Deregulated expression of CRLF2 (CRLF2-d) arises via its juxtaposition to the IGH@ enhancer or P2RY8 promoter. Among 865 BCP-ALL children treated on MRC ALL97, 52 (6%) had CRLF2-d, but it was more prevalent among Down syndrome patients (54%). P2RY8-CRLF2 (n ‫؍‬ 43) was more frequent than IGH@-CRLF2 (n ‫؍‬ 9). CRLF2-d was not associated with age, sex, or white cell count, but IGH@-CRLF2 patients were older than P2RY8-CRLF2 patients (median 8 vs 4 years, P ‫؍‬ .0017). Patients with CRLF2-d were more likely to present with enlarged livers and spleens (38% vs 18%, P < .001).

Intrachromosomal amplification of chromosome 21 (iAMP21) defines a distinct subgroup of childhood B-cell precursor acute lymphoblastic leukemia (BCP-ALL) that has a dismal outcome when treated with standard therapy. For improved diagnosis and risk stratification, the initiating genetic events need to be elucidated. To investigate the genetic basis of BCP-ALL, genomes of 94 iAMP21 patients were interrogated by arrays, FISH, and multiplex ligation-dependent probe amplification. Most copy number alterations targeted chromosome 21, reinforcing the complexity of this chromosome. The common region of amplification on chromosome 21 was refined to a 5.1-mb region that included RUNX1, miR-802, and genes mapping to the Down syndrome critical region. Recurrent abnormalities affecting genes in key pathways were identified: IKZF1 (22%), CDKN2A/B (17%), PAX5 (8%), ETV6 (19%), and RB1 (37%). Investigation of clonal architecture provided evidence that these abnormalities, and P2RY8-CRLF2, were secondary to chromosome 21 rearrangements. Patient outcome was uniformly poor with standard therapy irrespective of the presence or absence of these changes. This study has provided evidence that chromosome 21 instability is the only anomaly among those so far investigated that is common to all iAMP21 patients, and therefore the initiating event is likely to be found among the complex structural rearrangements of this abnormal chromosome.

Key Points• More than 75% of primary diagnostic BCP-ALL samples engraft in the CNS in xenograft models.• We find no evidence for selective trafficking to the CNS but show that CNS entry is a generic property of BCP-ALL cells.Prevention of central nervous system (CNS) relapse is critical for cure of childhood B-cell precursor acute lymphoblastic leukemia (BCP-ALL). Despite this, mechanisms of CNS infiltration are poorly understood, and the timing, frequency, and properties of BCP-ALL blasts entering the CNS compartment are unknown. We investigated the CNS-engrafting potential of BCP-ALL cells xenotransplanted into immunodeficient NOD.Cg-Prkdc scid Il2rg tm1Wjl /SzJ mice. CNS engraftment was seen in 23 of 29 diagnostic samples (79%): 2 of 2 from patients with overt CNS disease and 21 of 27 from patients thought to be CNS negative by diagnostic lumbar puncture. Histologic findings mimic human pathology and demonstrate that leukemic cells transit the blood-cerebrospinal fluid barrier situated close to the dural sinuses, the site of recently discovered CNS lymphatics. Retrieval of blasts from the CNS showed no evidence for chemokine receptor-mediated selective trafficking. The high frequency of infiltration and lack of selective trafficking led us to postulate that CNS tropism is a generic property of leukemic cells. To test this, we performed serial dilution experiments which showed CNS engraftment in 5 of 6 mice after transplant of as few as 10 leukemic cells. Clonal tracking techniques confirmed the polyclonal nature of CNS-infiltrating cells, with multiple clones engrafting in both the CNS and periphery. Overall, these findings suggest that subclinical seeding of the CNS is likely to be present in most BCP-ALL patients at original diagnosis, and efforts to prevent CNS relapse should concentrate on effective eradication of disease from this site rather than targeting entry mechanisms.

CRLF2-d and IGH@-t represent distinct subtypes of adolescent and adult ALL. Deletions of key B-cell differentiation and cell cycle control genes are highly prevalent but vary in frequency by cytogenetic subgroup. CRLF2-d, IGH@-t, and IKZF1 deletions are associated with poor outcome in adolescent and adult ALL.