PURPOSE:Fanconi anemia (FA) is a genetic disorder typically characterized by progressive bone marrow failure (BMF) during childhood, leading to diagnosis at that stage. In adolescence or adulthood, patients are predisposed to myelodysplastic syndrome (MDS), acute myeloid leukemia, and solid tumors. However, some individuals present atypically, delaying FA recognition and resulting in life-threatening complications. This study describes the distinctive phenotype associated with biallelic FANCM pathogenic variants. PATIENTS AND METHODS:Clinical and biologic data were analyzed from eight patients carrying biallelic germline FANCM pathogenic variants within a French cohort of 411 patients with FA (2.0%). Clinical outcomes were compared with those of patients with FA carrying non-FANCM variants. RESULTS:None of the eight FANCM patients developed BMF, contrasting with a 93.5% cumulative incidence among other FA genotypes (P < .0001). This absence of marrow failure resulted in delayed FA diagnosis (median age 23.5 v 6.8 years, P < .01). Instead, six patients initially presented with malignancy and exhibited marked toxicity to conventional cancer therapies, prompting FA testing. Malignancies included four oral cancers and, unexpectedly, two ALL: a ZNF384-rearranged B-cell precursor ALL and a BCL11B::HOXA13 early T-cell precursor ALL. No ALL cases occurred among the 403 non-FANCM patients with FA (P < .0001). The treatment courses of the two FANCM-related ALL cases are reported. CONCLUSION:Biallelic FANCM variants define a distinct FA subtype lacking early BMF, leading to missed diagnoses and severe toxicity upon malignancy. Recognizing this presentation is crucial for timely FA detection and for implementing adapted therapeutic and follow-up strategies.

ETV6::RUNX1-like ALL is defined by a gene expression signature similar to that of ETV6::RUNX1-positive ALL and absence of all genetic subtype-defining aberrations, including the ETV6::RUNX1 fusion. Within the International BFM Study Group, we assembled and analyzed a cohort of 100 patients (including 97 children) with ETV6::RUNX1-like ALL. We describe their diverse genetic landscape, centered around ETV6 aberrations with frequent IKZF1 disruptions, as previously shown, but including various rare non-ETV6/non-IKZF1 gene fusions, and rearrangements of CRLF2 (CRLF2r). We show that ETV6 and IKZF1 aberrations do not occur exclusively in this subtype, which hampers its classification based solely on genomic data. We confirm our previous observation of a strong association of the CD27-positive/CD44low-negative immunophenotype with ETV6::RUNX1(-like) subtype. Compared to ETV6::RUNX1-positive ALL, patients with ETV6::RUNX1-like ALL are younger, have higher white blood cell counts at diagnosis, and have an inferior early treatment response. While overall survival is comparable, event-free survival is significantly lower in patients with ETV6::RUNX1-like ALL, with NCI risk, early treatment response, IKZF1 deletions, CRLF2r, and JAK2 mutations having prognostic relevance. Notably, Down syndrome is highly prevalent and associated with a worse outcome in ETV6::RUNX1-like ALL. In conclusion, we provide biological, demographic, and clinical characteristics of the largest ETV6::RUNX1-like cohort presented to date.

Tisagenlecleucel (tisa-cel), an autologous anti-CD19 CAR T-cell therapy, has significantly improved outcomes in pediatric, adolescents and young adults with relapsed/refractory B-cell precursor acute lymphoblastic leukemia (R/R BCP-ALL). However, 30-50% experience early failure or relapse. We retrospectively analyzed 52 cases of early failures (n = 13) or relapses (n = 39), evaluating post-tisa-cel outcomes and prognostic factors. CD19 antigen loss was the only factor associated with a lower complete remission rate after salvage therapy (OR = 0.16, 95%CI [0.03-0.90], p = 0.04). Median overall survival (OS) was 14.5 months, with a 2-year OS of 37.4% (95%CI [24.4-50.4]), similar between early failure (38.5%) and relapse (37.0%) groups (p = 0.78). Patients with measurable residual disease only at salvage initiation had significantly improved 2-year OS (61.9%, (95%CI [38.1-78.8])) compared to those with overt disease (20.7%, (95%CI [8.4-36.7], p = 0.008)). Factors associated with inferior OS included high pre-infusion tumor burden (HR = 3.57, p < 0.01), and prior inotuzumab ozogamicin exposure (HR = 3.81, p < 0.01). Although salvage therapies and hematopoietic stem cell transplantation benefit some patients, 5 of 18 transplanted patients died from treatment-related toxicity, underscoring the significant associated risks. These findings highlight the poor prognosis of tisa-cel failures and the urgent need for novel strategies.

