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•Population-scale genomics (N = 937 939) allowed assembly of a large FVL/PTGM double-heterozygous cohort while mitigating ascertainment bias.•FVL/PTGM DH appeared to be more common than FVL homozygosity and conferred a similar risk of venous thrombosis.

Myeloid neoplasms with erythroid or megakaryocytic differentiation include pure erythroid leukemia, myelodysplastic syndrome with erythroid features, and acute megakaryoblastic leukemia (FAB M7) and are characterized by poor prognosis and limited treatment options. Here, we investigate the drug sensitivity landscape of these rare malignancies. We show that acute myeloid leukemia (AML) cells with erythroid or megakaryocytic differentiation depend on the antiapoptotic protein B-cell lymphoma (BCL)-XL, rather than BCL-2, using combined ex vivo drug sensitivity testing, genetic perturbation, and transcriptomic profiling. High-throughput screening of >500 compounds identified the BCL-XL–selective inhibitor A-1331852 and navitoclax as highly effective against erythroid/megakaryoblastic leukemia cell lines. In contrast, these AML subtypes were resistant to the BCL-2 inhibitor venetoclax, which is used clinically in the treatment of AML. Consistently, genome-scale CRISPR-Cas9 and RNAi screening data demonstrated the striking essentiality of BCL-XL-encoding BCL2L1 but not BCL2 or MCL1, for the survival of erythroid/megakaryoblastic leukemia cell lines. Single-cell and bulk transcriptomics of patient samples with erythroid and megakaryoblastic leukemias identified high BCL2L1 expression compared with other subtypes of AML and other hematological malignancies, where BCL2 and MCL1 were more prominent. BCL-XL inhibition effectively killed blasts in samples from patients with AML with erythroid or megakaryocytic differentiation ex vivo and reduced tumor burden in a mouse erythroleukemia xenograft model. Combining the BCL-XL inhibitor with the JAK inhibitor ruxolitinib showed synergistic and durable responses in cell lines. Our results suggest targeting BCL-XL as a potential therapy option in erythroid/megakaryoblastic leukemias and highlight an AML subgroup with potentially reduced sensitivity to venetoclax-based treatments.

•High-throughput screening of >500 drugs identifies BCL-XL inhibitor sensitivity and venetoclax resistance in erythroid/megakaryocytic AML.•BCL2L1(BCL-XL) is highly expressed in erythroid/megakaryocytic AML and is essential for their survival based on CRISPR screens.

•Blast-specific ex vivo venetoclax sensitivity correlated strongly with treatment responses and predicted longer survival.•VenEx trial showed that ex vivo drug sensitivity is beneficial for selecting patients with R/R or sAML for venetoclax-azacitidine therapy.

Introduction: Clonal hematopoiesis (CH) becomes common with age and can play a role in the pathophysiology of inflammatory diseases by modifying immune cell function. CH is characterized by the presence of somatic mutations in genes such as DNMT3Aand TET2(CH of indeterminate potential, CHIP), and as mosaic chromosomal alterations (mCAs). While CHIP, particularly in the context of TET2mutations, can promote inflammation in cardiovascular diseases, the association between clonal hematopoiesis and autoimmunity has not been comprehensively studied to date. Rheumatoid arthritis (RA) is one of the most common autoimmune diseases primarily affecting joints. Seropositive RA is defined by the presence of anti-cyclic citrullinated peptide antibodies (ACPA) and/or rheumatoid factor (RF), whereas seronegative RA is a more heterogeneous inflammatory disease entity involving patients with clinical RA manifestations in the absence of ACPA and RF. Understanding of the association between CH and inflammatory processes may yield novel insights into prevention and management of autoimmune diseases.

Background:Cartilage-hair hypoplasia (CHH) is an autosomal recessive disorder characterized by short stature, hypotrichosis, combined immunodeficiency, and macrocytic anemia. CHH is caused by germline mutations in the RMRPgene encoding the RNA component of the mitochondrial RNA processing endoribonuclease. CHH is heavily enriched in the Finnish (1 in 23000 births) and the Amish (1 in 1340 births) populations. Transient macrocytic anemia is common in children with CHH, whereas severe hypoplastic macrocytic anemia is only present in 10% of pediatric CHH patients and can be treated with allogeneic hematopoietic stem cell transplantation (HSCT). The mechanism behind hypoplastic anemia in CHH is unknown: while erythroid differentiation from hematopoietic stem cells is impaired in vitroin CHH, immune dysregulation may also play a role in this phenotype.

