Donor-derived malignancy and transplant morbidity: Risks of patient and donor genetics in allogeneic hematopoietic stem cell transplant

Abstract

Allogeneic hematopoietic stem cell transplant (allo-HSCT) remains a key treatment option for hematologic malignancies (HMs), although it carries significant risks. Up to 30% of patients relapse after allo-HSCT, of which up to 2–5% are donor-derived malignancies (DDM). DDM can arise from a germline genetic predisposition allele or clonal hematopoiesis (CH) in the donor. Increasingly, genetic testing reveals that patient and donor genetic factors contribute to the development of DDM and other allo-HSCT complications. Deleterious germline variants in CEBPA, DDX41, GATA2, and RUNX1 predispose to inferior allo-HSCT outcomes. DDM has been linked to donor acquired somatic CH variants in DNMT3A, ASXL1, JAK2, and IDH2, often with additional new variants. We do not yet have evidence to standardize donor genetic sequencing prior to allo-HSCT. The presence of hereditary HM disorders should be considered in patients with myeloid malignancies and their related donors, and screening of unrelated donors should include family and personal history of cytopenias and HMs. Excellent multidisciplinary care is critical to ensure efficient timelines of screening and necessary discussions among medical oncologists, genetic counselors, recipients, and potential donors. After allo-HSCT, HM relapse monitoring with genetic testing effectively results in genetic sequencing of the donor as the transplanted hematopoietic system is donor derived, which presents ethical challenges for disclosure to patients and donors. We encourage consideration of the recent National Marrow Donor Program policy that allows donors to opt in for notification about detection of their genetic variants after allo-HSCT, with appropriate genetic counseling when feasible. We look forward to prospective investigation of the impact of germline and acquired somatic genetic variants on hematopoietic stem cell mobilization/engraftment, graft versus host disease, and DDM to facilitate improved outcomes through knowledge of genetic risk.

Graphical Abstract

allo-HSCT allogeneic hematopoietic stem cell transplant, DDM donor-derived malignancy, HHMs hereditary hematologic malignancies. Created with BioRender.com.

Introduction

Allogeneic hematopoietic stem cell transplant (allo-HSCT) remains an important treatment option for hematologic malignancies (HMs) when standard treatment approaches do not provide an option for cure. Accounting for over one third of allo-HSCTs worldwide, acute myeloid leukemia (AML) is the most common indication.¹ Access to curative allo-HSCT has increased with the use of partially histocompatibility leukocyte antigen (HLA)-matched (haploidentical) donors, most commonly siblings or parents of the patient with the HM. Outcomes after allo-HSCT have improved over time due to personalized approaches to preparative regimens, improved detection of pre-allo-HSCT disease burden, and enhanced supportive care. However, the risks of allo-HSCT remain significant. The most common cause of death remains relapse of the patient’s original HM (up to 50% of deaths depending on original disease type and conditioning regimen), though at least 0.1–0.5% of post-allo-HSCT malignancies occur in the transplanted donor cells, and are likely underestimated due to inconsistent investigation of cell of origin at time of malignancy detection after allo-HSCT. ^2–4 The remainder of allo-HSCT mortalities occur due to infection (16–20%) and severe graft versus host disease (12–17%) after allo-HSCT.⁵

Although infrequent, donor-derived malignancy (DDM) is a challenging diagnosis. Patients with DDM experience lower efficacy and higher toxicity with subsequent treatment, and prognosis is dismal⁶. Series of DDM cases suggest mortality rates ranging from 56–75% with varied follow up,^4,6,7 similar to 77% mortality rates at one year after relapse of original leukemia post-allo-HSCT.⁸ Patient and donor factors contribute to DDM, although pathogenesis is not completely understood. Donor selection for allo-HSCT relies heavily on HLA typing and age, historically prioritizing fully HLA-matched and younger donors. Recently, data have emerged that donor genetic variants influence transplant outcomes, including poor stem cell mobilization, allograft dysfunction, impaired immune reconstitution, and subsequent DDM.^6,9–11 However, genetic screening is inconsistently incorporated in donor selection. Hereditary hematologic malignancies (HHMs) and acquired clonal hematopoiesis (CH) increase the likelihood of premalignant clones that can be transferred through allo-HSCT. Here, we describe the current understanding of the genetic and bone marrow microenvironment factors from the donor and recipient that predispose allo-HSCT complications and DDM. We also discuss the ethical considerations regarding disclosure of genetic results to donors and recipients, and the ways in which genetic analysis may improve outcomes if incorporated into donor selection algorithms in the future. (Figure 1)