Abstract LZTR1 negatively regulates RAS family proteins via proteasomal degradation. Germline loss-of-function variants cause Noonan syndrome, with emerging evidence implicating LZTR1 in predisposition to childhood acute lymphoblastic leukemia (ALL), though its role in hematopoiesis remains poorly defined. Screening 1,587 children with ALL identified LZTR1 variants in 44 patients (2.8%). Germline variants were detected in 32 patients (2.0%), a frequency comparable to that observed in the general population (1.75%; 1,925/110,017; p=0.50). Somatic LZTR1 alterations were identified in 22 patients (1.4%) and were predominantly bi-allelic, arising through either a germline-plus-somatic or dual somatic configuration. They persisted at relapse. Despite enrichment in favorable-risk subtypes ( ETV6::RUNX1 , high-hyperdiploid, ERG/DUX4), bi-allelic LZTR1 -mutated cases showed delayed minimal residual disease clearance and higher late relapse risk, identifying a subgroup unsuitable for treatment de-escalation. LZTR1 expression was increased in most wild-type leukemias, consistent with a compensatory response to aberrant RAS pathway activation. Bi-allelic LZTR1 inactivation abolished RAS regulation, leading to deregulated canonical RAS expression and ectopic expression of the non-canonical RIT1 protein, whose involvement in ALL has not previously been reported. These findings establish LZTR1 as a classical tumor suppressor in ALL via a two-hit model. Monoallelic alterations show insufficient signaling perturbation and low germline penetrance, whereas bi-allelic inactivation acts as a driver event linked to a high risk of late relapse despite favorable genomics.

Measurable residual disease (MRD) is a key prognostic factor in pediatric acute lymphoblastic leukemia (ALL), but current gold-standard methods based on immunoglobulin/T-cell receptor (IG/TCR) rearrangements are complex and not informative in a subset of patients. Genomic breakpoints of oncogenic fusions (GFBs) are stable, biologically grounded markers that may overcome these limitations and improve MRD assessment. In a cohort of 403 patients with known or suspected fusions, a short-read, capture-based next-generation sequencing strategy coupled with an open-source pipeline identified patient-specific GFBs in 97% of cases. GFB-based MRD assays implemented by qPCR or ddPCR showed very high specificity and allowed lower detection thresholds, resulting in improved sensitivity relative to IG/TCR-based assays. Longitudinal monitoring in 104 patients across multiple pediatric ALL fusion subtypes demonstrated excellent overall concordance between methods in non-BCR::ABL ALL, while revealing fusion-dependent differences. GFB-based MRD was particularly informative in ETV6::RUNX1- and MEF2D-rearranged ALL, where IG/TCR tracking is frequently suboptimal or unreliable due to ongoing rearrangements or the absence of VDJ recombination, respectively. Altogether, this large-scale study establishes GFBs as robust, clinically implementable MRD markers and supports their integration into clinical strategies, provided that subtype-specific biology is considered when selecting markers and interpreting results.

ABSTRACT:Juvenile myelomonocytic leukemia (JMML) is a rare, aggressive pediatric myeloproliferative neoplasm for which hematopoietic stem cell transplantation (HSCT) is currently the only established curative therapy. However, a watch-and-wait (W&W) approach has shown promise for long-term survival in selected cases. In this real-world study, we analyzed outcomes of patients with JMML initially managed with a W&W strategy within a nationwide cohort of 161 genetically characterized cases. W&W was chosen for 35 patients, with increasing adoption over time, reaching 39% in the 2016-to-2021 period. Most patients carried mutations in CBL (43%), NRAS (34%), or homozygous germ line SH2B3 (14%). Over a median follow-up of 6.5 years, 30 of 35 (86%) achieved long-term survival with partial or complete resolution of myeloproliferative symptoms, although clonal hematopoiesis persisted in nearly all survivors (18/20). Disease progression occurred in 5 patients (CBL, n = 3; NRAS, n = 1; PTPN11, n = 1), mostly within 2 years after diagnosis. Overall, in the W&W cohort, the 5-year overall and event-free survivals were 93.1% and 84.5%, respectively. In NRAS-mutated cases, age of <30 months, normal to slightly elevated fetal hemoglobin, platelet counts of >45 × 109/L, the absence of additional somatic mutations, and low DNA methylation profile were associated with favorable outcomes. In CBL-driven JMML, no predictive factor of adverse evolution was identified. Notably, W&W was effective in all patients with homozygous germ line SH2B3. These findings support W&W as a viable alternative in up to 30% of patients with JMML, potentially sparing them from HSCT-associated risks. Given the persistence of clonal hematopoiesis and the risk of extrahematological complications, long-term monitoring remains essential.

