Evaluation of minimal residual disease by real-time quantitative PCR of Wilms' tumor 1 expression in patients with acute myelogenous leukemia after allogeneic stem cell transplantation: correlation with flow cytometry and chimerism

Biomarkers of Response to Venetoclax Therapy in Acute Myeloid Leukemia

Recent progress in the use of massive sequencing technologies has greatly enhanced our understanding of acute myeloid leukemia (AML) pathology. This knowledge has in turn driven the development of targeted therapies, such as venetoclax, a BCL-2 inhibitor approved for use in combination with azacitidine, decitabine, or low-dose cytarabine for the treatment of newly diagnosed adult patients with AML who are not eligible for intensive chemotherapy. However, a significant number of AML patients still face the challenge of disease relapse. In this review, we will explore biomarkers that may predict disease progression in patients receiving venetoclax-based therapy, considering both clinical factors and genetic changes. Despite the many advances, we conclude that the identification of molecular profiles for AML patients who will respond optimally to venetoclax therapy remains an unmet clinical need.

Prognostic Impact of Copy Number Alterations' Profile and AID/RAG Signatures in Acute Lymphoblastic Leukemia (ALL) with BCR::ABL and without Recurrent Genetic Aberrations (NEG ALL) Treated with Intensive Chemotherapy

Adult acute lymphoblastic leukemia (ALL) is associated with poor outcomes. ALL is initiated by primary aberrations, but secondary genetic lesions are necessary for overt ALL. In this study, we reassessed the value of primary and secondary aberrations in intensively treated ALL patients in relation to mutator enzyme expression. RT-PCR, genomic PCR, and sequencing were applied to evaluate primary aberrations, while qPCR was used to measure the expression of RAG and AID mutator enzymes in 166 adult ALL patients. Secondary copy number alterations (CNA) were studied in 94 cases by MLPA assay. Primary aberrations alone stratified 30% of the patients (27% high-risk, 3% low-risk cases). The remaining 70% intermediate-risk patients included BCR::ABL1pos subgroup and ALL lacking identified genetic markers (NEG ALL). We identified three CNA profiles: high-risk bad-CNA (CNAhigh/IKZF1pos), low-risk good-CNA (all other CNAs), and intermediate-risk CNAneg. Furthermore, based on RAG/AID expression, we report possible mechanisms underlying the CNA profiles associated with poor outcome: AID stratified outcome in CNAneg, which accompanied most likely a particular profile of single nucleotide variations, while RAG in CNApos increased the odds for CNAhigh/IKZF1pos development. Finally, we integrated primary genetic aberrations with CNA to propose a revised risk stratification code, which allowed us to stratify 75% of BCR::ABL1pos and NEG patients.

Measurable Residual Disease (MRD) by Flow Cytometry in Adult B-Acute Lymphoblastic Leukaemia (B-ALL) and Acute Myeloid Leukaemia (AML): Correlation with Molecular MRD Testing and Clinical Outcome at One Year

Measurable residual disease (MRD) detected by flow cytometry (FC) is well established in paediatric B- lymphoblastic leukaemia (B-ALL) and adult chronic lymphocytic leukaemia (CLL), but its utility in adult B-ALL and adult acute myeloid leukaemia (AML) is less clear. In this prospective MRD study, one of the largest in Australia to date, we examined consecutive bone marrow aspirates from adult participants with B-ALL (n = 47) and AML (n = 87) sent for FC-MRD testing at a quaternary referral hospital in Sydney. FC-MRD results were correlated to corresponding Mol-MRD testing where available and clinical outcomes at three-month intervals over 1 year. B-ALL showed a moderate positive correlation (rs = 0.401, p < 0.001), while there was no correlation between FC-MRD and Mol-MRD for AML (rs = 0.13, p = 0.237). Five FC-MRD patterns were identified which had significant associations with relapse (X2(4) = 31.17(4), p > 0.001) and survival (X2(4) = 13.67, p = 0.008) in AML, but not in B-ALL. The three-month MRD results were also strongly associated with survival in AML, while the association in B-ALL was less evident. There was a moderate correlation between FC-MRD and Mol-MRD in B-ALL but not AML. The association of FC-MRD with relapse and survival was stronger in AML than in B-ALL. Overall, these findings suggest divergent utilities of FC-MRD in AML and B-ALL.

Intracranial Myeloid Sarcoma Mimicking Hypertensive Intracerebral Hemorrhage

We herein report a case of intracranial myeloid sarcoma mimicking hypertensive intracerebral hemorrhage. A 71-year-old man with a history of acute myeloid leukemia was admitted with acute-onset dysarthria. A hematoma-like lesion was found on computed tomography in the left putamen. Magnetic resonance imaging (MRI) and cerebrospinal fluid cytology confirmed the diagnosis of intracranial myeloid sarcoma. The patient showed a favorable response to chemotherapy, and follow-up MRI revealed shrinkage of the tumor. Since the computed tomography findings resemble those of intracerebral hemorrhage, it is important to suspect intracranial neoplasm, particularly in cases with a history of hematologic diseases.

Measurable residual disease in adult acute myeloid leukaemia: evaluation of a multidimensional 'radar' flow cytometric plot analysis method

Measurable residual disease (MRD) monitoring in acute myeloid leukaemia (AML) is becoming increasingly important and is predominantly performed by multiparameter flow cytometry (MFC) or quantitative polymerase chain reactions (RT-qPCR). We investigated the use of multidimensional plots (MD-MFC) for AML MRD monitoring in an adult cohort. AML MRD was determined using a novel MD-MFC method for 115 MRD samples. Results were correlated with traditional two-dimensional MFC (2D-MFC) and molecular methods. Using the standard cut-off of 0.1% CD45+ cells, concordance was 99/115 (p=0.332). Eighty-four of 115 were concordant using a very low reporting limit of 0.01% (p=0.216). MRD <0.1% by either method was present in 40 of 115 samples. Fifteen of 40 were MD-MFC positive and 2D-MFC negative. Of these two of 15 had a molecular MRD marker and both were positive. Molecular MRD markers were available in 36 of 115 cases. Twenty-one of 36 (58%) were concordant with MD-MFC. Eight of 36 had detectable molecular MRD only and eight of 36 had positive MD-MFC only. There was no correlation between either the MFC method and the molecular results. In summary, there is good correlation between MD- and 2D-MFC-MRD and no correlation between the MFC and molecular methods.

