Philadelphia-like acute lymphoblastic leukemia (Ph-like ALL) is a type of B-cell ALL that has a similar gene expression profile to Ph-positive ALL, however, no Philadelphia chromosome is present. Genetically, this is a very heterogeneous disease. Several different alterations that lead to deregulation of cytokine receptors and tyrosine kinases can be found. This makes the diagnosis of Ph-like ALL challenging yet of vital importance for treatment selection. It requires a systematic approach and use of various methods. Alterations in the CRLF2 gene are the most common alterations in Ph-like ALL.
CRLF2 gene is located in the pseudoautosomal region 1 on the short arms of chromosomes X and Y. It encodes a member of the type I cytokine receptor family, which is a part of the receptor for thymic stromal lymphopoietin. The activation of this receptor leads to the activation of several signaling pathways that control processes such as cell proliferation and development of the hematopoietic system. Alterations in the CRLF2 gene lead to uncontrolled proliferation and survival of B-cell progenitors.
The purpose of this project was to determine the frequency of CRLF2 rearrangements in Slovenian patients with B-ALL and to evaluate the usefulness of FISH analysis in identification of Ph-like ALL patients. The rearrangements were detected using a fluorescently labeled DNA probe Cytocell CRLF2 Breakapart Probe which detects two CRLF2 rearrangements on the short arms of the sex chromosomes.
We determined the cut-off value to be 3,8 % for the translocation (usually results in IGH-CRLF2) and 3,8 % for the deletion (results in P2RY8-CRLF2) as well. We analyzed 49 bone marrow samples from Slovenian B-ALL patients that did not have recurring genetic alterations. CRLF2 rearrangements were found in four samples which accounts for 8,2 % (4/49) of all analyzed samples and 3 % (4/133) of all Slovenian B-ALL patients (from January 2012 to June 2019). Three patients carried a translocation and one carried a deletion. One patient with the translocation also had a slightly higher CRLF2 gene expression (detected by flow cytometry).
FISH analysis proved to be efficient in identification of Ph-like ALL patients and its introduction into routine diagnostics is simple. We suggest the use of both flow cytometry and FISH to identify patients with CRLF2 rearrangements. We also suggest an introduction of the CRLF2 DNA probe into the DNA probe choice algorithm for B-ALL patients. This probe should be used, if all other recurring alterations are ruled out.
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