Cup-like nuclei are a distinctive morphological feature observed in certain cases of acute lymphoblastic leukemia (ALL). We provide evidence that they characterize DUX4/ERG ALL independently of IKZF1 deletion and reveal marked mitochondrial accumulation in this ALL subset.

Juvenile myelomonocytic leukemia (JMML) is an aggressive clonal myeloproliferative neoplasm that affects infants and young children. The narrow window of onset suggests that age-related factors are involved in leukemogenesis. To investigate whether ontogeny-related features are involved in JMML oncogenesis, we compared the gene expression profile of hematopoietic progenitor cells isolated from JMML patients with that of healthy individuals at different stages of ontogeny. This analysis identified two main groups of JMML patients. In the first group, JMML progenitors exhibited a gene expression profile similar to that of embryo-fetal progenitors. Progenitors showed a strong monocytic identity as evidenced by the overexpression of monocytic/dendritic, inflammasome, and innate immune markers. This resembled the monocyte-predominant myelopoiesis characteristic of normal fetal hematopoiesis. However, in the second group, despite evidence of developmental dysregulation as indicated by the aberrant signature of the master oncofetal regulator LIN28B, JMML clustered separately from healthy prenatal and postnatal fractions. These findings highlight the intricate relationship between JMML and development, which will help inform future therapeutic approaches for this rare but severe form of leukemia.

T cell acute lymphoblastic leukemia (T-ALL) is an aggressive malignancy characterized by high rates of induction failure and relapse, and effective targeted immunotherapies are lacking. Despite promising clinical progress with genome-edited CD7-directed CAR-T cells, which present significant logistical and regulatory issues, CAR-T cell therapy in T-ALL remains challenging due to the shared antigen expression between malignant and healthy T cells. This can result in CAR-T cell fratricide, T cell aplasia, and the potential for blast contamination during CAR-T cell manufacturing. Recently described CAR-T cells target non-pan-T antigens, absent on healthy T cells but expressed on specific T-ALL subsets. These antigens include CD1a (NCT05679895), which is expressed in cortical T-ALL, and CCR9. We show that CCR9 is expressed on >70% of T-ALL patients (132/180) and is maintained at relapse, with a safe expression profile in healthy hematopoietic and non-hematopoietic tissues. Further analyses showed that dual targeting of CCR9 and CD1a could benefit T-ALL patients with a greater blast coverage than single CAR-T cell treatments. We therefore developed, characterized, and preclinically validated a novel humanized CCR9-specific CAR with robust and specific antileukemic activity as a monotherapy in vitro and in vivo against cell lines, primary T-ALL samples, and patient-derived xenografts. Importantly, CCR9/CD1a dual-targeting CAR-T cells showed higher efficacy than single-targeting CAR-T cells, particularly in T-ALL cases with phenotypically heterogeneous leukemic populations. Dual CD1a/CCR9 CAR-T therapy may prevent T cell aplasia and obviate the need for allogeneic transplantation and regulatory-challenging genome engineering approaches in T-ALL.

Genetic alterations are the cornerstone of risk stratification in B-cell precursor acute lymphoblastic leukemia (BCP-ALL), and their accurate identification is critical for optimal treatment. Most cases with ABL-class fusion are classified as high-risk yet display good responses to tyrosine kinase inhibitors (TKIs). Current clinical protocols recommend adding a TKI to chemotherapy as soon as possible, making it mandatory to rapidly identify these alterations. We investigated here whether the identification of immunophenotypic features associated with these molecular alterations could be a valuable screening tool. CD36 expression was shown to be a characteristic feature of ABL- or JAK-class kinase fusions. The main genetic subgroups clustering in the subset with Philadelphia (Ph)-like features were also found to display specific immunophenotypic characteristics. A predictive multiparameter scoring system was generated, segregating genetic subtypes with aberrant kinase activation (PAX5/CRLF2alt, BCR::ABL1, ABL/JAK-class). The most robust markers identified were the TSLPR with CD19/22/9/38/81/304 and CD49f. As TKI adjunction is currently limited to the ABL-class kinase fusions, immunophenotypes distinguishing ABL from JAK-class were also investigated. The flow cytometry method reported here, accessible to most hematology departments, is thus a new useful tool to quickly screen for Ph-like kinase fusion with a good sensitivity (95%) and specificity (96%).