Pediatric Myeloid Sarcoma, More than Just a Chloroma: A Review of Clinical Presentations, Significance, and Biology

Myeloid sarcomas (MS), commonly referred to as chloromas, are extramedullary tumors of acute myeloid leukemia (AML) with varying incidence and influence on outcomes. Pediatric MS has both a higher incidence and unique clinical presentation, cytogenetic profile, and set of risk factors compared to adult patients. Optimal treatment remains undefined, yet allogeneic hematopoietic stem cell transplantation (allo-HSCT) and epigenetic reprogramming in children are potential therapies. Importantly, the biology of MS development is poorly understood; however, cell-cell interactions, epigenetic dysregulation, cytokine signaling, and angiogenesis all appear to play key roles. This review describes pediatric-specific MS literature and the current state of knowledge about the biological determinants that drive MS development. While the significance of MS remains controversial, the pediatric experience provides an opportunity to investigate mechanisms of disease development to improve patient outcomes. This brings the hope of better understanding MS as a distinct disease entity deserving directed therapeutic approaches.

FLT3-ITD Expression as a Potential Biomarker for the Assessment of Treatment Response in Patients with Acute Myeloid Leukemia

FLT3-internal tandem duplication (ITD) analysis is not typically performed in cDNA samples and is not considered an appropriate marker for monitoring measurable residual disease (MRD). The aims of this study were to compare FLT3-ITD mutation analysis in DNA and cDNA samples at diagnosis and to demonstrate the usefulness of its expression measurement as an MRD marker after allogeneic stem cell transplantation (allo-HSCT) or FLT3 inhibitor (FLT3i) administration. A total of 46 DNA and cDNA diagnosis samples, 102 DNA and cDNA post-allo-HSCT samples from 34 patients and 37 cDNA samples from 7 patients with refractory/relapse AML treated with FLT3i were assessed for the FLT3-ITD mutation through fragment analysis. In terms of sensitivity, the analysis of cDNA was superior to that of DNA, quantifying higher allelic ratio values in most cases at diagnosis, and thus optimizing the detection of minor clones and prognostic classification. Regarding the last sample before post-HSCT relapse, cDNA analysis anticipated relapse in most cases, unlike DNA analyses. With regard to the post-FLT3i follow-up, FLT3-ITD expression was reduced after the first FLT3i cycle when the treatment was effective, whereas it was not reduced in refractory patients. FLT3-ITD expression could be a useful additional biomarker at diagnosis and for the assessment of MRD after allo-HSCT and FLT3i in AML.

Possible prognostic impact of WT1 mRNA expression at day + 30 after haploidentical peripheral blood stem cell transplantation with posttransplant cyclophosphamide for patients with myeloid neoplasm: a multicenter study from the Okayama Hematological Study Group

BACKGROUND

Previous studies have revealed that relapse of myeloid neoplasms after allogeneic hematopoietic stem cell transplantation (allo-HSCT) could be predicted by monitoring Wilms' tumor 1 (WT1) mRNA expression. However, only a few studies have investigated patients who received human leukocyte antigen-haploidentical stem cell transplantation with posttransplant cyclophosphamide (PTCY-haplo). In this study, we investigated the relationship between WT1 mRNA levels and clinical outcomes in the PTCY-haplo group, and compared them with those in the conventional graft-versus-host disease prophylaxis group (conventional group).

METHODS

We retrospectively analyzed 130 patients who received their first allo-HSCT between April 2017 and December 2020, including 26 who received PTCY-haplo.

RESULTS

The WT1 mRNA expression level at day + 30 after allo-HSCT associated with increased risk of 1-year cumulative incidence of relapse (CIR) was ≥ 78 copies/μg RNA in the conventional group (p < 0.01) and ≥ 50 copies/μg RNA in the PTCY-haplo group (p = 0.03).

CONCLUSIONS

The appropriate cutoff level of WT1 mRNA at day + 30 after allo-HSCT for predicting prognosis in patients treated with PTCY-haplo may be < 50 copies/μg RNA.

Prognostic impact of pretransplant measurable residual disease assessed by peripheral blood WT1-mRNA expression in patients with AML and MDS

OBJECTIVE

As peripheral blood (PB) Wilm's Tumor 1 (WT1)-mRNA expression is established as MRD-marker during conventional AML chemotherapy, impact of pretransplant WT1 expression remains unclear. Therefore, we aimed to assess prognostic impact of pretransplant WT1 expression on post-transplant outcome in patients with AML/MDS.

METHODS

In 64 AML/MDS patients, pretransplant WT1 expression was retrospectively analyzed using a standardized assay offering high sensitivity, specificity, and a validated cut-off. Patients were divided into three groups determined by pretransplant remission and WT1 expression. Post-transplant outcome of these groups was compared regarding cumulative incidence of relapse (CIR), relapse-free (RFS), and overall survival (OS).

RESULTS

Pretransplant forty-six patients (72%) showed hematologic remission, including 21 (46%) MRD-negative and 25 (54%) MRD-positive patients indicated by WT1 expression, while 18 refractory patients (28%) showed active disease. Two-year estimates of post-transplant CIR, RFS, and OS were similar in MRD-positive (61%, 37%, 54%) and refractory patients (70%, 26%, 56%), but significantly inferior compared with MRD-negative patients (10%, 89%, 90%). After multivariable adjustment, pretransplant MRD negativity measured by WT1 expression retained its prognostic impact on CIR (P = .008), RFS (P = .005), and OS (P = .049).

CONCLUSIONS

PB WT1 expression represents a useful method to estimate pretransplant MRD, which is highly predictable for post-transplant outcome and may help improving peri-transplant management in AML/MDS patients.

WT1 Expression Levels Combined with Flow Cytometry Blast Counts for Risk Stratification of Acute Myeloid Leukemia and Myelodysplastic Syndromes

Wilm's tumor 1 (WT1), a zinc-finger transcription factor and an epigenetic modifier, is frequently overexpressed in several hematologic disorders and solid tumors, and it has been proposed as diagnostic and prognostic marker of acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS). However, the exact role of WT1 in leukemogenesis and disease progression remains unclear. In this real-world evidence retrospective study, we investigated prognostic role of WT1-mRNA expression levels in AML and MDS patients and correlations with complete blood counts, flow cytometry counts, and molecular features. A total of 71 patients (AML, n = 46; and MDS, n = 25) were included in this study, and WT1 levels were assessed at diagnosis, during treatment and follow-up. We showed that WT1 expression levels were inversely correlated with normal hemopoiesis in both AML and MDS, and positively associated with blast counts. Flow cytometry was more sensitive and specific in distinguishing normal myeloid cells from neoplastic counterpart even just using linear parameters and CD45 expression. Moreover, we showed that a simple integrated approach combining blast counts by flow cytometry, FLT3 mutational status, and WT1 expression levels might be a useful tool for a better prognostic definition in both AML and MDS patients.

