Genetic Events in Ikaros Gene Family (IKZF1 and IKZF2) in Pediatric T-Cell Lymphoblastic Leukemia/Lymphoma

Abstract 4622

The Ikaros gene family, which includes Ikaros (IKZF1), Helios (IKZF2), and Aiolos (IKZF3), is a primary regulator of lymphocyte differentiation and is involved in the development of acute lymphoblastic leukemia (ALL). In particular, loss of IKZF1 is primarily associated with T-cell ALL (T-ALL) development in mouse models. Moreover, Helios expression has been detected in early T-cell lineage and is involved in human lymphoid proliferative disease. We sought to determine whether the Ikaros gene family is associated with leukemogenesis in T-ALL and T-cell non-Hodgkin lymphoma (T-NHL) in Japanese children.

Fifty-eight Japanese children (39 boys and 19 girls; age, <18 years) with T-ALL (n = 50) and T-NHL (n = 8) were evaluated. The median age at diagnosis was 8 years (1–15 years). Samples were collected at Japanese Nagoya First Red Cross Hospital and Nagoya University Hospital. Blasts from bone marrow (n = 40), peripheral blood (n = 9), pleural effusions (n = 6), and lymph nodes (n=3) were subjected to DNA extraction. All coding exons in IKZF1 were amplified by polymerase chain reaction using the genomic DNA of patients with T-ALL/T-NHL. After amplification using a BigDye Terminator Cycle Sequencing kit (Life Technologies, Carlsbad, CA, USA) in the second step, products were analyzed in an ABI/PRISM 3130xl Genetic Analyzer (Life Technologies). Multiplex ligation-dependent probe amplification (MLPA) analyses were performed using a SALSA MLPA reaction mixture according to the manufacturer's protocol [P202 kit; IKZF1(IKAROS); MRC-Holland, Amsterdam, Netherlands]. Amplification products were quantified and identified by capillary electrophoresis on an ABI/PRISM 3130xl Genetic Analyzer. Informed consent was received from parents of the patients. Approval for this study, including molecular analyses, was obtained from the institutional ethics committee of Nagoya University Graduate School of Medicine.

An IKZF1 mutation was found in only 1 of 59 (2%) patients (exon 5, c.476 A>G, p.N159S; patient number 41) who was diagnosed with primary immune deficiency after birth. She received regular immunoglobulin replacement therapy just before the development of T-ALL at 13 years of age. Her IKZF1 mutation was confirmed as a germline mutation by sequencing of genomic DNA from her fingernails. In the IKZF1 null mouse model, Ikaros inactivation is closely linked to T-ALL development, which is highly associated with acquisition of a somatic NOTCH1 mutation. As in the mouse model, we could identify a somatic NOTCH1 mutation (exon 26, c.4750, insGAG) in the sample upon T-ALL development. IKZF1 mutations were not found in the other 57 samples. This finding implied that the IKZF1 mutation was a rare genetic event in de novo T-ALL/T-NHL in children. Furthermore, we analyzed all samples by MLPA to determine if monoallelic genomic deletions were present. Although 59 samples exhibited normal biallelic IKZF1 and IKZF3 chromosomal regions, we identified IKZF2 microdeletions in 2 (3.4%) patients (numbers 9 and 49). One of them with an IKZF2 microdeletion died of leukemia relapse after allogeneic bone marrow transplantation from a human leukocyte antigen (HLA)-matched sibling donor. Overexpression of full-length Helios (IKZF2) blocks αβ T-cell differentiation at the CD4-CD8- stage in the thymus, and results in the increased frequencies of γδ T and natural killer (NK) cells in peripheral lymphoid organs. Overexpression of dominant negative Helios leads to increased T-cell proliferation upon T-cell receptor stimulation and the development of T lymphoma. Furthermore, dominant negative Helios isoforms or allelic losses have been detected in some human T-ALL/T-NHL. These results suggest that Helios is an essential regulator of T-cell homeostasis and a tumor suppressor.

In general, IKZF1 mutation or deletion is a rare event in pediatric patients with T-ALL/T-NHL. We could identify a secondary T-ALL patient after primary immunodeficiency who shared genetic findings with an IKZF1 null mouse model (germline IKZF1 mutation and somatic NOTCH1 mutation). We found IKZF2 microdeletion in 2 of 59 patients. The genetic events in the Ikaros gene family (IKZF1 and IKZF2) are associated with leukemogenesis in a few pediatric patients T-ALL/T-NHL.