ETV6: Its Role in Mouse Development and Leukaemogenesis Using Knock-Out and Knock-In Models.

Disruption of the transcription factor ETV6 (TEL) is believed to play a pivotal role in the development of many types of leukaemia. As well as there being more than 15 ETV6 fusion gene partners described to date, there are other genetic alterations of ETV6 including deletions, point mutations and disruption in the ETV6 promoter region that may contribute to the malignant phenotype. We generated two Etv6 knock-out mouse models (Met1 and Met43) to investigate the role of Etv6 in mouse embryonic development and two knock-in models carrying the t(12;21) and t(9;21) fusion genes to investigate the role of the Etv6-RUNX1 and Etv6-ABL fusion oncoproteins in leukaemogenesis. Etv6-Met1^−/− was compatible with full embryonic development. However, from a total of 382 animals generated from the Etv6-Met1^+/− cross only 39 were identified as homozygous suggesting some embryonic lethality. A second knock-out construct (Etv6-Met43) was generated in which the second transcription start site at AA position 43 was removed. Of the 235 offspring genotyped from the Etv6-Met43^+/− cross, no homozygous animals were identified. Further investigation revealed that Etv6-Met43^−/− foetuses lacked definitive haematopoiesis and died between dE10.5–11.5. These data indicate that an Etv6 transcript generated from the methionine at AA position 43 can partially rescue the Etv6-Met1^−/− phenotype. No mice homozygous for the Etv6-RUNX1 were identified from the 628 offspring genotyped from the Etv6-RUNX1^+/− cross. Further investigation revealed that Etv6-RUNX1^−/− foetuses displayed a similar phenotype to the Etv6-Met43^−/− animals and died in-utero at a similar time. Etv6-RUNX1^+/− animals were fertile and remained well for the first year of life. However, after this time in a group of 91 animals it was noticed that among 41 Etv6-RUNX1^+/− animals, the majority were dying from a blastic haematological tumour formation best identified in spleens, at a higher incidence than in the 51 wild-type littermates. Chimeric mice targeted with the Etv6-ABL construct remained well for a short period after birth but all subsequently sickened and died within the first year of life. Upon examination peripheral blood as well as post-mortem organs showed a homogeneous infiltration of granulocytic cells resembling a chronic myeloid proliferation. RT-PCR on RNA isolated from the peripheral blood, bone marrow and spleen of three chimeric animals showed the presence of the fusion gene. Our study demonstrates that Etv6 is essential for mouse embryonic development. The role of Etv6 in malignant transformation is complex and would appear to be dependent on the function of the partner fusion gene rather than on the common Etv6 part of the protein.