The Proportional Relationship Between Pretransplant WT1 mRNA Levels and Risk of Mortality After Allogeneic Hematopoietic Cell Transplantation in Acute Myeloid Leukemia Not in Remission

BACKGROUND

The relationship between the expression levels of Wilms' tumor-1 gene (WT1) mRNA in peripheral blood before allogeneic hematopoietic cell transplantation (allo-HCT) and risk of mortality in acute myeloid leukemia (AML) patients in noncomplete remission (non-CR) remains quite elusive.

METHODS

We retrospectively assessed the impact of the pretransplant WT1 mRNA level on survival after allo-HCT in non-CR AML patients.

RESULTS

A total of 125 AML patients, including 46 non-CR patients (36.8%), were analyzed. On multivariate analysis of non-CR AML patients, WT1 mRNA ≥5000 copies/μg RNA was significantly related to increased risk of mortality (hazard ratio, 2.7; 95% confidence interval, 1.3-5.5; P = 0.008). Furthermore, in the entire cohort, log10-transformed WT1 mRNA before allo-HCT was found to be significantly associated with the increased risk of mortality irrespective of whether the disease status was CR or non-CR, using Akaike's information criterion. As the pretransplant WT1 mRNA level elevated, the hazard ratio of mortality monotonically increased in a nonlinear manner regardless of remission status, suggesting that WT1 mRNA level in peripheral blood might reflect tumor burden.

CONCLUSIONS

This study demonstrated that the pretransplant WT1 mRNA level was a powerful prognostic factor in allo-HCT even for non-CR AML patients, and there may be a WT1 mRNA threshold in non-CR patients for benefiting from allo-HCT.

Lysyl oxidase expression is associated with inferior outcome and Extramedullary disease of acute myeloid leukemia

BACKGROUND

Lysyl oxidase (LOX) has been described as necessary for premetastatic niche formation in epithelium-derived malignancies and its expression level therefore correlates with risk of metastatic disease and overall survival. However, its role in acute myeloid leukemia (AML) has not been sufficiently analyzed.

METHODS

We investigated LOX plasma expression in 683 AML patients (age 17-60 years) treated within the prospective AML2003 trial (NCT00180102). The optimal cut-off LOX value was determined using a minimal-p-value method dichotomizing patients into a LOX-high group (> 109 ng/mL, n = 272, 40%) and a LOX-low group (≤ 109 ng/mL, n = 411, 60%).

RESULTS

Higher LOX expression was associated with lower peripheral white blood cells, lower serum LDH, and a lower frequency of FLT3-ITD and NPM1 mutations at diagnosis. Higher LOX expression was found significantly more frequently in patients with secondary AML and therapy-related AML, in patients with French-American-British M5 subtypes, and in patients with adverse-risk cytogenetics. Comparing patients in the LOX-high group and the LOX-low group revealed a 3-year overall survival (OS) of 47 and 53% (p = 0.022) and 3-year event-free survival (EFS) of 27 and 35% (p = 0.005), respectively. In the LOX-high group significantly more patients had extramedullary AML compared to the LOX-low group (p = 0.037). Combining extramedullary AML and LOX as interacting factors in a multivariate analysis resulted in an independent impact on survival for the LOX-high-extramedullary interaction for OS (HR = 2.25, p = 0.025) and EFS (HR = 2.48, p = 0.008). Furthermore, in patients with extramedullary disease (n = 59) the LOX level predicted survival. Patients within the LOX-low group had an OS of 43% and EFS of 36% as compared to the LOX-high group with an OS of 13% and EFS of 6% (p = 0.002 and p = 0.008, respectively).

CONCLUSION

We hypothesize LOX expression to be a new potential biomarker to predict outcome in AML, specifically in AML subgroups such as the prognostic heterogeneous group of AML patients with extramedullary disease.

TRIAL REGISTRATION

This retrospective study was performed with patient samples registered within the prospective AML2003 trial (NCT00180102). Patients were enrolled between December 2003 and November 2009.

Quantitative chimerism in CD3-negative mononuclear cells predicts prognosis in acute myeloid leukemia patients after hematopoietic stem cell transplantation

Relapse is a major complication of acute myeloid leukemia (AML) after allogeneic hematopoietic stem cell transplantation (SCT). The objective of our study was to evaluate chimerism monitoring on the CD3-negative mononuclear cells by RQ-PCR to predict relapse of patients allografted for AML and to compare its performance with WT1 quantification. A cohort of 100 patients undergoing allogenic SCT for AML was retrospectively analyzed in a single institution. Patients without complete chimerism, defined as less than 0.01% of recipient's DNA in CD3-negative cells, had a significantly higher risk of relapse and a lower overall survival (p < 0.001). An increase in the percentage of recipient DNA in CD3-negative cells was associated with an increased risk of relapse (p < 0.001) but not with overall survival. Comparable performances between monitoring of CD3-negative cell chimerism and WT1 expression to predict relapse was observed up to more than 90 days before hematological relapse, with sensitivity of 82% and 78%, respectively, and specificity of 100% for both approaches. Quantitative specific chimerism of the CD3-negative mononuclear fraction, enriched in blastic cells, is a new and powerful tool for monitoring measurable residual disease and could be used for AML patients without available molecular markers.

Subgroup Analysis Can Optimize the Relapse-Prediction Cutoff Value for WT1 Expression After Allogeneic Hematologic Stem Cell Transplantation in Acute Myeloid Leukemia

High WT1 expression after allogeneic hematologic stem cell transplantation (allo-HSCT) can strongly predict relapse in acute myeloid leukemia (AML). However, the cutoff values obtained have been inconsistent. Precise cutoff values may be optimized through subtype analysis; the RUNX1-RUNX1T1 fusion transcript provides an ideal reference. RUNX1-RUNX1T1 and WT1 transcript levels were simultaneously measured in 1299 bone marrow samples serially collected from 176 t(8;21) AML patients after allo-HSCT. The upper limit of the normal bone marrow WT1 level was 0.6%, which we previously reported to be the cutoff value for significant relapse prediction in AML as a whole. WT1 cutoff values of 0.6%, 1.2%, and 1.8% significantly differentiated patients in relapse after allo-HSCT. Nonetheless, patients with WT1 levels of 0.6% to 1.2% and those with levels of >1.2% and 1.8% after HSCT had rates of cumulative incidence of relapse similar to those with a continuous WT1 level of ≤0.6%, and both were significantly lower than that in patients with a WT1 level of >1.8%. WT1 expression was significantly related to RUNX1-RUNX1T1 transcript levels at WT1 levels of >1.8% but not at levels of 0.6% to 1.2% or >1.2% to 1.8%. Therefore, subgroup analysis can optimize the relapse-prediction cutoff value of WT1 expression. A cutoff level of 1.8% more accurately differentiates t(8;21) AML patients in relapse after allo-HSCT than does a cutoff level of 0.6%.