Figure 1:

DDM Etiology. DDM is predisposed by deleterious donor genetic factors, including germline variants and acquired clonal hematopoiesis in the setting of heavily treated bone marrow microenvironment with rapid hematopoietic expansion after ALLO-HSCT.

ALLO-HSCT allogeneic stem cell transplant, CH clonal hematopoiesis, HHMs hereditary hematologic malignancies Created with BioRender.com.

Methods

This review article was developed in response to a request for proposals for review articles from the Center for International Blood and Marrow Transplant Research (CIBMTR) Donor Health and Safety Working Committee (DSWC). After the proposal was elected by the DSWC for collaboration, the proposing authors and DSWC convened a writing committee made up of invited experts in the areas of allo-HSCT, CH, and HHM disorders. Literature review was conducted through PubMed searches for articles describing donor derived malignancy and genetic abnormalities in donors and recipients of allo-HSCT. Articles were selected that investigate genetic abnormalities in allo-HSCT, differentiated by variant origin of germline and CH disorders, with emphasis on balanced representation of the impacts of these abnormalities. Individual cases of donor-derived malignancy have been detailed in other review articles^6,12,13 and were not extensively detailed in this review.

Donor-derived malignancy

The first case of presumed DDM was reported in 1971, in a female who underwent transplant for refractory acute lymphoblastic leukemia (ALL) from her HLA-matched brother.¹⁴ ALL recurred ~2 months post-transplant with an XY karyotype suggesting it was donor-derived.¹⁴ Data on DDM are restricted to case reports and small case series. Thus, rates of DDM remain imprecise and are likely underestimated. A study from the European Bone Marrow Transplant (EBMT) registry identified 38 patients with DDM, with an incidence of 0.4% by 25 years after allo-HSCT⁴. Notably, definitive confirmation that malignancy post-transplant represents relapse of the recipient’s disease is often not ascertained. Thus, rates of DDM may be underestimated.

MDS and AML are the most reported DDMs. Fewer cases of donor-derived ALL, chronic myelogenous leukemia (CML), multiple myeloma, lymphoproliferative disease, sarcomas, and occasional non-malignant myelopoiesis have also been identified.^6,12 There is only one reported occurrence of transplantation of occult AML,¹⁵ but there are many instances of probable transplantation of premalignant clones.⁶ Inadvertent transplantation of CML and T cell lymphoma have also occurred.¹⁶ The latency period to develop donor-derived AML/MDS post-allo-HSCT varies from 1 month to 24 years, with a median latency of 28 months.⁶ DDM cases have been reported from all stem cell sources, including bone marrow, peripheral blood, and cord blood units, without clear difference in risk profile.^7,17,18

There is a paucity of data to develop guidelines regarding which patients should be tested for a donor-derived malignancy (versus relapse) and what tests to recommend. The original cases of DDM were indicated by cytogenetic analysis in gender-mismatched allo-HSCT, although this approach does not show definitive cell of origin.⁶ Standard testing after allo-HSCT includes chimerism studies for donor myeloid and T cell engraftment after allo-HSCT in combination with monitoring for disease relapse by NGS.¹⁹ These may suggest malignancy after allo-HSCT of donor origin, although targeted testing for DDM in the bone marrow by DNA chimerism analysis is not performed routinely.