Characterizing the heterogeneity in human leukemias is critical for understanding mechanisms of disease onset, uncovering biomarkers for disease diagnosis, and guiding research into novel treatment approaches. While extensive research focus has been placed on characterizing the molecular heterogeneity in leukemia by modern 'omics technologies, comparatively less emphasis has been placed on studying the diversity in holistic morphological changes that occur in single cells in the context of disease. Here, we report on a novel approach to understanding and characterizing different hematological malignancies using ghost cytometry, a recently developed high-content flow cytometric method that leverages high-speed, artificial intelligence (AI) driven morphological characterization and analysis of single cells.

Chronic lymphoproliferative disorders of natural killer cells (CLPD-NKs) and T-cell large granular lymphocytic leukemias (T-LGLs) are clonal lymphoproliferations arising from either natural killer cells or cytotoxic T lymphocytes (CTLs). We have investigated for distribution and functional significance of mutations in 50 CLPD-NKs and 120 T-LGL patients by direct sequencing, allele-specific PCR, and microarray analysis. STAT3 gene mutations are present in both T and NK diseases: approximately one-third of patients with each type of disorder convey these mutations. Mutations were found in exons 21 and 20, encoding the Src homology 2 domain. Patients with mutations are characterized by symptomatic disease (75%), history of multiple treatments, and a specific pattern of STAT3 activation and gene deregulation, including increased expression of genes activated by STAT3. Many of these features are also found in patients with wild-type STAT3, indicating that other mechanisms of STAT3 activation can be operative in these chronic lymphoproliferative disorders. Treatment with STAT3 inhibitors, both in wild-type and mutant cases, resulted in accelerated apoptosis. STAT3 mutations are frequent in large granular lymphocytes suggesting a similar molecular dysregulation in malignant chronic expansions of NK and CTL origin. STAT3 mutations may distinguish truly malignant lymphoproliferations involving T and NK cells from reactive expansions.

Background

Kuusanmäki: AbbVie: Research Funding. Heckman: Novartis: Research Funding; Orion Pharma: Research Funding; Celgene/BMS: Research Funding; Oncopeptides: Consultancy, Research Funding; Kronos Bio, Inc.: Research Funding. Ettala: Novartis: Membership on an entity's Board of Directors or advisory committees; Pfizer: Membership on an entity's Board of Directors or advisory committees; Amgen: Other: Conference expensens; Sanofi: Other: Conference expensens. Pyörälä: Pfizer: Honoraria; Novartis: Honoraria; AbbVie: Honoraria. Rimpiläinen: Astellas: Membership on an entity's Board of Directors or advisory committees; AbbVie: Membership on an entity's Board of Directors or advisory committees; Pfizer: Membership on an entity's Board of Directors or advisory committees. Siitonen: amgen: Honoraria; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Brystol Myers Squibb: Consultancy; Janssen-Cilag: Consultancy, Membership on an entity's Board of Directors or advisory committees; abbvie: Membership on an entity's Board of Directors or advisory committees; Pfizer: Consultancy, Membership on an entity's Board of Directors or advisory committees; celgene: Membership on an entity's Board of Directors or advisory committees. Kontro: Astellas: Consultancy, Membership on an entity's Board of Directors or advisory committees; Jazz Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; AbbVie: Membership on an entity's Board of Directors or advisory committees, Research Funding.

Heckman: Celgene: Research Funding; Novartis: Research Funding; Orion Pharma: Research Funding. Porkka:Novartis: Honoraria, Research Funding; Celgene: Honoraria, Research Funding. Wennerberg:Novartis: Research Funding.

Background

Porkka: Novartis: Honoraria, Research Funding; Celgene: Honoraria, Research Funding. Marques Ramos:Novartis: Employment. Pallaud:Novartis: Employment. Aittokallio:Novartis: Research Funding. Wennerberg:Novartis: Research Funding.

