Canonical BAF complex regulates the oncogenic program in human T-cell acute lymphoblastic leukemia.

ABSTRACT: Acute leukemia cells require bone marrow microenvironments, known as niches, which provide leukemic cells with niche factors that are essential for leukemic cell survival and/or proliferation. However, it remains unclear how the dynamics of the leukemic cell-niche interaction are regulated. Using a genome-wide CRISPR screen, we discovered that canonical BRG1/BRM-associated factor (cBAF), a variant of the switch/sucrose nonfermenting chromatin remodeling complex, regulates the migratory response of human T-cell acute lymphoblastic leukemia (T-ALL) cells to a niche factor CXCL12. Mechanistically, cBAF maintains chromatin accessibility and allows RUNX1 to bind to CXCR4 enhancer regions. cBAF inhibition evicts RUNX1 from the genome, resulting in CXCR4 downregulation and impaired migration activity. In addition, cBAF maintains chromatin accessibility preferentially at RUNX1 binding sites, ensuring RUNX1 binding at these sites, and is required for expression of RUNX1-regulated genes, such as CDK6; therefore, cBAF inhibition negatively impacts cell proliferation and profoundly induces apoptosis. This anticancer effect was also confirmed using T-ALL xenograft models, suggesting cBAF as a promising therapeutic target. Thus, we provide novel evidence that cBAF regulates the RUNX1-driven leukemic program and governs migration activity toward CXCL12 and cell-autonomous growth in human T-ALL.

The orally bioavailable GSPT1/2 degrader SJ6986 exhibits in vivo efficacy in acute lymphoblastic leukemia.

Advancing cure rates for high-risk acute lymphoblastic leukemia (ALL) has been limited by the lack of agents that effectively kill leukemic cells, sparing normal hematopoietic tissue. Molecular glues direct the ubiquitin ligase cellular machinery to target neosubstrates for protein degradation. We developed a novel cereblon modulator, SJ6986, that exhibits potent and selective degradation of GSPT1 and GSPT2 and cytotoxic activity against childhood cancer cell lines. Here, we report in vitro and in vivo testing of the activity of this agent in a panel of ALL cell lines and xenografts. SJ6986 exhibited similar cytotoxicity to the previously described GSPT1 degrader CC-90009 in a panel of leukemia cell lines in vitro, resulting in apoptosis and perturbation of cell cycle progression. SJ6986 was more effective than CC-90009 in suppressing leukemic cell growth in vivo, partly attributable to favorable pharmacokinetic properties, and did not significantly impair differentiation of human CD34+ cells ex vivo. Genome-wide CRISPR/Cas9 screening of ALL cell lines treated with SJ6986 confirmed that components of the CRL4CRBN complex, associated adaptors, regulators, and effectors were integral in mediating the action of SJ6986. SJ6986 is a potent, selective, orally bioavailable GSPT1/2 degrader that shows broad antileukemic activity and has potential for clinical development.

Application of prime editing system to introduce TP53 R248Q hotspot mutation in acute lymphoblastic leukemia cell line.

In childhood acute lymphoblastic leukemia (ALL), TP53 gene mutation is associated with chemoresistance in a certain population of relapsed cases. To directly verify the association of TP53 gene mutation with chemoresistance of relapsed childhood ALL cases and improve their prognosis, the development of appropriate human leukemia models having TP53 mutation in the intrinsic gene is required. Here, we sought to introduce R248Q hotspot mutation into the intrinsic TP53 gene in an ALL cell line, 697, by applying a prime editing (PE) system, which is a versatile genome editing technology. The PE2 system uses an artificial fusion of nickase Cas9 and reverse-transcriptase to directly place new genetic information into a target site through a reverse transcriptase template in the prime editing guide RNA (pegRNA). Moreover, in the advanced PE3b system, single guide RNA (sgRNA) matching the edited sequence is also introduced to improve editing efficiency. The initially obtained MDM2 inhibitor-resistant PE3b-transfected subline revealed disrupted p53 transactivation activity, reduced p53 target gene expression, and acquired resistance to chemotherapeutic agents and irradiation. Although the majority of the subline acquired the designed R248Q and adjacent silent mutations, the insertion of the palindromic sequence in the scaffold hairpin structure of pegRNA and the overlap of the original genomic DNA sequence were frequently observed. Targeted next-generation sequencing reconfirmed frequent edit errors in both PE2 and PE3b-transfected 697 cells, and it revealed frequent successful edits in HEK293T cells. These observations suggest a requirement for further modification of the PE2 and PE3b systems for accurate editing in leukemic cells.