The approach to treatment options for patients with DDM is challenging both from disease and patient perspectives. Regarding the disease itself, it is unclear how to classify and treat this malignancy in the setting of therapy for original disease and allo-HSCT. Equally unclear is whether these malignancies behave as anticipated by their cytogenetic/molecular features, or whether they are more aggressive. In case series, most patients were treated for their DDM with chemotherapy, with nearly half achieving disease remission.^4,7 Patients in remission who proceeded to second allo-HSCT demonstrated >50% survival at median follow up time of 3.7 years.⁷ However, the rest of the patients never achieving remission (18/33) demonstrated poor survival regardless of attempt for second allo-HSCT.⁷

Donor outcomes after DDM

Outcome data on the donors in cases of DDM are limited.²⁰ Within EBMT registry data from 1982–2003, among 14 DDM cases identified, no malignancies within donors were reported.³ A later EBMT survey found 38 DDM cases and reported 25 donors with available follow up data, of which 2 donors (8%) developed AML and 5 (20%) developed chronic leukemias.⁴ Among 163 donor-derived AML and MDS in case reports and series in the literature, five donors developed AML, two developed MDS, and three developed other malignancies, including B-cell lymphoma, breast cancer, and bronchogenic carcinoma.⁶ Donor follow up in the literature remains incomplete and later donor development of HM is likely underestimated.

Patient factors that predispose DDM

The pathogenesis of DDM is poorly understood and likely is impacted by both donor and recipient factors. The contribution of intrinsic patient factors remains speculative and may be inferred from the lower rate of HM after prolonged follow up of donors compared to the recipients in DDM.³ Previous treatment damage to the bone marrow microenvironment of the allo-HSCT recipient likely plays a role in DDM. T-cell depletion seemed associated with increased DDM in one large series, which may be related to impaired immune surveillance by reduced functional T-lymphocytes.⁴ However, another series found no influence of GVHD prophylaxis on DDM incidence.⁷ In one series, twenty percent of donor-derived AML and MDS were characterized by chromosome 7 abnormalities, a karyotypic finding in therapy-related myeloid malignancies⁶. Conditioning modalities may disrupt stromal and endothelial cell function,²¹ creating a selective environment for the outgrowth of cells with pro-leukemia variants.²² Whether the hematopoietic stem and progenitor cell (HSPC) niches are abnormal due to underlying HHMs is not yet known. However evidence from murine models suggests that germline genetics can create an unfavorable microenvironment and may promote leukemogenesis.^23–25 Patients with inherited bone marrow failure syndromes (IBMFS) had worse survival after allo-HSCT compared to patients transplanted for aplastic anemia (HR 2.13, p=0.0004), suggesting that underlying germline hematopoietic disorders may negatively influence allo-HSCT outcomes distinct from DDM.²⁶

Transplantation of germline mutations and predisposition to HHM

Deleterious germline variants in related and unrelated donors have also been linked to DDM in recipients, including: CEBPA²⁷, DDX41²⁸, GATA2²⁹, RUNX1³⁰, XPD³¹, and XRCC1.³¹, CHEK2³², and TERT⁹. Stem cell donors carrying deleterious germline DNA variants have also demonstrated inferior allo-HSCT outcomes distinct from DDM, including poor stem cell mobilization,^9,10 and delayed or failure of engraftment.^10,11

In some cases, poor stem cell mobilization provides the first indication of germline donor abnormality. Unrecognized donor germline TERC mutations, which may mediate late onset dyskeratosis congenita, have been associated with suboptimal stem cell mobilization and delayed engraftment after allo-HSCT.¹⁰ In another study of 328 HLA-matched related donors for allo-HSCT, 28 poor mobilizers (in the lowest quartile for stem cell mobilization) underwent sequencing, with two showing known deleterious variants: the first with a novel germline TERT variant and the second acquired deleterious variants in TET2 and SF3B1.⁹

Among families with inherited RUNX1 variants, patients who received transplants from siblings prior to genetic testing experienced higher allo-HSCT complications including relapse of original disease, failure of engraftment, and death from PTLD.¹¹ Similar outcomes after allo-