Extramedullary leukemia behaving as solid cancer: clinical, histologic, and genetic clues to chemoresistance in organ sites

Recent studies of leukemic tumors in individual extramedullary sites showed they adopt the clinical and metastatic behavior of solid cancers originating in those sites. To elucidate features of leukemic tumors that render them resistant to agents effective against marrow leukemia, we analyzed a series of AML breast tumors by histology, immunohistochemistry, and RNA sequencing. Striking histologic similarities to solid cancers were found: a single-filing architectural pattern virtually identical to that of invasive lobular breast carcinoma and dense desmoplastic keloid-like fibrosis similar to colon, gallbladder, and pancreas carcinomas. Sequencing found 2157 genes significantly downregulated in AML breast tumors compared to normal breast. Comparison to triple-negative breast cancer found 859 genes similarly downregulated. At least 30 of these genes have been associated with poor prognosis in breast cancers. Five were reported in AML marrow studies to correlate with poor prognosis. The findings of this pilot study suggest the seed-and-soil interaction recognized in solid cancer growth may help explain how leukemic cells, in some patients, adopt solid tumor behavior in non-marrow sites. Transformed cells that metastasize from tumor to marrow can impart chemoresistance and be an unrecognized cause of treatment failure and death. Further studies comparing leukemic tumor to simultaneous marrow could potentially identify biomarkers that predict extramedullary resistance and lead to new therapeutic targets. Recognizing the potential for leukemia to adopt solid tumor phenotype, and implementation of body scanning and ablative tumor treatment, could decrease the persistently high rates of marrow resistance and treatment failure.

Relapse of Acute Myeloid Leukemia after Allogeneic Stem Cell Transplantation: Prevention, Detection, and Treatment

Acute myeloid leukemia (AML) is a phenotypically and prognostically heterogeneous hematopoietic stem cell disease that may be cured in eligible patients with intensive chemotherapy and/or allogeneic stem cell transplantation (allo-SCT). Tremendous advances in sequencing technologies have revealed a large amount of molecular information which has markedly improved our understanding of the underlying pathophysiology and enables a better classification and risk estimation. Furthermore, with the approval of the FMS-like tyrosine kinase 3 (FLT3) inhibitor Midostaurin a first targeted therapy has been introduced into the first-line therapy of younger patients with FLT3-mutated AML and several other small molecules targeting molecular alterations such as isocitrate dehydrogenase (IDH) mutations or the anti-apoptotic b-cell lymphoma 2 (BCL-2) protein are currently under investigation. Despite these advances, many patients will have to undergo allo-SCT during the course of disease and depending on disease and risk status up to half of them will finally relapse after transplant. Here we review the current knowledge about the molecular landscape of AML and how this can be employed to prevent, detect and treat relapse of AML after allo-SCT.

A 5-day cytoreductive chemotherapy followed by haplo-identical hsct (FA5-BUCY) as a tumor-ablative regimen improved the survival of patients with advanced hematological malignancies

Haplo-HSCT has been used when HLA-matched siblings are not available. Conditioning regimens aim to reduce tumor burden prior to HSCT and provide sufficient immunoablation. We report the outcome of haplo-HSCT in 63 consecutive patients from 2/2013 to 12/2015 (19 females/44 males) with high-risk or relapsed/refractory hematological malignancies (n=29-AML; 8-sAML; 19-ALL; 5-advanced-MDS; 2-CML-BC). Median age was 20 years (range: 1.1-49). Twenty-one patients achieved remission prior to transplant, while 42 did not. Patients received FA5-BUCY, i.e., 5-day salvage chemotherapy (Fludarabine/Ara-C) and conditioning (Busulfan/Cyclophosphamide). GvHD prophylaxis included ATG, CsA, MMF and short-term MTX. All patients received stem cells from bone marrow and peripheral blood, and achieved successful engraftment, except two who died before. With a median follow-up of 269 days (120-1081), 42/63 patients are still alive and disease-free. Two-year OS and RFS were similar in patients not in remission and in those in complete remission (61.3% vs 56.3%, p=0.88; 58.3% vs 56.3%, p=0.991). Non-relapse mortality and relapse incidence were 22.2% and 11.1%, respectively. Severe acute-GvHD occurred in 4/63 patients. Transplant-related mortality was low at day+100 (17.5%) and for the entire study period (20.6%). Unexpectedly, few patients experienced mild-to-moderate toxicity, and main causes of death were infection and GvHD. BM blast counts, age, and donor-recipient gender-pairs did not affect the outcome. Less chemotherapy cycles prior to HSCT might result in more favorable outcome. Thus, haplo-HSCT with FA5-BUCY appears promising for advanced disease, especially when TBI and amsacrine, used for FLAMSA, are not available and in pediatric patients for whom TBI is not recommended.

The progress and current status of immunotherapy in acute myeloid leukemia

Recently, there has been remarkable progress in basic and preclinical studies of acute myeloid leukemia (AML). The improved outcomes of AML can largely be attributed to advances in supportive care and hematopoietic cell transplantation as opposed to conventional chemotherapy. However, as the 5-year survival rate remains low due to a high incidence of relapse, novel and effective treatments are urgently needed. Increasing attention is focusing on identifying suitable immunotherapeutic strategies for AML. Here, we describe the immunological features, mechanisms of immune escape, and recent progress in immunotherapy for AML. Problems encountered in the clinic will also be discussed. Although current outcomes may be limited, ongoing preclinical or clinical efforts are aimed at improving immunotherapy modalities and designing novel therapies, such as vaccines, monoclonal antibody therapy, chimeric antibody receptor-engineered T cells (CAR-T), TCR-engineered T cells (TCR-T), and checkpoint inhibitors, which may provide promising and effective therapies with higher specificity and efficacy for AML.

