging individuals. The patient we report was over 60 years old at the time of her second HSCT. TP53 frameshift mutation and PTPN11 mutation have been detected prior to HSCT and during disease relapse post HSCT, respectively. The age of her sibling donor remains unknown to us. Some germline variants, particularly those involving ANKRD26, CEBPA, DDX41, ETV6, GATA2, and RUNX1, have been reported as hereditary predispositions to developing myeloid malignancies [28][29][30], including donorcell derived malignancies [31]. All these six genes are included in our clinical NGS-based 81-gene leukaemia mutation panel, and none of them had variant alleles.

Conclusions
We report a case of donor cell-derived AML with 3q26.2 involvement/MECOM rearrangement. This genetic abnormality is often associated with a poor prognosis. Possible factors involved in leukemogenesis of DCL are also discussed. This case illustrates the importance of considering genetic evaluation of donors as part of screening for HSCT.

Author Contributions
Conceptualization, Jun Gu, Wei Wang and Zhenya Tang; data curation: Jun Gu, Wei Wang and Zhenya Tang; methodology, Wei Wang, Guilin Tang