The RAS-RAF-MEK-ERK (MAPK) pathway is deregulated in many cancers including acute myeloid leukemia (AML), where it facilitates cancer progression, chemo-resistance and cancer relapse. Therefore, the inhibition of this pathway has been explored for the treatment of AML, especially in the ~10% of AML cases where the leukemic cells carry activating mutations of the NRAS or KRAS oncogenes. However, MEK inhibitors, the best explored class of MAPK inhibitors, have not proven effective in the clinical setting. A novel clinically investigated drug class in this context are the paradox-breaker pan-RAF inhibitors that prevent the reactivation of the MAPK-pathway and have exhibited single agent efficacy in preclinical testing for solid cancers. Therefore, it is relevant to explore the utility of this class of drugs for AML treatment.

Hemminki: Oncos Therapeutics Ltd: Shareholder Other; TILT BioTherapeutics Ltd: Employment, Shareholder, Shareholder Other. Porkka:BMS and Novartis: Honoraria, Research Funding; Pfizer: Research Funding. Mustjoki:Bristol-Myers Squibb: Honoraria, Research Funding; Novartis: Honoraria, Research Funding.

Background:Dasatinib is a 2ndgeneration tyrosine kinase inhibitor (TKI) used in the treatment of chronic myeloid leukemia (CML). Its kinase inhibition profile is broad and includes several kinases important in the immune cell function such as SRC kinases. Furthermore, it is known that dasatinib has immunomodulatory effects in vivo. Recently, we observed that dasatinib induces a rapid and marked mobilization of lymphocytes, which closely follows the drug plasma concentration. The phenomenon is accompanied by an increase of NK-cell cytotoxicity. In addition, we have shown that dasatinib alters T-cell responses long-term favoring Th1 type of responses. Interestingly, the dasatinib induced immune effects have been associated with better treatment responses. We now aimed to characterize the dasatinib-induced antitumor immune responses in a syngeneic murine melanoma model to address whether dasatinib-induced immunoactivation affects tumor growth.

Koskela: Novartis: Honoraria; BMS: Honoraria; Janssen-Cilag: Honoraria. Mufti:Celgene: Consultancy, Research Funding. Mustjoki:Bristol-Myers Squibb: Honoraria, Research Funding; Novartis: Honoraria. Maciejewski:NIH: Research Funding; Aplastic Anemia&MDS International Foundation: Research Funding.

Abstract 1302

T-cell large granular lymphocyte (T-LGL) leukemia is a rare, clonal disease characterized by the expansion of mature CD3+CD8+ cytotoxic T-cells. It is often associated with autoimmune disorders and immune-mediated cytopenias. Our recent findings suggest that up to 40% of T-LGL patients harbor mutations in the STAT3 gene (Koskela et al, NEJM, 2012). In the remaining T-LGL patients, the pathogenetic mutations are not known.To identify additional somatic mutations, we chose two STAT3 mutation negative T-LGL leukemia patients for exome sequencing. CD8+ T-cells were used as test cells and matched CD4+ T-cells as control. The exome was captured with the Nimblegen SeqCap EZ Exome Library v2.0 and the sequencing was performed with the Illumina HiSeq2000 sequencing platform. Candidate somatic mutations were identified with a bioinformatics pipeline consisting of BWA for sequence alignment, Samtools for alignment filtering and Varscan for somatic mutation calling.Index patient 1 was diagnosed with T-LGL leukemia at the age of 70 and a TCR repertoire assay revealed one minor T- cell clone in the leukemic sample (Vβ7.1: 28 %). Exome sequencing revealed 10 nonsynonymous nucleotide variants with p-values lower than 0.01, of which the tumor suppressor gene Protein tyrosine phosphatase (PTP) receptor T (PTPRT) had a variant frequency of 14%. PTPRT has previously been found to reverse Tyr705 phosphorylation on STAT3, a modification associated with STAT3 deactivation. In this novel mutation, a highly conserved hydrophobic valine residue is converted into methionine (V995M). The mutation occurs in the cytoplasmic part of the protein, within the tyrosine-protein phosphatase 1 domain. The PTPRT V995M mutation may thereby affect STAT3 activity by reducing dephosphorylation of Tyr705, thus increasing the expression of STAT3 target genes.Index patient 2 was a 40 year-old male with untreated T-LGL leukemia. A TCR repertoire assay showed one predominant T-cell clone in the leukemic T-cells (Vβ13.2: 70%). Exome sequencing revealed 8 nonsynonymous nucleotide variants with p-values lower than 0.01. The missense mutation K436E in Angiopoietin-2 (ANGPT2), presenting with the lowest somatic p-value (1,06−09) and highest variant frequency (34%), was the most relevant candidate involved in the pathogenesis of leukemia. The mutation occurs on the surface of ANGPT2 within the well-conserved fibrinogen C-terminal domain. This domain binds the receptor TIE2 and the change in the polarity induced by K436E mutation is likely to affect the binding of TIE2 by ANGPT2. Overexpression of ANGPT2 has previously been shown to confer an adverse prognostic factor in other forms of leukemia. While these mutations appear biologically relevant and exciting, we have not yet seen them in other LGL patients screened so far (n=80).Somatic mutations in the PTPRT and ANGPT2 genes may represent rare genetic causes for T-LGL leukemia. Screening for these mutations in a larger cohort of patients is warranted. The mutation in the PTPRT gene is particularly exciting as it may directly impact the STAT3 pathway, which is a common pathogenetic event in T-LGL leukemia. Inactivating mutations of the PTPRT gene may have the same functional consequence as activating mutations of STAT3 in LGL patients.Koskela: Novartis: Honoraria; BMS: Honoraria; Janssen-Cilag: Honoraria. Kallioniemi:TEKES-FiDiPro: Research Funding. Porkka:Bristol-Myers Squibb: Honoraria, Research Funding; Novartis: Honoraria, Research Funding. Maciejewski:NIH: Research Funding; Aplastic Anemia&MDS International Foundation: Research Funding. Mustjoki:Bristol-Myers Squibb: Honoraria, Research Funding; Novartis: Honoraria.