Quantitative assessment of Wilms tumor 1 expression by real-time quantitative polymerase chain reaction in patients with acute myeloblastic leukemia

BACKGROUND

The Wilms tumor 1 (WT1) gene is originally defined as a tumor suppressor gene and a transcription factor that overexpressed in leukemic cells. It is highly expressed in more than 80% of acute myeloid leukemia (AML) patients, both in bone marrow (BM) and in peripheral blood (PB), and it is used as a powerful and independent marker of minimal residual disease (MRD); we have determined the expression levels of the WT1 by real-time quantitative polymerase chain reaction (RQ-PCR) in PB and BM in 126 newly diagnosed AML patients.

MATERIALS AND METHODS

This study was done in molecular pathology and cancer research center from April 2014 to June 2015, RQ-PCR method was used to determine the WT1 gene expression in BM and/or PB samples from 126 patients of AML, we cloned both WT1 and ABL genes for creating a standard curve, and we calculate copy number of WT1 genes in patients.

RESULTS

A total of 126 AML patients consist of 70 males (55.6%) and 56 females (44.4%), with a median age of 26 years; 104 (81%) patients out of 126 show overexpression of WT1 gene. We also concomitant monitoring of fusion transcripts (PML RARa, AML1-ETO, MLL-MLL, CBFb-MYH11, or DEK-CAN) in our patients, the AML1-ETO group showing remarkably low levels of WT1 compared with other fusion transcript and the CBFB-MYH11 showing high levels of WT1.

CONCLUSION

We conclude that WT1 expression by RQ-PCR in AML patients may be employed as an independent tool to detect MRD in the majority of normal karyotype AML patients.

Impact of Wilms' tumor 1 expression on outcome of patients undergoing allogeneic stem cell transplantation for AML

The monitoring of the minimal residual disease by Wilms' tumor 1 expression (MRD^WT1) is a standardized test, which can be used in over 80% of patients with AML. To investigate the prognostic value of MRD^WT1 in patients undergoing allogeneic stem cell transplantation (allo-SCT) for AML, MRD^WT1 was monitored 3 months after transplantation in 139 patients. MRD^WT1 positivity did not lead to any therapeutic intervention. Median follow-up was 39.3 (6.4-99.8) months. Patients with positive MRD^WT1 at 3 months experienced more often post-transplant relapse (27/30, 90%) than those with negative MRD^WT1 (16/109, 14.7%) (P<0.0001). Similarly, a shorter 3-year event-free survival (EFS) was observed in MRD^WT1-positive patients (10% vs 72.3% in MRD^WT1-negative patients, P<0.0001). The correlation between relapse and MRD^WT1 was stronger in blood than in bone marrow samples. Multivariate analysis confirmed the detrimental role of 3-month positive MRD^WT1 for relapse (hazard ratio (HR): 15.42; 95% confidence interval (CI): 7.53-31.59; P<0.0001) and EFS (HR: 10.71; 95% CI: 5.41-21.21; P<0.0001). Interestingly, 3-month chimerism was less predictive of relapse than positive MRD^WT1. In conclusion, our results demonstrate the usefulness of peripheral blood MRD^WT1 monitoring in identifying very high-risk patients, who could benefit from an early preemptive treatment, and those who do not need such an intervention.

Minimal Residual Disease in Acute Myeloid Leukemia of Adults: Determination, Prognostic Impact and Clinical Applications

Pretreatment assessment of cytogenetic/genetic signature of acute myeloid leukemia (AML) has been consistently shown to play a major prognostic role but also to fail at predicting outcome on individual basis, even in low-risk AML. Therefore, we are in need of further accurate methods to refine the patients’ risk allocation process, distinguishing more adequately those who are likely to recur from those who are not. In this view, there is now evidence that the submicroscopic amounts of leukemic cells (called minimal residual disease, MRD), measured during the course of treatment, indicate the quality of response to therapy. Therefore, MRD might serve as an independent, additional biomarker to help to identify patients at higher risk of relapse. Detection of MRD requires the use of highly sensitive ancillary techniques, such as polymerase chain reaction (PCR) and multiparametric flow cytometry(MPFC). In the present manuscript, we will review the current approaches to investigate MRD and its clinical applications in AML management.

The impact of minimal residual disease prior to unmanipulated haploidentical hematopoietic stem cell transplantation in patients with acute myeloid leukemia in complete remission

The minimal residual disease (MRD) before and after a haploidentical hematopoietic stem cell transplantation (HSCT) of 86 patients with acute myeloid leukemia (AML) in complete remission (CR) was measured using flow cytometry (FCM) and Wilms tumor 1 (WT1). In all, 18 patients met the criteria of pre-MRD + before HSCT. The FCM + (p = .028) and the combinative criteria for positive MRD (MRDco+) (p = .022) post-transplantation were significantly correlated to relapse in univariate analysis. A multivariate analysis showed that only MRDco + post-transplantation was an independent risk factor of leukemia relapse (p = .022, HR = 4.653, 95% CI: 1.249-17.334). A significant difference in the cumulative incidence of relapse between patients with or without post-transplant MRDco + was observed (p < .001). However, the results suggested that the MRD status before transplantation might not affect leukemia relapse of patients with AML in CR after haploidentical HSCT. These types of patients might not need to receive further intensive chemotherapy or active intervention concerning HSCT.

Should minimal residual disease guide therapy in AML?

The prognostic power of minimal residual disease after therapy for acute leukemias is not in question. It is only logical that the finding of leukemic blast cells after therapy predicts for impending relapse or at least the need for additional treatment. Which level of what is called minimal residual disease (MRD) is clinically relevant, however, depends on the efficacy of the initial treatment as well as the treatment strategies available to target MRD. There are a multitude of additional factors that can alter the clinical significance of MRD, including the genotype of the patient's leukemic cells. The fact that methodologies of MRD detection are not standardized and thresholds for defining MRD positivity vary depending upon MRD detection method and the operator's skills or convictions only add to the complexity of MRD interpretation. While enormous efforts are devoted to enhancing the sensitivity of MRD detection, eg, by next-generation sequencing, improvements of methods for detecting MRD per se will not automatically lead to a more reliable estimation of total tumor burden. Most importantly, even the best assay will yield accurate MRD results only if the tissue source for MRD determination is of good quality. Another aspect of potentially crucial importance is the heterogenous distribution of leukemic cells throughout the skeleton after treatment, recently demonstrated for acute myeloid leukemia (AML) by bone marrow imaging. Once technical difficulties of MRD measurement are resolved and better MRD-targeting drugs are developed, we still need to learn about alternate proposed mechanisms to explain MRD-independent prognostication, well described in acute lymphoid leukemia, before MRD can be included routinely in the guidance of therapy in AML.