HSCT have been reported for families harboring CEPBA mutations, and transplantation from an unknowingly afflicted related donor has resulted in DDM.^27,30

HHM alleles are being recognized in an increasing number of individuals with HMs, and present risk for allo-HSCT complications when transplanted inadvertently from donors^33–35 (Table 1). Paired NGS and skin biopsy samples in 391 adult AML patients showed 13.6% with likely germline pathogenic variants, with CHEK2 and DDX41 most commonly involved.³⁶ Among 404 pairs of MDS patients and their related donors, 7% of these patients shared likely pathogenic variants with their related donor, with presumed germline variants present in another 4% of patients.³³

Table 1:

Select key hereditary predispositions to hematologic malignancies.

Condition	Gene	Inheritance	Age of Onset	Malignancy	Other features
ANKRD26-Related Thrombocytopenia	ANKRD26	AD	Variable	MDS & AML	Thrombocytopenia
CEBPA-Associated Familial AML	CEBPA	AD	Variable	AML
DDX41-Associated Familial MDS and AML	DDX41	AD	Adult	MDS, AML, Lymphoma
RUNX1 Familial Platelet Disorder (FPD)	RUNX1	AD	Childhood through Adult	MDS, AML some ALLs	Thrombocytopenia and qualitative platelet defects
ETV6 Thrombocytopenia and Predisposition to Leukemia	ETV6	AD	Childhood through Adult	B-ALL, MDS, AML	Thrombocytopenia
GATA2 Deficiency	GATA2	AD	Adolescent	MDS, AML	Immunodeficiency, lymphedema, sensorineural hearing loss, extragenital warts
Shwachman-Diamond syndrome	SBDS, DNAJC21, EFL1, SRP54	AR	Childhood most commonly	Bone marrow failure, Aplastic Anemia, MDS, AML	Cytopenias (neutropenia), exocrine pancreas insufficiency

AD autosomal dominant; AR autosomal recessive, AML acute myelogenous leukemia, MDS myelodysplastic syndrome

For the majority of these conditions, inheritance is autosomal dominant, so having one deleterious allele is sufficient to confer risk (Table 1). The germline nature of a DNA variant can be determined by testing of DNA derived from cultured skin fibroblasts (or other tissues considered equivalent to the germline) or by familial segregation. Asymptomatic related potential donors can have targeted genetic testing of the genetic variant of concern using hair bulbs, fingernails, buccal cells, or possibly peripheral blood.^37,38 The high frequency of somatic reversion within hematopoietic tissues makes the use of peripheral blood or bone marrow less desirable, especially in conditions such as germline mutations in SAMD9/SAMD9L in which somatic revision events are common.³⁹ If not conducted at time of tumor analysis, germline genetic testing and counseling should be conducted as early as possible in patients with concern for HHM based on personal or family history, or if there are HHM-associated variants found on tumor sequencing. Ideally, tumor NGS testing should include HHM-associated variants which would signal cascade to germline testing. However, we do not advocate using hematopoietic tumor-based molecular profiling as the sole means of detecting germline genetic disorders, since somatic reversion is common in some of these conditions, such as SAMD9/SAMD9L, resulting in false negative germline genetic testing results in hematopoietic tissues. It may be prudent to test multiple family members simultaneously to minimize the delays to transplant. Importantly, unrelated donors may carry such alleles as well, especially considering the relatively high frequency of deleterious germline variants in CHEK2 and DDX41 in some populations.⁴⁰ The risk of germline syndromes in unrelated donors is thought to be low, although further research is warranted to clarify the incidence.