Acquired copy number amplification at the MYC enhancer in human B-precursor acute lymphoblastic leukemia cell lines.

Our study highlights the discovery of recurrent copy number alterations in noncoding regions, specifically blood enhancer cluster (BENC-CNA), in B-precursor acute lymphoblastic leukemia (BCP-ALL) cell lines. We demonstrate that BENC-CNA acts as a super-enhancer, driving MYC expression and possibly contributing to the immortalization and proliferative advantage of BCP-ALL cells in vitro.

Genome-wide CRISPR-Cas9 screen identifies rationally designed combination therapies for CRLF2-rearranged Ph-like ALL.

Acute lymphoblastic leukemia (ALL) harboring the IgH-CRLF2 rearrangement (IgH-CRLF2-r) exhibits poor clinical outcomes and is the most common subtype of Philadelphia chromosome-like acute lymphoblastic leukemia (Ph-like ALL). While multiple chemotherapeutic regimens, including ruxolitinib monotherapy and/or its combination with chemotherapy, are being tested, their efficacy is reportedly limited. To identify molecules/pathways relevant for IgH-CRLF2-r ALL pathogenesis, we performed genome-wide CRISPR-Cas9 dropout screens in the presence or absence of ruxolitinib using 2 IgH-CRLF2-r ALL lines that differ in RAS mutational status. To do so, we employed a baboon envelope pseudotyped lentiviral vector system, which enabled, for the first time, highly efficient transduction of human B cells. While single-guide RNAs (sgRNAs) targeting CRLF2, IL7RA, or JAK1/2 significantly affected cell fitness in both lines, those targeting STAT5A, STAT5B, or STAT3 did not, suggesting that STAT signaling is largely dispensable for IgH-CRLF2-r ALL cell survival. We show that regulators of RAS signaling are critical for cell fitness and ruxolitinib sensitivity and that CRKL depletion enhances ruxolitinib sensitivity in RAS wild-type (WT) cells. Gilteritinib, a pan-tyrosine kinase inhibitor that blocks CRKL phosphorylation, effectively killed RAS WT IgH-CRLF2-r ALL cells in vitro and in vivo, either alone or combined with ruxolitinib. We further show that combining gilteritinib with trametinib, a MEK1/2 inhibitor, is an effective means to target IgH-CRLF2-r ALL cells regardless of RAS mutational status. Our study delineates molecules/pathways relevant for CRLF2-r ALL pathogenesis and could suggest rationally designed combination therapies appropriate for disease subtypes.

Tumor-intrinsic and -extrinsic determinants of response to blinatumomab in adults with B-ALL.

Blinatumomab, a bispecific antibody that directs CD3+ T cells to CD19+ tumor cells, shows variable efficacy in B-progenitor acute lymphoblastic leukemia (B-ALL). To determine tumor-intrinsic and -extrinsic determinants of response, we studied 44 adults with relapsed or refractory B-ALL (including 2 minimal residual disease positive) treated with blinatumomab using bulk tumor and single-cell sequencing. The overall response rate in patients with hematological disease was 55%, with a high response rate in those with CRLF2-rearranged Philadelphia chromosome-like ALL (12 [75%] of 16). Pretreatment samples of responders exhibited a tumor-intrinsic transcriptomic signature of heightened immune response. Multiple mechanisms resulted in loss of CD19 expression, including CD19 mutations, CD19-mutant allele-specific expression, low CD19 RNA expression, and mutations in CD19 signaling complex member CD81. Patients with low hypodiploid ALL were prone to CD19- relapse resulting from aneuploidy-mediated loss of the nonmutated CD19 allele. Increased expression of a CD19 isoform with intraexonic splicing of exon 2, CD19 ex2part, at baseline or during therapy was associated with treatment failure. These analyses demonstrate both tumor-intrinsic and -extrinsic factors influence blinatumomab response. We show that CD19 mutations are commonly detected in CD19- relapse during blinatumomab treatment. Identification of the CD19 ex2part splice variant represents a new biomarker predictive of blinatumomab therapy failure.

Degradation of Janus kinases in CRLF2-rearranged acute lymphoblastic leukemia.