Introducing minimal residual disease in acute myeloid leukemia

PURPOSE OF REVIEW

Acute myeloid leukemia (AML) is a heterogeneous disease. Detection of minimal residual disease (MRD) has the potential to improve risk stratification, and its routine monitoring may allow timely therapeutic actions such as allogeneic hematopoietic stem cell transplantation. The current review will discuss challenges and available evidence for clinical application of MRD detection in AML management.

RECENT FINDINGS

The heterogeneous nature of AML, variations in genetic aberrations and immunophenotypes among patients and between malignant subclones coexisting within a single patient, is a challenge for the development of a reliable MRD test in AML. MRD value was demonstrated in subtypes of AML in which reliable leukemia-specific genetic marker is present (e.g., core-binding leukemia, AML positive for NPM1 mutation). Multicolor flow cytometry and quantitative PCR monitoring for Wilms tumor 1 gene transcript have also been shown to correlate with disease progression. MRD results should always be interpreted within patient-specific clinical context considering other risk factors and timing of MRD eradication.

SUMMARY

Introduction of MRD testing into routine clinical practice is a challenge in AML. An improvement in laboratory techniques along with identification of additional leukemia-specific markers is required.

An integrated genotyping approach for HLA and other complex genetic systems

Clinical immunogenetics laboratories performing routine sequencing of human leukocyte antigen (HLA) genes in support of hematopoietic cell transplantation are motivated to upgrade to next-generation sequencing (NGS) technology by its potential for cost savings as well as testing accuracy and flexibility. While NGS machines are available and simple to operate, there are few systems available that provide comprehensive sample preparation and data analysis workflows to complete the process. We report on the development and testing of the Integrated Genotyping System (IGS), which has been designed to specifically address the challenges associated with the adoption of NGS in clinical laboratories. To validate the system for a variety of sample DNA sources, we have tested 336 DNA specimens from whole blood, dried blood spots, buccal swabs, and lymphoblastoid cell lines. HLA class I and class II genotypes were derived from amplicon sequencing of HLA-A, -B, -C for exons 1-7 and HLA-DPA1, -DPB1, -DQA1, -DQB1, -DRB1, -DRB3, -DRB4, -DRB5 for exons 1-4. Additionally, to demonstrate the extensibility of the IGS to other genetic loci, KIR haplotyping of 93 samples was carried out in parallel with HLA typing using a workflow based on the HLA system. These results are discussed with respect to their applications in the clinical setting and consequent potential for advancing precision medicine.

A multigene array for measurable residual disease detection in AML patients undergoing SCT

AML is a diagnosis encompassing a diverse group of myeloid malignancies. Heterogeneous genetic etiology, together with the potential for oligoclonality within the individual patient, have made the identification of a single high-sensitivity marker of disease burden challenging. We developed a multiple gene measurable residual disease (MG-MRD) RQ-PCR array for the high-sensitivity detection of AML, retrospectively tested on 74 patients who underwent allo-SCT at the NHLBI in the period 1994-2012. MG-MRD testing on peripheral blood samples prior to transplantation demonstrated excellent concordance with traditional BM-based evaluation and improved risk stratification for post-transplant relapse and OS outcomes. Pre-SCT assessment by MG-MRD predicted all clinical relapses occurring in the first 100 days after allo-SCT compared with 57% sensitivity using WT1 RQ-PCR alone. Nine patients who were negative for WT1 prior to transplantation were correctly reclassified into a high-risk MG-MRD-positive group, associated with 100% post-transplant mortality. This study provides proof of principle that a multiple gene approach may be superior to the use of WT1 expression alone for AML residual disease detection.

Minimal residual disease testing in hematologic malignancies and solid cancer

Minimal residual disease (MRD) assays are of a great value to assess treatment efficacy and may provide prognostic information. This is particularly relevant in the era of targeted therapy where the introduction of MRD monitoring has fundamentally transformed the way in which cancer patients are managed. While MRD guidelines are well-established for chronic myeloid leukemia, acute promyelocytic leukemia and acute lymphoblastic leukemia, areas for continuing development are available. High level of standardization and regular external quality control rounds and recommendations for data interpretation remain essential to improve MRD monitoring. In this review, we describe the different applications of MRD assays in most frequent hematologic malignancies and solid cancer and provide an overview of the strengths and potential weaknesses of each method.

Reprint of: Back to the future! The evolving role of maintenance therapy after hematopoietic stem cell transplantation

Relapse is a devastating event for patients with hematologic cancers treated with hematopoietic stem cell transplantation. In most situations, relapse treatment options are limited. Maintenance therapy offers the possibility of delaying or avoiding disease recurrence, but its role remains unclear in most conditions that we treat with transplantation. Here, Dr. Hourigan presents an overview of minimal residual disease (MRD) measurement in hematologic malignancies and the applicability of MRD-based post-transplantation interventions. Dr. McCarthy reviews current knowledge of maintenance therapy in the autologous transplantation context, with emphasis on immunologic interventions and immune modulation strategies designed to prevent relapse. Dr. de Lima discusses current lines of investigation in disease recurrence prevention after allogeneic transplantation, focusing on acute myeloid leukemia and myelodysplastic syndrome.

Real-time assessment of relapse risk based on the WT1 marker in acute leukemia and myelodysplastic syndrome patients after hematopoietic cell transplantation

Relapse is the major cause of treatment failure after allogeneic hematopoietic cell transplantation (alloHCT) for acute leukemia and myelodysplastic syndrome (MDS). Wilms' tumor Ag (WT1) is overexpressed in the majority of acute leukemia and MDS patients and has been proposed as a universal diagnostic marker for detection of impending relapse. Comprehensive studies have shown that WT1 transcript levels have predictive value in acute leukemia patients in CR after chemotherapy. However, the focus of this study is the period after alloHCT for predicting relapse onset. We analyzed the accumulation of WT1 mRNA transcripts in PB of 82 leukemia and MDS patients and defined specific molecular ratios for relapse prediction. The extensively validated WT1/c-ABL ratio was used to normalize increases in WT1 transcript levels. The observed lead time of crossing or exceeding set WT1 levels is presented along with linear interpolation to estimate the calculated day the WT1 thresholds were crossed. The WT1/c-ABL transcript ratio of 50 or above yielded 100% specificity and 75% sensitivity reliably predicting future relapse with an observed average of 29.4 days (s.d.=19.8) and a calculated average of 63 days (s.d.=29.3) lead time before morphologic confirmation. A lower ratio of 20 or above gave lower specificity, but higher sensitivity (84.8% and 87.5%, respectively) identified more patients who relapsed, at earlier times, providing an earlier warning with actual average lead time of 49.1 days (s.d.=30.8) and calculated average of 78 days (s.d.=28.8). WT1 transcript levels serve as a diagnostic relapse test with greater sensitivity than the morphologic approach used in the clinic as a readout.