Investigation on the unique risk profiles of HHM-associated genes continues to reveal more nuances, illustrated by the case of DDX41. In a large study of 1,371 patients with AML or MDS, 13 were found to have a suspicious DDX41 somatic and/or germline mutation with half having both somatic and germline DDX41 variants and about 40% having a germline only variant.⁴¹ These data suggest that if a somatic mutation is present there is an approximately 75% chance the patient also harbors a germline mutation. In contrast, the risk of germline DDX41 in patients without a somatic DDX41 mutation is approximately 2%. Additional data show that when NGS identifies a DDX41 variant in hematologic malignancy in a patient with history of cytopenias and family or personal history of another cancer, virtually all patients have a germline DDX41 pathogenic variant.⁴² These patients also remain at risk for difficult post-transplant courses, as patients with deleterious germline DDX41 variants show higher incidence of severe (grades 3–4) GVHD compared to patients with other HHM variants or those without germline variants.²⁵ DDX41 variants are correlated with late-onset MDS, with increased risk in males and thus would likely preclude consideration as a donor if known prior to allo-HSCT.⁴²

HHMs should be considered in, but not limited to, patients and donors with a family history of HM or bleeding disorder.⁴³ Failure to screen donors could result in adverse transplant complications. Importantly, there may be some deleterious germline variants that are permissive to allo-HSCT, at least in the short term,⁴⁴ whereas others, including those in RUNX1 and CEBPA, may affect early recipient allo-HSCT outcomes.^11,27,30 The process of hematopoietic cell procurement may also confer added risk for donors who have pathogenic mutations linked to HM, as leukemias in related donors with germline mutations have been reported after stem cell mobilization/collection.^27,30 Peripheral blood stem cell collection involves hyperproliferation and expansion of the hematopoietic stem cell pool, which could increase the likelihood of a subsequent HM in those with genetic predisposition. Donors with genetic predispositions should be avoided if possible, however, if there are no other suitable options, it may be prudent to collect HSPCs via bone marrow harvest.

Transplantation of donor clonal hematopoiesis

CH is characterized by the presence of a recurrent somatic mutation in the HSPCs of an individual without HM. CH is classically characterized by the presence of a recurrent somatic mutation of over 2% variant allele frequency (VAF),⁴⁵ in genes associated with myeloid malignancies (e.g., DNMT3A, TET2, ASXL1, etc.).^46,47 The prevalence of CH in healthy populations increases exponentially with age and varies depending on a study’s limit of detection.⁴⁸ For example, in populations over the age of 65, CH occurs in approximately 20% of individuals at a VAF cutoff of 2% and close to 100% at a cutoff of 0.5%.^49–51 While the clinical relevance of very low VAF CH mutations is uncertain, in allo-HSCT where donor HSPCs rapidly expand in the recipient, it may be hypothesized that even very low VAF CH may be clinically relevant. CH is linked to a variety of poor health outcomes, most robustly HM and cardiovascular diseases.^46,52,53 CH can be detected many years, if not decades prior, to the development of HM^52–55 and often defines the dominant clone at progression⁵⁴, suggesting that CH is the origin of many cases of HMs.

As in the general population, CH is common in hematopoietic stem cell donors with frequency increasing with age.^54,56–58 Donors related to patients with myeloid malignancies may have a higher prevalence of CH than donors related to patients with lymphoid malignancies.⁵⁷ When present in the donor, CH usually engrafts in the recipient.^56,57 Donor-derived CH differentially impacts allo-HSCT outcomes and may confer an increased risk of DDM, with numerous case studies reporting CH in donors progressing to DDM in recipients.^57,59 Prospective cohort studies with donor genetic testing prior to transplant have shown inconsistent effects of donor CH on allo-HSCT outcomes, which may be limited by size of cohorts.^57,59,60