Pediatric Blood & CancerVolume 71, Issue 3 e30842 LETTER TO THE EDITOR A PAX5 P80R pediatric B acute lymphoblastic leukemia with monocytic lineage switch at diagnosis: Deciphering classification ambiguity Marion Strullu, Marion Strullu Pediatric Hematology and Immunology Department, Robert-Debré Hospital, APHP, Paris, France University of Paris Cité, Paris, France Inserm U1131, IUH, Paris, FranceSearch for more papers by this authorAurélie Caye-Eude, Aurélie Caye-Eude Inserm U1131, IUH, Paris, France Genetic Department, Robert-Debré Hospital, APHP, Paris, FranceSearch for more papers by this authorOdile Fenneteau, Odile Fenneteau Hematology Laboratory, Robert-Debré Hospital, Assistance Publique-Hôpitaux de Paris (APHP), Paris, FranceSearch for more papers by this authorChloé Arfeuille, Chloé Arfeuille University of Paris Cité, Paris, France Inserm U1131, IUH, Paris, France Genetic Department, Robert-Debré Hospital, APHP, Paris, FranceSearch for more papers by this authorWendy Cuccuini, Wendy Cuccuini Hematology Laboratory, Saint-Louis Hospital, APHP, Paris, FranceSearch for more papers by this authorHélène Cavé, Hélène Cavé University of Paris Cité, Paris, France Inserm U1131, IUH, Paris, France Genetic Department, Robert-Debré Hospital, APHP, Paris, FranceSearch for more papers by this authorAndré Baruchel, André Baruchel Pediatric Hematology and Immunology Department, Robert-Debré Hospital, APHP, Paris, France University of Paris Cité, Paris, France URP-3518, Institut de Recherche Saint-Louis, Université Paris Cité, Paris, FranceSearch for more papers by this authorElodie Lainey, Corresponding Author Elodie Lainey [email protected] orcid.org/0000-0002-0224-4338 University of Paris Cité, Paris, France Inserm U1131, IUH, Paris, France Hematology Laboratory, Robert-Debré Hospital, Assistance Publique-Hôpitaux de Paris (APHP), Paris, France Correspondence Elodie Lainey, Service d'Hématologie Biologique, Hôpital Robert Debré, 48, Boulevard Sérurier, 75019 Paris, France. Email: [email protected]Search for more papers by this author Marion Strullu, Marion Strullu Pediatric Hematology and Immunology Department, Robert-Debré Hospital, APHP, Paris, France University of Paris Cité, Paris, France Inserm U1131, IUH, Paris, FranceSearch for more papers by this authorAurélie Caye-Eude, Aurélie Caye-Eude Inserm U1131, IUH, Paris, France Genetic Department, Robert-Debré Hospital, APHP, Paris, FranceSearch for more papers by this authorOdile Fenneteau, Odile Fenneteau Hematology Laboratory, Robert-Debré Hospital, Assistance Publique-Hôpitaux de Paris (APHP), Paris, FranceSearch for more papers by this authorChloé Arfeuille, Chloé Arfeuille University of Paris Cité, Paris, France Inserm U1131, IUH, Paris, France Genetic Department, Robert-Debré Hospital, APHP, Paris, FranceSearch for more papers by this authorWendy Cuccuini, Wendy Cuccuini Hematology Laboratory, Saint-Louis Hospital, APHP, Paris, FranceSearch for more papers by this authorHélène Cavé, Hélène Cavé University of Paris Cité, Paris, France Inserm U1131, IUH, Paris, France Genetic Department, Robert-Debré Hospital, APHP, Paris, FranceSearch for more papers by this authorAndré Baruchel, André Baruchel Pediatric Hematology and Immunology Department, Robert-Debré Hospital, APHP, Paris, France University of Paris Cité, Paris, France URP-3518, Institut de Recherche Saint-Louis, Université Paris Cité, Paris, FranceSearch for more papers by this authorElodie Lainey, Corresponding Author Elodie Lainey [email protected] orcid.org/0000-0002-0224-4338 University of Paris Cité, Paris, France Inserm U1131, IUH, Paris, France Hematology Laboratory, Robert-Debré Hospital, Assistance Publique-Hôpitaux de Paris (APHP), Paris, France Correspondence Elodie Lainey, Service d'Hématologie Biologique, Hôpital Robert Debré, 48, Boulevard Sérurier, 75019 Paris, France. Email: [email protected]Search for more papers by this author First published: 08 January 2024 https://doi.org/10.1002/pbc.30842Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat REFERENCES 1Khoury JD, Solary E, Abla O, et al. The 5th edition of the World Health Organization classification of haematolymphoid tumours: myeloid and histiocytic/dendritic neoplasms. 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T cell acute lymphoblastic leukemia (T-ALL) is an aggressive malignancy characterized by high rates of induction failure and relapse, and effective targeted immunotherapies are lacking. Despite promising clinical progress with genome-edited CD7-directed CAR-T cells, which present significant logistical and regulatory issues, CAR-T cell therapy in T-ALL remains challenging due to the shared antigen expression between malignant and healthy T cells. This can result in CAR-T cell fratricide, T cell aplasia, and the potential for blast contamination during CAR-T cell manufacturing. Recently, CAR-T cells have been described that target non-pan-T antigens, absent on healthy T cells but expressed on specific T-ALL subsets. These antigens include CD1a ([NCT05679895][1]), which is expressed in cortical T-ALL, and CCR9. We show that CCR9 is expressed on >70% of T-ALL patients (132/180) and is maintained at relapse, with a safe expression profile in healthy hematopoietic and non-hematopoietic tissues. Further analyses showed that dual targeting of CCR9 and CD1a could benefit ~86% of patients with T-ALL, with a greater blast coverage than single CAR-T cell treatments. We therefore developed, characterized, and preclinically validated a novel humanized CCR9-specific CAR with robust and specific antileukemic activity as a monotherapy in vitro and in vivo against cell lines, primary T-ALL samples, and patient-derived xenografts. Importantly, CCR9/CD1a dual-targeting CAR-T cells showed higher efficacy than single-targeting CAR-T cells, particularly in T-ALL cases with phenotypically heterogeneous leukemic populations. Dual CCR9/CD1a CAR-T therapy may prevent T cell aplasia and obviate the need for allogeneic transplantation and regulatory-challenging genome engineering approaches in T-ALL. ### Competing Interest Statement PM is a cofounder of OneChain Immunotherapeutics, a spin-off company from the Josep Carreras Leukemia Research Institute which has licensed the CCR9 binder (PCT/EP2024/053734). The remaining authors report no conflicts of interest in this work. [1]: /lookup/external-ref?link_type=CLINTRIALGOV&access_num=NCT05679895&atom=%2Fbiorxiv%2Fearly%2F2024%2F09%2F03%2F2024.09.02.610843.atom