CRLF2-rearranged (CRLF2r) acute lymphoblastic leukemia (ALL) accounts for more than half of Philadelphia chromosome-like (Ph-like) ALL and is associated with a poor outcome in children and adults. Overexpression of CRLF2 results in activation of Janus kinase (JAK)-STAT and parallel signaling pathways in experimental models, but existing small molecule inhibitors of JAKs show variable and limited efficacy. Here, we evaluated the efficacy of proteolysis-targeting chimeras (PROTACs) directed against JAKs. Solving the structure of type I JAK inhibitors ruxolitinib and baricitinib bound to the JAK2 tyrosine kinase domain enabled the rational design and optimization of a series of cereblon (CRBN)-directed JAK PROTACs utilizing derivatives of JAK inhibitors, linkers, and CRBN-specific molecular glues. The resulting JAK PROTACs were evaluated for target degradation, and activity was tested in a panel of leukemia/lymphoma cell lines and xenograft models of kinase-driven ALL. Multiple PROTACs were developed that degraded JAKs and potently killed CRLF2r cell lines, the most active of which also degraded the known CRBN neosubstrate GSPT1 and suppressed proliferation of CRLF2r ALL in vivo, e.g. compound 7 (SJ988497). Although dual JAK/GSPT1-degrading PROTACs were the most potent, the development and evaluation of multiple PROTACs in an extended panel of xenografts identified a potent JAK2-degrading, GSPT1-sparing PROTAC that demonstrated efficacy in the majority of kinase-driven xenografts that were otherwise unresponsive to type I JAK inhibitors, e.g. compound 8 (SJ1008030). Together, these data show the potential of JAK-directed protein degradation as a therapeutic approach in JAK-STAT-driven ALL and highlight the interplay of JAK and GSPT1 degradation activity in this context.

BCL6 inhibition ameliorates resistance to ruxolitinib in CRLF2-rearranged acute lymphoblastic leukemia.

Philadelphia chromosome-like acute lymphoblastic leukemia (Ph-like ALL) is an intractable disease and most cases harbor genetic alterations that activate JAK or ABL signaling. The commonest subtype of Ph-like ALL exhibits a CRLF2 gene rearrangement that brings about JAK1/2-STAT5 pathway activation. However, JAK1/2 inhibition alone is insufficient as a treatment, so combinatorial therapies targeting multiple signals are needed. To better understand the mechanisms underlying the insufficient efficacy of JAK inhibition, we explored gene expression changes upon treatment with a JAK1/2 inhibitor (ruxolitinib) and found that elevated BCL6 expression was one such mechanism. Upregulated BCL6 suppressed the expression of TP53 along with its downstream cell cycle inhibitor p21 (CDKN2A) and pro-apoptotic molecules, such as FAS, TNFRSF10B, BID, BAX, BAK, PUMA, and NOXA, conferring cells some degree of resistance to therapy. BCL6 inhibition (with FX1) alone was able to upregulate TP53 and restore the TP53 expression that ruxolitinib had diminished. In addition, ruxolitinib and FX1 concertedly downregulated MYC. As a result, FX1 treatment alone had growth-inhibitory and apoptosis- sensitizing effects, but the combination of ruxolitinib and FX1 more potently inhibited leukemia cell growth, enhanced apoptosis sensitivity, and prolonged the survival of xenografted mice. These findings provide one mechanism for the insufficiency of JAK inhibition for the treatment of CRLF2-rearranged ALL and indicate BCL6 inhibition as a potentially helpful adjunctive therapy combined with JAK inhibition.

Prenatal Torsion of Radial Polydactyly: A Gangrenous Mass at the Base of the Thumb.

A patient was born with a mass at the base of the thumb approximately 1.5 cm in diameter on the radial side of the fingers. The mass had globular swelling filled with hemorrhagic fluid and was dark red. X-rays and histology of the excised specimen suggested the diagnosis of gangrene and torsion of polydactyly. Prenatal torsion of polydactyly is not a common occurrence; moreover, prenatal torsion of polydactyly has only been found in ulnar polydactyly. Our case is a novel case of radial polydactyly that was gangrenous at birth owing to prenatal torsion. Diagnosing such a mass at the base of the thumb is important.

Prominence of NUDT15 genetic variation associated with 6-mercaptopurine tolerance in a genome-wide association study of Japanese children with acute lymphoblastic leukaemia.