Abstract 646

By affecting not only BCR-ABL1 but also a wide variety of other tyrosine kinases, tyrosine kinase inhibitors (TKIs) have potential impact on the immune defense system. As patients could use TKIs even for decades, immunological off-target effects may be of significance in clinical practice. The aim of this project was to study the effects of TKI therapy on B-cell-mediated immunity in vivo in CML patients.Peripheral blood (PB) and bone marrow (BM) samples were collected from newly diagnosed chronic phase CML patients and follow-up samples were obtained during TKI therapy at 1, 3, 6, and 12 months after therapy start (imatinib n=21, dasatinib n=15 and nilotinib n=7). Samples from healthy volunteers served as controls (n=10). Plasma immunoglobulin (Ig) levels were measured with an immunoturbidometric method and BM and PB lymphocyte subclasses were analyzed with multicolor flow cytometry. Ig heavy chain rearrangement PCR according to BIOMED-2 protocol was used to identify clonal B-cell populations among sorted CD19pos B lymphocytes.At diagnosis, before TKI therapy, plasma Ig levels were similar to healthy controls. During imatinib treatment IgA and IgG levels decreased continuously and were significantly lower at 12 months compared either to diagnostic samples or to healthy controls: median IgA level at 12 months 1.22 vs. 1.51 g/l at diagnosis (p=0.040) or vs. 1.61 g/L in healthy controls (p=0.023); median IgG at 12 months 7.00 vs. 8.80 g/l at diagnosis (p=0.009) or vs. 8.55 g/l in healthy controls (p=0.009). Similarly plasma IgM levels decreased: 0.59 g/ at 12 months vs. 1.06 g/l at diagnosis (p=0.0002), but did not differ significantly from healthy controls (0.75 g/l, p=0.331). Positive correlation was found between plasma IgG and IgM levels at 3 months (r=0.678, p=0.045). One imatinib patient developed severe constant hypogammaglobulinemia, which was first noticed at 3 months, and imatinib was discontinued at 15 months due to recurrent respiratory infections.In dasatinib-treated patients, only plasma IgM levels decreased notably, and at 12 months, they were significantly lower (0.39 g/l) compared to diagnosis (0.86 g/l, p=0.011) or healthy controls (0.75 g/l, p=0.037).No statistically significant changes were observed in nilotinib-treated patients, possibly due to small number of patients.The CD45 expression analyzed by flow cytometry was used to detect different B-cell subclasses (CD45high as mature B-cells, CD45intermediate as maturating B-cells and CD45low as immature B-cells). In imatinib treated patients the proportion of mature B-cells in BM from all CD19+ cells was significantly lower at 3 months than in dasatinib group (55% vs. 84%, p<0.05), whereas the proportion of maturating B-cells was higher (34% vs. 12%, p<0.05). This could be related to the observed low plasma Ig levels in imatinib patients, as only mature B-cells are capable of secreting immunoglobulins. BM B-cell profile of nilotinib patients resembled that of imatinib patients.Although clonal T-cells are found in the majority of CML patients (Kreutzman et al. Blood 2010), Ig heavy chain rearrangement analysis did not reveal any clonal B-cell populations in CML patients at diagnosis (n=2) or during imatinib (n=2) or dasatinib (n=9) treatment. Similarly, no marked abnormalities were observed in serum electrophoresis or immunofixation analysis.Imatinib-treated patients have reduced plasma Ig levels and the proportion of mature immunoglobulin-secreting B-cells was lower in BM. Unexpectedly, dasatinib-induced changes were much less prominent, although dasatinib inhibits a broader spectrum of kinases, some of which are clearly involved in B lymphocyte biology, such as the Lyn-kinase. Further studies are needed to understand which kinase inhibition plays the major role in imatinib-induced reduction of Ig levels. However, the measurement of plasma Ig levels is warranted in patients with recurrent respiratory infections and possibility of hypogammaglobulinemia needs to be taken into account.Koskela: Novartis: Honoraria. Richter:Bristol-Myers Squibb: Honoraria; Novartis: Honoraria. Hjorth-Hansen:Novartis: Honoraria; Bristol-Myers Squibb: Honoraria. Porkka:Novartis: Honoraria; Bristol-Myers Squibb: Honoraria. Mustjoki:Novartis: Honoraria; Bristol-Myers Squibb: Honoraria.