CAALL-F01 is a French prospective multicentric cohort study focused on children and adolescents, aged 1-17 years, with acute lymphoblastic leukemia (ALL). Patients with BCP-ALL were initially stratified based on age, white blood cell count, central nervous system (CNS) and/or testis involvement, genetics, and response to a prednisone prephase (PP). This stratification defined three risk groups: standard (B-SR), medium (B-MR), and high risk (B-HR). T-cell ALL patients were stratified into two groups, T-SR and T-HR, depending on CNS status and response to PP. As asparaginase is a major drug for childhood ALL and the pegylated form, pegaspargase (PEG), was not registered in the EU, a two-parallel-arm randomized clinical trial (RCT) was conceived and embedded in the cohort study. Randomization occurred before the first PEG infusion (D12), in the B-SR, B-MR, and T-SR groups. Patients were allocated (1:1) to either receive one infusion of PEG 2500 IU/m² on D12 (Arm A) or two infusions of 1250 IU/m² on D12 and D26 (Arm B) during induction. The dose assigned at randomization was used in subsequent treatment phases. Patients in the B-HR and T-HR groups received two infusions of PEG 2500 IU/m² during induction and 2500 IU/m² per infusion afterwards. The study had two primary objectives: 1) to assess the superiority of the fractionated scheme in terms of pharmacokinetics, 2) to assess the equivalence in tolerance of the two schemes. The co-primary endpoints were: 1) the incidence of adequate (>100 IU/L) asparaginase activity (AA) at the end of induction (D33 ± 1 day), 2) the incidence of severe (CTCAE Grade ≥ 3) asparaginase-related toxicities (CNS thrombosis, pancreatitis, severe allergy/anaphylaxis, and bilirubin elevation) from D12 of induction to D49. Results: From September 2016 to February 2022, 2032 patients (mean age, 5.4 years, 57% male) were included. Among these, 1624 patients in the B-SR (942 pts), B-MR (518 pts), or T-SR (164 pts) groups were randomly allocated to Arm A (814 pts) or Arm B (810 pts). The 2 arms were well balanced for baseline characteristics and prognostic features. The median follow-up was 44.6 months. The D33 AA was evaluable in 1475 (86.7%) randomized patients. The two-infusion arm was superior to the one-infusion arm, as D33 AA was ≥ 100 U/L in 87.9% of the patients in Arm A (653/743) versus 96.6% in Arm B (707/732) (p<.0001). The cumulative incidence of any of the four targeted toxicities was 15.9% in Arm A (129/814 pts) versus 14.1% in Arm B (114/810 pts) (difference, 1.78%, 95% confidence interval (CI) -1.70 to +5.26%); as the equivalence margin was set to 4%, equivalence was not demonstrated. Minimal residual disease levels assessed by clone-specific IG-TCR PCR at the end of induction and consolidation did not differ between the 2 arms, nor did the 48-month event-free survival (EFS, 95.5% CI 95 94.7-97.4 vs 97% CI 95.78-98.12). Post hoc analyses revealed that 48-month EFS in Arms A and B were not significantly different for patients with B-ALL (94.0%, 95CI 92.0-95.9 for Arm A vs 95.8%, 95CI 94.2-97.4 for Arm B, HR= 0.78, 95CI 0.51-1.20, p=0.26), nor for patients with T-cell ALL (88.0%, 95CI 81.2-95.2 for ArmA vs 87.2%, 95CI 80.1-94.9 for Arm B, HR=1.16, 96CI 0.49-2.72; p=0.74). Conclusion: For non-high-risk ALL, representing 80% of the study population, a fractionated scheme of pegaspargase (2 doses of 1250 IU/m²) is at least as well tolerated as a one-infusion scheme (1 dose of 2500 IU/m²) and more effective in terms of maintaining adequate AA throughout induction. However, at the current MFU of 44 months, this did not translate into superior long-term outcomes.