Inherited genetic variation is associated with 6-mercaptopurine (6-MP) dose reduction and frequent toxicities induced by 6-MP. However, the tolerable dose for 6-MP is not fully predicted by the known variation in NUDT15 and TPMT among Asian children with acute lymphoblastic leukaemia (ALL). We performed a genome-wide association study (GWAS) related to 6-MP dose among Japanese children with ALL. This GWAS comprised 224 patients previously enrolled in Tokyo Children's Cancer Study Group clinical studies with replication attempted in 55 patients. Genome-wide single nucleotide polymorphism (SNP) genotypes were evaluated for association with average 6-MP dose during the initial 168 days of maintenance therapy. Possible associations were observed across five gene-coding regions, among which only variants at 13q14.2 were significant and replicated genome-wide (rs116855232, NUDT15, ß = -10.99, p = 3.7 × 10-13 ). Notable findings were observed for variants in AFF3 (rs75364948, p = 2.05 × 10-6 ) and CHST11 (rs1148407, p = 2.09 × 10-6 ), but were not replicated possibly due to small numbers. A previously reported candidate SNP in MTHFR was associated with higher average 6-MP dose (rs1801133, p = 0.045), and FOLH1 (rs12574928) was associated in an evaluation of candidate regions (padjust  = 0.013). This study provides strong evidence that rs116855232 in NUDT15 is the genetic factor predominantly associated with 6-MP tolerable dose in children in Japan.

CN470 is a BET/CBP/p300 multi-bromodomain inhibitor and has an anti-tumor activity against MLL-rearranged acute lymphoblastic leukemia.

Acute lymphoblastic leukemia with chromosomal rearrangements involving the mixed-lineage leukemia (MLL) gene (MLL-r ALL) remains an incurable disease. Thus, development of a safe and effective therapeutic agent to treat this disease is crucial to address this unmet medical need. BRD4, a member of the bromodomain and extra-terminal domain (BET) protein family, and cyclic AMP response element binding protein binding protein (CBP) and p300, two paralogous histone acetyltransferases, are all considered cancer drug targets and simultaneous targeting of these proteins may have therapeutic advantages. Here, we demonstrate that a BET/CBP/p300 multi-bromodomain inhibitor, CN470, has anti-tumor activity against MLL-r ALL in vitro and in vivo. CN470, potently inhibited ligand binding to the bromodomains of BRD4, CBP, and p300 and suppressed the growth of MLL-r ALL cell lines and patient-derived cells with MLL rearrangements. CN470 suppressed mRNA and protein expression of MYC and induced apoptosis in MLL-r ALL cells, following a cell cycle arrest in the G1 phase. Moreover, CN470 reduced BRD4 binding to acetylated histone H3. The in vivo effects of CN470 were investigated using SEMLuc/GFP cells expressing luminescent markers in an orthotopic mouse model. Mice administered CN470 daily had prolonged survival compared to the vehicle group. Further, CN470 also showed anti-tumor effects against an MLL-r ALL patient-derived xenograft model. These findings suggest that inhibition of BET/CBP/p300 by the multi-bromodomain inhibitor, CN470, represents a promising therapeutic approach against MLL-r ALL.

Association of aberrant ASNS imprinting with asparaginase sensitivity and chromosomal abnormality in childhood BCP-ALL.

Karyotype is an important prognostic factor in childhood B-cell precursor acute lymphoblastic leukemia (BCP-ALL), but the underlying pharmacogenomics remain unknown. Asparaginase is an integral component in current chemotherapy for childhood BCP-ALL. Asparaginase therapy depletes serum asparagine. Normal hematopoietic cells can produce asparagine by asparagine synthetase (ASNS) activity, but ALL cells are unable to synthesize adequate amounts of asparagine. The ASNS gene has a typical CpG island in its promoter. Thus, methylation of the ASNS CpG island could be one of the epigenetic mechanisms for ASNS gene silencing in BCP-ALL. To gain deep insights into the pharmacogenomics of asparaginase therapy, we investigated the association of ASNS methylation status with asparaginase sensitivity. The ASNS CpG island is largely unmethylated in normal hematopoietic cells, but it is allele-specifically methylated in BCP-ALL cells. The ASNS gene is located at 7q21, an evolutionally conserved imprinted gene cluster. ASNS methylation in childhood BCP-ALL is associated with an aberrant methylation of the imprinted gene cluster at 7q21. Aberrant methylation of mouse Asns and a syntenic imprinted gene cluster is also confirmed in leukemic spleen samples from ETV6-RUNX1 knockin mice. In 3 childhood BCP-ALL cohorts, ASNS is highly methylated in BCP-ALL patients with favorable karyotypes but is mostly unmethylated in BCP-ALL patients with poor prognostic karyotypes. Higher ASNS methylation is associated with higher L-asparaginase sensitivity in BCP-ALL through lower ASNS gene and protein expression levels. These observations demonstrate that silencing of the ASNS gene as a result of aberrant imprinting is a pharmacogenetic mechanism for the leukemia-specific activity of asparaginase therapy in BCP-ALL.

