IGH@ Translocations Involving the Pseudoautosomal Region 1 (PAR1) of Both Sex Chromosomes Deregulate the Cytokine Receptor-Like Factor 2 (CRLF2) Gene in B Cell Precursor Acute Lymphoblastic Leukemia (BCP-ALL)

We have recently described recurrent IGH@ translocations in BCP-ALL, which result in deregulated expression of the translocated oncogene. We carried out screening for novel IGH@ translocations in BCP-ALL by fluorescence in situ hybridization (FISH) and identified two cryptic translocations involving PAR1 of either sex chromosomes in 29 patients: t(X;14)(p22;q32) (n=18) and t(Y;14)(p11;q32) (n=7) (X or Y unknown, n= 4) and 2 BCP-ALL cell lines with t(Y;14)(p11;q32). The median age of the patients was 18 years (range 4–76 years) and median WBC was 70 ×10⁹/l (range 1–342). Follow-up data was available for 16 patients. Among 5 children diagnosed 1996–2001, 1 failed to remit, 1 suffered an early death and 3 relapsed (16m, 54m, and 64m post diagnosis). To date, all 5 children diagnosed 2004–2005 are in first continuing complete remission (CCR) after 33–46 months, suggesting an improved outcome on the current childhood ALL treatment trial. All 6 adults were diagnosed 2004–2006: 2 died within a year (1 in remission and 1 post transplant); 2 relapsed and died within 1 year and 2 are in 1^st CCR after 20 and 36 months. These observations indicate a variable outcome among adults. Translocation breakpoint cloning from IGHJ by long distance inverse PCR and subsequent FISH mapping identified CRLF2, also known as thymic stromal-derived lymphopoietin receptor, at Xp22 and Yp11, to be juxtaposed to IGH@ in these translocations. Breakpoints were cloned in 11 patients and clustered between 2–27kb centromeric of the 3′ UTR of CRLF2. Expression levels of CRLF2, measured by qRT-PCR in 4 patients and the cell lines, was 500–6,500 fold higher than the control cohort of BCP-ALL patients without the translocation. This overexpression confirmed CRLF2 to be the target gene. CRLF2 and IL2RGC bind to IL7RA to form functional receptors for TSLP and IL7, respectively. However, neither IL2RGC nor IL7RA were aberrantly expressed. The presence of associated genomic copy number alterations was shown by array-based comparative genomic hybridization and FISH. As described for BCP-ALL in general, the significant changes included deletions of the B cell differentiation genes: PAX5 and IKAROS, as well as the cell cycle control gene, CDKN2A. The cell lines revealed some biochemical insights into the functional consequences of this translocation involving PAR1. Cell surface expression of CRLF2 was demonstrated, together with constitutive tyrosine phosphorylation of JAK2 and STAT5, the signaling intermediates of the classical JAK-STAT pathway. Further evidence of increased STAT5 activation came from retroviral infection of normal mouse fetal liver cells with human CRLF2, which sustained the proliferation of B-cell precursors in vitro. Conversely, initial studies of CRLF2 knockdown in the cell lines using shRNA demonstrated decreased proliferation. Together, these results show for the first time, the involvement of both sex chromosomes in an IGH@ translocation in leukemia, resulting in deregulated expression of CRLF2 and constitutive JAK-STAT activation. These data suggest CRLF2 as a potential therapeutic target in this subset of patients.

Laboratories of MJSD, RS and CJH contributed equally to this work.