When comparing incidence of DDM by donor CH status, patients who underwent allo-HSCT from donors with CH seemed more likely to progress to DDM than from donors without CH (2/82 or 2.4% vs. 0/426 or 0%, respectively, p=0.026).⁵⁷ Although the age of donors with CH is usually higher, there are reports of CH in younger donors (< 50 years old) transplanted into allo-HSCT recipients.⁵⁸ Similar to non-allo-HSCT populations, the transition of CH to DDM is characterized by CH expansion and the acquisition of additional mutations at diagnosis of DDM, supporting a “multiple-hit” hypothesis promoting DDM development.^57,61–63 The relationship between the CH variant or donor VAF and latency to DDM is still largely unknown, but current data suggest that donor CH variants in TP53 and splicing factors (e.g., SF3B1, SRSF2, U2AF1), although infrequent, appear to confer an elevated risk for DDM development.^57,59 Interestingly, in cases where both the recipient and donor progress to malignancy (i.e., DDM in recipient and primary malignancy in the donor), the CH mutations persist in both but follow different evolutionary trajectories and acquire distinct mutations before diagnosis.^58,64–66 Occasionally cases of DDM are diagnosed due to 100% donor chimerism in the absence of apparent donor CH or germline predisposition mutations,^59,62 raising the question of whether CH was below the limit of detection (e.g., VAF < 0.5%), or if there was an unknown germline predisposition or other unknown cause (e.g., cytogenetic abnormality) in the donor.

CH mutations result in preferential expansion of mutated HSPCs leading to differentiation skewing and a proinflammatory profile.^59,67,68 Recipients of donor DNMT3A-CH clones show higher inflammatory cytokines after allo-HSCT.⁵⁹ Patients with DNMT3A-CH also have higher rates of GVHD and reduced relapse, presumably mediated by robust graft-versus-leukemia effect.⁵⁹ In summary, the influence of donor CH varies with mutation involved, and requires further study to weigh the positive and negative effects and determine the impact on allo-HSCT outcomes.

Current state of donor screening and selection for allo-HSCT in the US

With increasing numbers of allo-HSCTs, regulatory agencies, national, and international registries have codified donor evaluation procedures to ensure donor safety in collection of stem cells and to minimize the risk of the product to the recipient. Pretransplant donor assessment is designed to determine eligibility and suitability of the donor, which includes the HLA-match between recipient and donor, to ensure the quality and safety of donation. It is worth mentioning that the donor eligibility and suitability criteria for related donors, who comprise almost half of all donors⁶⁹ are less strict than for unrelated donors and vary significantly between centers.⁷⁰

In order to be eligible for stem cell donation, allo-HSCT donors must meet specific eligibility criteria outlined by the FDA in 21 CFR 1271 Subpart C. Based on FDA guidance, allo-HSCT donors must be screened and tested for relevant communicable disease agents or diseases. This prevention of donor-derived infection transmission underscores the value of preventing transmitting donor diseases to the recipient. Assessing donor genetic risk factors is not yet standard though testing is broadly accessible and costs continue to decrease.

Over the past decade, the use of reduced intensity conditioning regimens has expanded access to allo-HSCT for older recipients or recipients with comorbidities. The increase in older patients undergoing transplant has come with an increase in sibling donor age. Although the NMDP prioritizes younger donors (age under 45)^71–73, the likelihood of finding a match in the registry varies widely based on ethnic background and it is uncertain at present if selection of a younger mismatched unrelated donor is associated with equivalent or better transplant outcomes than a sibling matched donor. Therefore, the best available donor option for many patients remains an older donor such as an HLA-identical sibling, or even a haploidentical relative.

Ethical complexities related to genetic testing in allo-HSCT

Broad implementation of NGS after allo-HSCT can lead to the identification of germline variants in the donor. How best to operationalize and navigate the ethical considerations surrounding germline genetic testing and return of results, while simultaneously preserving the rights and safety of donors remain open questions. Most would agree that it is not ethical to withhold genetic information from the donor that may be potentially relevant to the donor and/or their families, and that disclosing results of germline genetic testing for HM predisposition to donors without prior consent is also unethical. Increased access to direct-to-consumer (e.g., 23andMe, Ancestry.com) and patient initiated genetic testing options have already allowed some transplant recipients to identify their donor or donor family members.⁷⁴ During allo-HSCT donor work-up, donors should be informed of the potential implications of genetic testing to one’s own health and that of their family and the possibility that results could surface during their donor work-up or at some subsequent point in the recipients’ care. Clarifying the donor’s preference for return of this information is needed. Moreover, if upfront screening for germline cancer risk is recommended during donor evaluation, obtaining informed consent from the donor for this testing is necessary. These conversations would best take place as part of the pre-transplant consent process and should cover the complexities of genetic testing briefly reviewed below and the medical, emotional, and financial implications of germline genetic testing to the donor.