Prognostic impact of WT1 expression prior to hematopoietic stem cell transplantation in children with malignant hematological diseases

PURPOSE

Malignant hematological diseases represent the most common pediatric cancer. As they cannot always be cured by chemotherapy alone, leukemia and myelodysplastic syndrome (MDS) are frequent medical indications for hematopoietic stem cell transplantation, yet even this treatment is not capable of preventing relapse for certain. Therefore, molecular markers are used to monitor minimal residual disease (MRD) to be enabled to react early to an impeding relapse. As specific markers are not always available, Wilms' tumor gene 1 (WT1) has been suggested as a universal marker, but has not yet been established clinically.

METHODS

We determined the level of WT1 gene expression in 130 children, adolescents and young adults with malignant hematological diseases prior to transplantation and evaluated its impact on patients' outcome. A real-time quantitative RT-PCR was used for this purpose.

RESULTS

The relationship between a high level of WT1 and the cumulative incidence of relapse, event-free survival and overall survival proved to be highly significant in univariate and multivariate analyses. Forty-eight percent of all patients with high WT1 levels suffered from a relapse, whereas only eight percent showing normal WT1 levels before transplantation relapsed. The most convincing result was found for acute myeloid leukemia (AML) and MDS.

CONCLUSION

We conclude that WT1 expression prior to transplantation qualifies as an independent prognostic factor and should be further evaluated for MRD monitoring. It might especially be useful for patients with AML or MDS missing specific markers.

Peripheral blood WT1 expression predicts relapse in AML patients undergoing allogeneic stem cell transplantation

To evaluate if WT1 expression may predict relapse after allo-SCT, we analyzed WT1 levels on peripheral blood (PB) and bone marrow (BM) before and after allo-SCT in 24 AML patients with WT1 overexpression at diagnosis. Five copies of WT1/ABL × 10⁴ from PB were identified as the threshold value that correlated with relapse after allo-SCT. The same correlation was not identified when WT1 expression was assessed from bone marrow (BM). Eight out of 11 (73%) patients with a pre-allo-SCT PB-WT1 ≥ 5 and 4/13 (31%) patients with a pre-allo-SCT PB-WT1 < 5 relapsed, respectively (P = 0.04). The incidence of relapse was higher in patients with PB-WT1 ≥ 5 measured after allo-SCT, at the 3rd (56% versus 38%; P = 0.43) and at the 6th month (71% versus 20%; P = 0.03). Patients with pretransplant PB-WT1 < 5 had significantly better 2-year OS and LFS than patients with a PB-WT1 ≥ 5 (81% versus 0% and 63% versus 20%) (P = 0.02). Our data suggest the usefulness of WT1 monitoring from PB to predict the relapse in allotransplanted AML patients and to modulate the intensity of conditioning and/or the posttransplant immunosuppression in an attempt to reduce the posttransplant relapse risk.

Comparison of chimerism and minimal residual disease monitoring for relapse prediction after allogeneic stem cell transplantation for adult acute lymphoblastic leukemia

Little data are available on the relative merits of chimerism and minimal residual disease (MRD) monitoring for relapse prediction after allogeneic hematopoietic stem cell transplantation (HCT). We performed a retrospective analysis of serial chimerism assessments in 101 adult HCT recipients with acute lymphoblastic leukemia (ALL) and of serial MRD assessments in a subgroup of 22 patients. All patients had received myeloablative conditioning. The cumulative incidence of relapse was significantly higher in the patients with increasing mixed chimerism (in-MC) compared with those with complete chimerism, low-level MC, and decreasing MC, but the sensitivity of in-MC detection with regard to relapse prediction was only modest. In contrast, MRD assessment was highly sensitive and specific. Patients with MRD positivity after HCT had the highest incidence of relapse among all prognostic groups analyzed. The median time from MRD positivity to relapse was longer than the median time from detection of in-MC, but in some cases in-MC preceded MRD positivity. We conclude that MRD assessment is a powerful prognostic tool that should be included in the routine post-transplantation monitoring of patients with ALL, but chimerism analysis may provide additional information in some cases. Integration of these tools and clinical judgment should allow optimal decision making with regard to post-transplantation therapeutic interventions.

Relapse assessment following allogeneic SCT in patients with MDS and AML

Options to pre-emptively treat impending relapse of myelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML) after allogeneic haematopoietic stem cell transplantation (allo-SCT) continuously increase. In recent years, the spectrum of diagnostic methods and parameters to perform post-transplant monitoring in patients with AML and MDS has grown. Cytomorphology, histomorphology, and chimaerism analysis are the mainstay in any panel of post-transplant monitoring. This may be individually combined with multiparameter flow cytometry (MFC) for the detection of residual cells with a leukaemia phenotype and quantitative real-time polymerase chain reaction (RQ-PCR) to assess gene expression, e.g., of WT1 or the residual mutation load (e.g., in case of an NPM1 mutation). Data evaluating the aforementioned methods alone or in combination are discussed in this review with particular emphasis on data pointing towards their suitability to steer pre-emptive post-transplant interventions such as immunotherapy, chemotherapy or therapy with demethylating agents.

Hypermethylation of the alternative AWT1 promoter in hematological malignancies is a highly specific marker for acute myeloid leukemias despite high expression levels

BACKGROUND

Wilms tumor 1 (WT1) is over-expressed in numerous cancers with respect to normal cells, and has either a tumor suppressor or oncogenic role depending on cellular context. This gene is associated with numerous alternatively spliced transcripts, which initiate from two different unique first exons within the WT1 and the alternative (A)WT1 promoter intervals. Within the hematological system, WT1 expression is restricted to CD34+/CD38- cells and is undetectable after differentiation. Detectable expression of this gene is an excellent marker for minimal residual disease in acute myeloid leukemia (AML), but the underlying epigenetic alterations are unknown.

METHODS

To determine the changes in the underlying epigenetic landscape responsible for this expression, we characterized expression, DNA methylation and histone modification profiles in 28 hematological cancer cell lines and confirmed the methylation signature in 356 cytogenetically well-characterized primary hematological malignancies.