[Application of genome editing technology for the validation of pharmacogenomics in acute lymphoblastic leukemia].

Therapeutic outcome in childhood acute lymphocytic leukemia has been dramatically improved by recent developments in treatment. However, disease relapse is still observed in approximately 10-15% of the patients. Moreover, adverse effects associated with intensified chemotherapy and hematopoietic stem cell transplantation remain important clinical issues for some survivors. Personalized medicine is valuable, under these circumstances, to reduce adverse effects and further improve the therapeutic outcome. Thus, identifying pharmacogenomic backgrounds associated with individual variation in drug sensitivity of leukemia cells and chemotherapy-induced adverse effects is important for precision medicine development. Recent advances in genome-editing technologies, such as CRISPR/Cas9 system, enable direct confirmation of associations between drug sensitivities and genetic backgrounds, such as polymorphisms and mutations, in the intrinsic genes of leukemia cells. Consequently, genome-editing systems are an ideal tool to develop in vitro and in vivo experimental models of drug sensitivity or resistance. The usefulness of the CRISPR/Cas9 system for the validation of pharmacogenomics in the selection of chemotherapeutic agents for acute lymphocytic leukemia has been discussed with specific examples in this review.

NUDT15 polymorphism and NT5C2 and PRPS1 mutations influence thiopurine sensitivity in acute lymphoblastic leukaemia cells.

In chemotherapy for childhood acute lymphoblastic leukaemia (ALL), maintenance therapy consisting of oral daily mercaptopurine and weekly methotrexate is important. NUDT15 variant genotype is reportedly highly associated with severe myelosuppression during maintenance therapy, particularly in Asian and Hispanic populations. It has also been demonstrated that acquired somatic mutations of the NT5C2 and PRPS1 genes, which are involved in thiopurine metabolism, are detectable in a portion of relapsed childhood ALL. To directly confirm the significance of the NUDT15 variant genotype and NT5C2 and PRPS1 mutations in thiopurine sensitivity of leukaemia cells in the intrinsic genes, we investigated 84 B-cell precursor-ALL (BCP-ALL) cell lines. Three and 14 cell lines had homozygous and heterozygous variant diplotypes of the NUDT15 gene, respectively, while 4 and 2 cell lines that were exclusively established from the samples at relapse had the NT5C2 and PRPS1 mutations, respectively. Both NUDT15 variant genotype and NT5C2 and PRPS1 mutations were significantly associated with DNA-incorporated thioguanine levels after exposure to thioguanine at therapeutic concentration. Considering the continuous exposure during the maintenance therapy, we evaluated in vitro mercaptopurine sensitivity after 7-day exposure. Mercaptopurine concentrations lethal to 50% of the leukaemia cells were comparable to therapeutic serum concentration of mercaptopurine. Both NUDT15 variant genotype and NT5C2 and PRPS1 mutations were significantly associated with mercaptopurine sensitivity in 83 BCP-ALL and 23 T-ALL cell lines. The present study provides direct evidence to support the general principle showing that both inherited genotype and somatically acquired mutation are crucially implicated in the drug sensitivity of leukaemia cells.

Inherited genetic variants associated with glucocorticoid sensitivity in leukaemia cells.

