DISEASE CHARACTERISTICS:

• Diagnosis of one of the following:

  • Acute myeloid leukemia (AML) or acute lymphoblastic leukemia

    • Less than 20% morphologic marrow blasts in an evaluable marrow sample (> 25% of normal cellularity for age) collected within the past month
    • Patients persistently aplastic for > 1 month after completing last chemotherapy are eligible
    • No AML in first complete remission with favorable prognostic cytogenetics (t[8;21], t[15;17], inv16)

  • Myelodysplastic syndromes (MDS)

    • Any 2001 WHO classification subtype, including the following:

      • Refractory anemia (RA)
      • RA with ringed sideroblasts (RS)
      • Refractory cytopenia with multilineage dysplasia (RCMD)
      • RCMD and RS
      • RA with excess blasts (RAEB)-1
      • RAEB-2 (may proceed directly to transplant, but may also be considered for induction chemotherapy before transplant)
    • No low-risk (IPSS 0) MDS
    • Patients with ≥ 20% morphologic marrow blasts require induction therapy to reduce morphologic marrow blasts below 5% before transplant

  • Juvenile myelomonocytic leukemia (JMML)

    • Patients are eligible with or without previous chemotherapy, but should be considered for pretransplant cytoreductive therapy if marrow blasts exceed 15-20%

  • Chronic myelogenous leukemia

    • All types allowed, except for refractory blast crisis
    • Chronic phase patients must have failed or been intolerant to imatinib mesylate or other tyrosine kinase inhibitors

• Meets one of the following criteria:

  • At low risk for graft failure, as defined by any of the following:

    • Has undergone autologous stem cell transplantation within the past 12 months
    • Received ≥ 2 courses of multiagent chemotherapy with ≥ 1 course of therapy administered within the past 3 months

  • At high risk for graft failure, as defined by any of the following:

    • Received < 2 courses of multiagent chemotherapy
    • Received no multiagent chemotherapy or immunosuppressive chemotherapy within the past 3 months
    • Received only a single induction therapy
• No CNS leukemic involvement that does not clear with intrathecal chemotherapy and/or cranial radiotherapy prior to initiation of study conditioning regimen (day -6)
• No available 5-6/6 HLA-A, B, DRB1-matched sibling donor or readily available 10/10-matched unrelated donor

• Matched (≥ 4/6 HLA-A, B, and DRB1 loci) unrelated umbilical cord blood (UCB) donor available

  • Each UCB unit must contain ≥ 1.5 x 10^7 total nucleated cells/kg
  • If two UCB units are used, they must be matched to each other for ≥ 3/6 HLA-A, B, and DRB1 loci

PATIENT CHARACTERISTICS:

• Karnofsky performance status (PS) 70-100% (adults)
• Lansky PS 50-100% (pediatrics)
• Comorbidity index ≤ 2 (for patients > 50 years of age)

• Creatinine ≤ 2.0 mg/dL (adults )

  • Adults with a creatinine > 1.2 mg/dL or history of renal dysfunction must have creatinine clearance > 40 mL/min
• Creatinine clearance > 40 mL/min (pediatrics)
• LVEF ≥ 35%
• DLCO > 70% predicted
• Not pregnant or nursing
• Negative pregnancy test
• Fertile patients must use effective contraception during and for ≥ 1 year after completion of study therapy
• No decompensated congestive heart failure
• No uncontrolled arrhythmia
• No requirement for oxygen

• None of the following hepatic conditions*:

  • Fulminant liver failure
  • Cirrhosis with evidence of portal hypertension or bridging fibrosis
  • Alcoholic hepatitis
  • Esophageal varices
  • History of bleeding esophageal varices
  • Hepatic encephalopathy
  • Correctable hepatic synthetic dysfunction evidenced by prolongation of the prothrombin time
  • Ascites related to portal hypertension
  • Bacterial or fungal abscess
  • Biliary obstruction
  • Chronic viral hepatitis with total serum bilirubin > 3 mg/dL
  • Symptomatic biliary disease
• No evidence of HIV infection or known HIV positivity
• No uncontrolled viral or bacterial infection
• No active or recent (within the past 6 months) invasive fungal infection that has not been cleared by infectious disease consult NOTE: *Patients with clinical or laboratory evidence of disease will be evaluated for the cause of liver disease and its clinical severity in terms of liver function, histology, and the degree of portal hypertension

PRIOR CONCURRENT THERAPY:

• See Disease Characteristics
• At least 3 months since prior myeloablative stem cell transplantation