Juvenile myelomonocytic leukemia (JMML) is a rare, generally aggressive myeloproliferative neoplasm affecting young children. It is characterized by granulomonocytic expansion, with monocytosis infiltrating peripheral tissues. JMML is initiated by mutations upregulating RAS signaling. Approximately 10% of cases remain without an identified driver event. Exome sequencing of two unrelated cases of familial JMML of unknown genetics and analysis of the French JMML cohort identified 11 patients with variants in SH2B3, encoding LNK, a negative regulator of the JAK-STAT pathway. All variants were absent from healthy population databases, and the mutation spectrum was consistent with a loss of function of the LNK protein. A stoploss variant was shown to affect both protein synthesis and stability. The other variants were either truncating or missense, the latter affecting the SH2 domain that interacts with activated JAK. Of the 11 patients, eight from five families inherited pathogenic bi-allelic SH2B3 germline variants from their unaffected heterozygous parents. These children represent half of the cases with no identified causal mutation in the French cohort. They displayed typical clinical and hematologic features of JMML with neonatal onset and marked thrombocytopenia. They had a hypomethylated DNA profile with fetal characteristics and did not have additional genetic alterations. All patients showed partial or complete spontaneous clinical resolution. However, progression to thrombocythemia and immunity-related pathologies may be of concern later in life. Bi-allelic SH2B3 germline mutations thus define a new condition predisposing to a JMML-like disorder, suggesting that JAK pathway deregulation is capable of initiating JMML, and opening new therapeutic options.

Acute leukemias of ambiguous lineage (ALAL) represent between 3 and 5% of childhood AL. This term encompasses many subtypes of AL that have been defined according to the immunophenotypic profile based on the expression of various lineage markers. This classification has been modified and enriched during the last decade thanks to the improvement of molecular biology techniques, which have led to reconsider the ontogenic proximity existing between certain forms of ALAL. This increasing diagnostic complexity justifies the establishment of a close communication between clinicians and biologists in the management of these rare forms of AL. Indeed, the initial classification remains the cornerstone of their management since it conditions the future choice of therapeutic protocol. Thus, with the notable exception of undifferentiated forms of AL or AUL (for acute undetermined leukemia), it is now accepted that ALAL benefit from a lymphoidbased therapy approach. As with the management of "classic" acute lymphoblastic leukemias (ALL), the evaluation of response to treatment will determine the modalities of therapeutic intensification. The objective of improving the prognosis of ALAL justifies, in the long term, their future inclusion in the international ALLTogether protocol while continuing in-depth molecular exploration of these patients to identify targeted therapies.