RESULTS

Despite high expression of WT1 and AWT1 transcripts in AML-derived cell lines, we observe robust hypermethylation of the AWT1 promoter and an epigenetic switch from a permissive to repressive chromatin structure between normal cells and AML cell lines. Subsequent methylation analysis in our primary leukemia and lymphoma cohort revealed that the epigenetic signature identified in cell lines is specific to myeloid-lineage malignancies, irrespective of underlying mutational status or translocation. In addition to being a highly specific marker for AML diagnosis (positive predictive value 100%; sensitivity 86.1%; negative predictive value 89.4%), we show that AWT1 hypermethylation also discriminates patients that relapse from those achieving complete remission after hematopoietic stem cell transplantation, with similar efficiency to WT1 expression profiling.

CONCLUSIONS

We describe a methylation signature of the AWT1 promoter CpG island that is a promising marker for classifying myeloid-derived leukemias. In addition AWT1 hypermethylation is ideally suited to monitor the recurrence of disease during remission in patients undergoing allogeneic stem cell transfer.

Back to the future! The evolving role of maintenance therapy after hematopoietic stem cell transplantation

Relapse is a devastating event for patients with hematologic cancers treated with hematopoietic stem cell transplantation. In most situations, relapse treatment options are limited. Maintenance therapy offers the possibility of delaying or avoiding disease recurrence, but its role remains unclear in most conditions that we treat with transplantation. Here, Dr. Hourigan presents an overview of minimal residual disease (MRD) measurement in hematologic malignancies and the applicability of MRD-based post-transplantation interventions. Dr. McCarthy reviews current knowledge of maintenance therapy in the autologous transplantation context, with emphasis on immunologic interventions and immune modulation strategies designed to prevent relapse. Dr. de Lima discusses current lines of investigation in disease recurrence prevention after allogeneic transplantation, focusing on acute myeloid leukemia and myelodysplastic syndrome.

Achieving stringent CR is essential before reduced-intensity conditioning allogeneic hematopoietic cell transplantation in AML

Reduced-intensity conditioning (RIC) allogeneic hematopoietic cell transplantation (allo-HCT) can cure patients with AML in CR. However, relapse after RIC allo-HCT may indicate heterogeneity in the stringency of CR. Strict definition of CR requires no evidence of leukemia by both morphologic and flow cytometric criteria. We re-evaluated 85 AML patients receiving RIC allo-HCT in CR to test if a strict definition of CR had direct implications for the outcome. These patients had leukemia immunophenotype documented at diagnosis and analyzed at allo-HCT. Eight (9.4%) had persistent leukemia by flow cytometric criteria at allo-HCT. The patients with immunophenotypic persistent leukemia had a significantly increased relapse (hazard ratio (HR): 3.7; 95% confidence interval (CI): 1.3-10.3, P=0.01) and decreased survival (HR: 2.9; 95% CI: 1.3-6.4, P<0.01) versus 77 patients in CR by both morphology and flow cytometry. However, the pre-allo-HCT bone marrow (BM) blast count (that is, 0-4%) was not significantly associated with risks of relapse or survival. These data indicate the presence of leukemic cells, but not the BM blast count affects survival. A strict morphologic and clinical lab flow cytometric definition of CR predicts outcomes after RIC allo-HCT, and therefore is critical to achieve at transplantation.

Minimal residual disease in acute myeloid leukaemia

Technological advances in the laboratory have led to substantial improvements in clinical decision making through the introduction of pretreatment prognostic risk stratification factors in acute myeloid leukaemia (AML). Unfortunately, similar progress has not been made in treatment response criteria, with the definition of 'complete remission' in AML largely unchanged for over half a century. Several clinical trials have demonstrated that high-sensitivity measurements of residual disease burden during or after treatment can be performed, that results are predictive for clinical outcome and can be used to improve outcomes by guiding additional therapeutic intervention to patients in clinical complete remission, but at increased relapse risk. We review these recent trials, the characteristics and challenges of the modalities currently used to detect minimal residual disease (MRD), and outline opportunities to both refine detection and improve clinical use of MRD measurements. MRD measurement is already the standard of care in other myeloid malignancies, such as chronic myelogenous leukaemia and acute promyelocytic leukaemia (APL). It is our belief that response criteria for non-APL AML should be updated to include assessment for molecular complete remission and recommendations for post-consolidation surveillance should include regular monitoring for molecular relapse as standard of care.

Minimal residual disease detectable by quantitative assessment of WT1 gene before allogeneic stem cell transplantation in patients in first remission of acute myeloid leukemia has an impact on their future prognosis

Overall 42 patients (pts) transplanted in hematological CR1 were retrospectively analyzed. Median follow-up was 15 months (range 2-77). The expression of WT1 gene was measured according to the European Leukaemia Net recommendations. At the time of allogeneic stem cell transplantation (allo-SCT) 29 pts were WT1-negative and 13 pts were WT1-positive. In the univariate analysis, significantly better results were observed in the group of WT1 neg in terms of progression-free survival (in three yr 77% vs. 27%, p = 0.001). In multivariate analysis, the only significant feature in terms of better OS was WT1 negativity (p = 0.029). Our results show that minimal residual disease status measured by quantitative assessment of WT1 gene in acute myeloid leukemia pts in CR1 significantly affects their future prognosis after allo-SCT.

Extramedullary relapse of acute myeloid leukemia after allogeneic hematopoietic stem cell transplantation: an easily overlooked but significant pattern of relapse

Acute myeloid leukemia may manifest as myeloid sarcoma in a variety of extramedullary (EM) tissues at diagnosis or at relapse. Although EM relapse after allogeneic hematopoietic stem cell transplantation (alloSCT) has been considered to be rare, recent studies have suggested that it occurs in 5% to 12% of patients who receive alloSCT, accounting for 7% to 46% of total relapses. The incidence of EM relapse after immunomodulation (eg, donor lymphocyte infusion) or a second SCT is even higher. Moreover, patients with EM relapse are more likely to have had preceding acute graft-versus-host disease or chronic graft-versus-host disease relative to those with bone marrow relapse. Collectively, these observations suggest that the preferential occurrence of the graft-versus-leukemia effect underlies the pathogenesis of EM relapse. Establishing an early diagnosis of EM relapse has been challenging because of the immense diversity in the relapse sites; however, recent studies have suggested the usefulness of (18)F-fluorodeoxyglucose positron emission tomography scans in the detection of EM relapse. As a treatment for EM relapse, a combination of local and systemic therapy should be considered, because local therapy alone often results in subsequent systemic relapse. The prognosis for patients who develop EM relapse after SCT remains poor but is slightly better than that after bone marrow relapse. In addition to an early diagnosis with new modalities, clinical studies using new agents that may offer systemic activity while preserving the graft-versus-leukemia effect are warranted as part of an effort to improve the clinical outcome.