Identification of genetic variants associated with glucocorticoids (GC) sensitivity of leukaemia cells may provide insight into potential drug targets and tailored therapy. In the present study, within 72 leukaemic cell lines derived from Japanese patients with B-cell precursor acute lymphoblastic leukaemia (ALL), we conducted genome-wide genotyping of single nucleotide polymorphisms (SNP) and attempted to identify genetic variants associated with GC sensitivity and NR3C1 (GC receptor) gene expression. IC50 measures for prednisolone (Pred) and dexamethasone (Dex) were available using an alamarBlue cell viability assay. IC50 values of Pred showed the strongest association with rs904419 (P = 4.34 × 10-8 ), located between the FRMD4B and MITF genes. The median IC50 values of prednisolone for cell lines with rs904419 AA (n = 13), AG (n = 31) and GG (n = 28) genotypes were 0.089, 0.139 and 297 µmol/L, respectively. For dexamethasone sensitivity, suggestive association was observed for SNP rs2306888 (P = 1.43 × 10-6 ), a synonymous SNP of the TGFBR3 gene. For NR3C1 gene expression, suggestive association was observed for SNP rs11982167 (P = 6.44 × 10-8 ), located in the PLEKHA8 gene. These genetic variants may affect GC sensitivity of ALL cells and may give rise to opportunities in personalized medicine for effective and safe chemotherapy in ALL patients.

Association of relapse-linked ARID5B single nucleotide polymorphisms with drug resistance in B-cell precursor acute lymphoblastic leukemia cell lines.

BACKGROUND: The genetic variants of the ARID5B gene have recently been reported to be associated with disease susceptibility and treatment outcome in childhood acute lymphoblastic leukemia (ALL). However, few studies have explored the association of ARID5B with sensitivities to chemotherapeutic agents. METHODS: We genotyped susceptibility-linked rs7923074 and rs10821936 as well as relapse-linked rs4948488, rs2893881, and rs6479778 of ARDI5B by direct sequencing of polymerase chain reaction (PCR) products in 72 B-cell precursor-ALL (BCP-ALL) cell lines established from Japanese patients. We also quantified their ARID5B expression levels by real-time reverse transcription PCR, and determined their 50% inhibitory concentration (IC50) values by alamarBlue assays in nine representative chemotherapeutic agents used for ALL treatment. RESULTS: No significant associations were observed in genotypes of the susceptibility-linked single nucleotide polymorphisms (SNPs) and the relapsed-linked SNPs with ARID5B gene expression levels. Of note, IC50 values of vincristine (VCR) (median IC50: 39.6 ng/ml) in 12 cell lines with homozygous genotype of risk allele (C) in the relapse-linked rs4948488 were significantly higher (p = 0.031 in Mann-Whitney U test) than those (1.04 ng/ml) in 60 cell lines with heterozygous or homozygous genotypes of the non-risk allele (T). Furthermore, the IC50 values of mafosfamide [Maf; active metabolite of cyclophosphamide (CY)] and cytarabine (AraC) tended to be associated with the genotype of rs4948488. Similar associations were observed in genotypes of the relapse-linked rs2893881 and rs6479778, but not in those of the susceptibility-linked rs7923074 and rs10821936. In addition, the IC50 values of methotrexate (MTX) were significantly higher (p = 0.023) in 36 cell lines with lower ARID5B gene expression (median IC50: 37.1 ng/ml) than those in the other 36 cell lines with higher expression (16.9 ng/ml). CONCLUSION: These observations in 72 BCP-ALL cell lines suggested that the risk allele of the relapse-linked SNPs of ARID5B may be involved in a higher relapse rate because of resistance to chemotherapeutic agents such as VCR, CY, and AraC. In addition, lower ARID5B gene expression may be associated with MTX resistance.

Successful hematopoietic stem cell transplantation from an HLA-mismatched parent for engraftment failure after unrelated cord blood transplantation in patients with juvenile myelomonocytic leukemia: Report of two cases.

JMML is an aggressive hematopoietic malignancy of early childhood, and allogeneic HSCT is the only curative treatment for this disease. Umbilical cord blood is one of donor sources for HSCT in JMML patients who do not have an HLA-compatible relative, but engraftment failure remains a major problem. Here, we report two cases of JMML who were successfully rescued by HSCT from an HLA-mismatched parent after development of primary engraftment failure following unrelated CBT. Both patients had severe splenomegaly and underwent unrelated CBT from an HLA-mismatched donor. Immediately after diagnosis of engraftment failure, both patients underwent HSCT from their parent. For the second HSCT, we used RIC regimens consisting of FLU, CY, and a low dose of rabbit ATG with or without TBI and additionally administered ETP considering their persistent severe splenomegaly. Both patients achieved engraftment without severe treatment-related adverse effects. After engraftment of second HSCT, their splenomegaly was rapidly regressed, and both patients showed no sign of relapse for over 4 years. These observations demonstrate that HSCT from an HLA-mismatched parent could be a feasible salvage treatment for primary engraftment failure in JMML patients.