Previously, there was no established infrastructure at the NMDP for implementing the return of germline genetic testing results to donors. In January 2023, the NMDP initiated a policy to have donors opt in at work-up if they want to be notified about their genetic changes detected in a recipient after transplantation.⁷⁵ Enacting this policy would ideally include mechanisms to incorporate donors whose stem cells were collected prior to this date as well as to connect donors to genetic counseling once deleterious genetic variants are found in recipient NGS after allo-HSCT. Studies of patients with cancer show that most people are interested in receiving genetic testing results and counseling where there are health implications, supporting the benefit of an opt-in approach for donors.^76–78

When genetic testing is elected, the types of variants and which genes should be tested remain open questions. The American College of Medical Genetics and Genomics (ACMG) has published guidelines for detection and reporting of secondary findings in genomic sequencing, which provides some framework to approach these questions.^79,80 The ACMG’s current list of actionable genes does not include many of the most common and highly penetrant HHM predisposition genes (e.g., RUNX1, GATA2, DDX41) and includes many genes not relevant to allo-HSCT.⁸⁰ Similarly, while it is clear that pathogenic variants in such genes should be reported, more guidance and careful consideration will be needed when it comes to evaluation of variants of uncertain significance (VUSs). One could imagine assembling tiers of germline mutations for both donor selection (e.g., mutations in Tier 1 genes exclude a donor from selection) and tiers of germline mutations for return of results to donors, where the donor may consent to receive the genetic results from some but not all tiers of genes (e.g., Tier 1 genes include those where surveillance has been shown to impact outcome). Important to this discussion is acknowledgement that evidence-based surveillance guidelines for HM predisposition are lacking for many of the germline disorders. Many mutations are likely to require patient-specific deliberation, weighing potential risk against transplant complexities such as the availability of another donor lacking the germline mutation. Donors should also be aware of the distinction between somatic and germline variants, and that germline confirmation may be required in some cases.

There are potential financial implications of genetic testing results for the donors. A recipient’s insurance may not cover the cost of donor testing, and moreover, management and cascade testing for family members will likely fall on the individuals which adds to the complexity. Further, there may be a concern about the misuse of these data and genetic discrimination in the employment and disability/life insurance contexts. Although the Genetic Information Nondiscrimination Act (GINA) prohibits the use of genetic information in workplace employment decisions or to deny medical insurance coverage, GINA’s protections do not apply to life insurance, disability insurance, or long-term care insurance.

The emotional implications of genetic findings to donors should also be discussed routinely in the allo-HSCT process. Counseling requires experienced practitioners who can support the psychological and emotional impacts to “healthy” donors, and a mechanism for cascade testing and surveillance of family members. At a minimum, discussions should cover potential findings and implications to the donor and their family, limitations of genetic testing, and the possibility for future discovery of meaningful information or identification. Special considerations should be applied to cases of cord blood donation or for donors who are minors, especially in cases of identification of risk for adult-onset disease.

Donor related ethical considerations may be even more complicated for pediatric recipients and donors. Although genetic testing of pediatric donors for underlying HHMs may be considered in the best interest of both the donor and the recipient, the donor’s right “to not know” is eliminated when tested as a child. This is especially relevant for adult-onset conditions, such as those associated with deleterious germline DDX41 variants. Risk factors associated with the quality of the graft, such as age, telomere length, and genetic variants may be amplified in pediatric recipients, for whom the duration of anticipated graft function is considerably longer, as the goal of transplantation is to achieve 50–60 or more quality years of life, compared to 20–30 years for an adult recipient.⁸¹ Further, with the increasing use of haplo-identical related donors, often parents of children (with ~30 year difference in age), there is an increased risk of age-related CH which may not be evident for years after allo-HSCT.⁴⁹ It will be important to perform long term studies to evaluate this potential risk and to include this consideration in the choice of donor selection for children undergoing allo-HSCT.