T-cell large granular lymphocyte (LGL) leukemia is an uncommon lymphoproliferative disorder characterized in most cases by expansion of mature, clonal CD3+CD8+ cytotoxic T lymphocytes (CTLs). The pathogenesis of LGL-leukemia is unknown, and leukemic cells closely resemble normal terminally differentiated effector memory CTLs. While resistance to apoptotic pathways (Fas/Fas ligand, sphingolipid) and activation of survival signaling pathways (Ras) have been implicated in LGL leukemia, the underlying genetic defects have not yet been elucidated. We aimed to identify somatic mutations in LGL leukemia by whole exome sequencing of leukemic and matched healthy control cells.Our index patient is a 70 year-old male with untreated CD8+ LGL leukemia diagnosed in 2009 with a clonal rearrangement in the T-cell receptor (TCR) delta and gamma gene. He has been asymptomatic with grade 2 neutropenia and an absolute lymphocyte count of 4–15 ×109/L. The patient had one large predominant T-cell clone: 94% of CD8+ cells consisted of a single Vβ16 clone, as assessed by flow cytometry. No clonal expansions were observed in the CD4+ fraction. DNA was extracted from FACS-sorted CD8+ (leukemic) and CD4+ (control) cells and sequenced by exome capture using an Agilent SureSelect All exon 50 MB capture kit and the Illumina GAII sequencing platform. Candidate somatic mutations were identified with a bioinformatics pipeline consisting of BWA for sequence alignment, Samtools for alignment filtering and Varscan for somatic mutation calling. Mutations were manually reviewed in IGV for alignment artifacts and validated by capillary sequencing.DNA samples from 8 additional untreated LGL-leukemia patients were used for further screening of confirmed somatic mutations by capillary sequencing. From six of these patients DNA was extracted from CD8 sorted cells and from two patients from whole blood.Whole exome sequencing of CD8+ leukemic DNA from the index patient identified a missense mutation in the STAT3 gene (D661V), which was subsequently confirmed by capillary sequencing. As STAT3 signaling has been associated with LGL leukemia pathogenesis previously, we next designed primers for the secondary screening of the six exomes of STAT3 SH2 region from the remaining patients. Another recurrent somatic missense mutation (STAT3 Y640F) was identified in two additional patients. Thus, three out of nine LGL patients (33%) showed evidence of mutations in the STAT3 SH2 region. Both missense mutations found (D661V and Y640F) were located in the area of the SH2 domain known to mediate STAT3 protein dimerization and activation. The Y640F mutation alters a conserved tyrosine residue leading to a hyperactivating STAT protein (Scarzello et al. Mol Biol Cell, 2007) and was recently found in a human inflammatory hepatocellular adenoma causing cytokine-independent tyrosine phosphorylation and activation as well as cytokine-dependent hyperactivation of STAT3 (Pitali et al., J Exp Med, 2011). The D661V mutation has not been described previously.Our data imply for the first time that STAT3 is a common mutational target in LGL leukemia, revealing insights to the molecular pathogenesis of this rare disease. Known structural and functional data on STAT biology imply that the mutations are leading to STAT3 hyperactivation and could also confer ligand-independent signaling. While confirmatory data from a larger series of patients are necessary, our results pinpoint STAT3 mutations and aberrations in the STAT3 pathway as key pathogenetic events in true clonal LGL leukemia. Detection of STAT3 mutations could therefore be applied in the diagnostic assessment, disease stratification and therapeutic monitoring of LGL patients.Koskela: Novartis: Honoraria. Kuittinen:Roche: Consultancy. Porkka:Novartis: Honoraria; Bristol-Myers Squibb: Honoraria. Mustjoki:Novartis: Honoraria; Bristol-Myers Squibb: Honoraria.

Abstract 1699

Abstract 936

Background: In contrast to other human endogenous retrovirus families which are highly defective, HERV-K proviruses possess all viral gene types of exogenous retroviruses. In malignancy (tumor cell lines, breast cancer, melanoma as well as leukemias and lymphomas) but also in many normal tissues, including placenta HERV-K mRNA expression has been detected. In hematopoietic stem cells, an interaction of HERV-K with elements from vectors which are used for gene therapy may lead to unforeseen consequences. The purpose of this study was to localize HERV-K mRNA expression in hematopoietic stem cells of different sources and to evaluate a potential association of HERV-expression with hematologic diseases and to define a possible risk of HERV-expression for transplantation. Methods: Quantitative real time PCR (RTQPCR) was used for detecting HERV-K expression in MNC from blood and/or bone marrow samples of patients with leukemias ( 10 MDS, 16 MPD, 14 AML and 1 CML blast crisis) and MNC from blood (n=9) and lymph nodes (n=3) from lymphoma patients. In addition, 8 blood samples and 2 bone marrow samples from healthy donors as well as 9 cord blood samples and 20 apheresates from lymphoma patients in remission containing PBSC were analyzed. Analysis of the same HERV-K gene (pol) in placenta and maternal blood as well as in genomic DNA from all above mentioned sources served as control. In HERV-K overexpressing samples expression of HERV in specific cells was tested by RNA in situ hybridisation (RISH), and combined staining with a CD34 specific monoclonal antibody. Results: Relative mRNA levels of HERV-K in apheresates as well as in blood and bone marrow samples of patients with MDS, MPD and AML from subtypes with less than 1% CD34 positive cells were in the range of less than 2% in relation to housekeeping genes. However, in bone marrow samples of two AML and a CML blast crisis (all of them with more than 50% CD34+ cells) as well as in CD34+ cells from cord blood (but not from apheresates or normal bone marrow) as well as in three lymph nodes of patients with B-cell lymphomas, an overexpression of HERV-K was detected and confirmed by RISH. Enhanced expression of HERV-K was detected in cord blood derived apoptotic CD34+ cells, whereas the relative amount of HERV in genomic DNA did not change in samples from different sources. Conclusions. Our data indicate that HERV-K may be activated in lymph nodes of lymphomas and in CD34+ cells from bone marrow of leukemia patients and from cord blood but not in stem cell apheresates from peripheral blood. In cord blood and malignancy expression appears to be associated with apoptosis. Similar amounts of HERV-K in different samples of genomic DNA of the same cells and the absence of HERV-K activation in normal bone marrow and PBSC indicate that activation of HERV-K is the result of regulation of gene expression rather than amplification. A possible specific role of HERV in the pathogenesis of hematologic diseases and in hematopoiesis (cord blood) remains to be established. Differences between peripheral blood stem cells and cord blood may support a critical stance regarding the use of cord blood derived stem cells for transplantation and gene therapy.

Background: In contrast to other human endogenous retrovirus families which are highly defective, HERV-K proviruses possess all viral gene types of exogenous retroviruses. In malignancy (tumor cell lines, breast cancer, melanoma as well as leukemias and lymphomas) but also in many normal tissues, including placenta HERV-K mRNA expression has been detected. In hematopoietic stem cells, an interaction of HERV-K with elements from vectors which are used for gene therapy may lead to unforeseen consequences. The purpose of this study was to localize HERV-K mRNA expression in hematopoietic stem cells of different sources and to evaluate a potential association of HERV-expression with hematologic diseases and to define a possible risk of HERV-expression for transplantation.