Knowledge Gaps and Future Directions

We are at the beginning of understanding the full impact of genetic factors in allo-HSCT. Currently, screening for germline disorders, CH, and occult malignancy are not routinely incorporated into donor evaluation at most centers. Indeed, there is not yet evidence to standardize donor genetic sequencing prior to allo-HSCT. Family cohorts and cases of DDM after related and unrelated donors indicate that deleterious germline variants carried by donors, in genes such as CEBPA, DDX41, GATA2, and RUNX1, may contribute to poor allo-HSCT outcomes.^10,11 Donors with variants in these genes should be avoided, and are not limited to related donors. In cases where there is a high suspicion of germline predisposition in the patient undergoing allo-HSCT, but when a genetic etiology has not been detected, use of an unrelated donor should be considered.

Acquired somatic variants in DNMT3A, ASXL1, IDH2, and JAK2 led to DDM after inadvertent transplantation from donors, often with additional new variants.⁶ Although studies show high rates of engraftment of donor derived CH^56,65 , there are conflicting results on the impact of age and donor CH on transplant-related outcomes.^57,60 Reports of adverse transplant outcomes associated with donor CH have raised concerns about the use of older donors and sparked debates about whether screening for CH in potential allogeneic donors older than age 50 years is needed, though not yet standard of care. When possible, we favor younger donors who generally have lower rates of CH, though we cannot assume younger donors lack CH.

Going forward, it will be important to study the rates of germline and acquired somatic variants in donors proceeding to allo-HSCT, with respect to donor age and for related and unrelated donor groups. We should prospectively assess the incidence of DDM, and the effects of donor genetic factors for stem cell mobilization, engraftment, and long-term allo-HSCT outcomes. The rate of germline deleterious variants in the unrelated donor population is unknown, though 11–13% of patients with myeloid disease harbor likely pathogenic germline variants.³⁶ We acknowledge that large patient cohorts will be needed to accurately detect these low incidence events, in which different gene abnormalities may mediate unique effects. This highlights the difficulty of obtaining sufficient data to inform policies for individual genetic changes.

If an intended donor is found to have a concerning mutation, patients with limited donor options will be most affected. This may affect minorities underrepresented in the donor registries disproportionately based on few alternative donor options available, and inability to use any family members with shared germline predispositions.^5,82 However, avoiding donors with the most deleterious genetic factors can prevent devastating allo-HSCT outcomes. Characterization of these risks will enable informed donor selection in situations where available donors carry genetic abnormalities. For genetic variants with longer latency, the age of the recipient may be a factor in deciding whether to use a donor with a deleterious germline variant, with more leniency given to older recipients who would be anticipated to have a shorter life expectancy than pediatric patients.

We should carry a high level of suspicion for HHMs for patients undergoing allo-HSCT and pursue further testing in patients with a family history of HM and in those with variants of interest detected on tumor NGS testing. To coordinate expedient allo-HSCT, patient genetic testing for acquired variants, and germline syndromes if suspected, should occur promptly at diagnosis. If deleterious germline variants are found, this testing should cascade quickly to family members under consideration as donors. (Figure 2) We need further information regarding the presence of deleterious genetic mutations in the unrelated donor pool. Most importantly, excellent multidisciplinary care is paramount to ensure efficient timelines of screening and necessary discussions among medical oncologists, genetic counselors, recipients, and potential donors. Consensus and evidence-based screening guidelines are also needed that can be adopted for implementation into routine practice.

Figure 2:

Timeline for genetic testing in hematologic malignancy.

HLA human leukocyte antigen, HM hematologic malignancy, NGS next-generation sequencing Created with BioRender.com.

Highlights:

• Pathogenic variants in donors and recipients may lead to allo-HSCT complications

• Donor-derived malignancy may arise from donor germline variants or CH

• We do not yet have evidence to standardize donor genetic sequencing before allo-HSCT

• Donor variants found after allo-HSCT require ethical disclosure