Retroviral Insertional Mutagenesis In Egr1^+/- mice, Haploinsufficient For a Human Del(5q) Myeloid Leukemia Gene, Develop Myeloid Neoplasms With Proviral Insertions In Genes Syntenic To Human 5q

Therapy-related myeloid neoplasm (t-MN) is a distinctive clinical syndrome occurring after treatment with chemotherapy and/or radiotherapy, typically for a primary malignant disease. Loss of the long arm of chromosome 5, del(5q), is the most common recurring cytogenetic abnormality and is observed in 40% of t-MN patients, as well as 10-15% of patients with primary MDS or AML de novo. These deletions typically encompass over 70Mb [spanning 5q14-q33] and numerous genes, making the identification of relevant del(5q) genes very challenging. In previous studies, we identified a commonly deleted region within 5q and identified Egr1 as a del(5q) haploinsufficient myeloid leukemia gene. Using Egr1^+/- mice, we previously showed that Egr1 cooperates with mutations, induced by the alkylating agent, ENU, to induce a myeloproliferative disorder with ineffective erythropoiesis (MPD). However, loss of Egr1 on its own was not sufficient for the development of MPD. To identify cooperating mutations in MPDs in Egr1 haploinsufficient mice, we conducted a retroviral insertional mutagenesis (RIM) screen. Egr1 WT (n=61) and Egr1^+/- (n=77) neonates were injected with the MOL4070LTR retrovirus. Although the overall survival of MOL4070LTR-treated Egr1^+/- and WT controls was similar, Egr1^+/- mice developed MPD or AML with a shorter latency and at a higher overall frequency than WT littermate controls. Forty-six percent of WT mice developed myeloid disease versus 61% for Egr1^+/- mice, with a median survival of 474 d for Egr1 WT and 389 d for Egr1^+/- mice (p=0.03).

We mapped the retroviral integration sites in myeloid neoplasms from 29 WT and 46 Egr1^+/- mice using barcoded splinkerette PCR and Illumina high-throughput sequencing. To identify and analyze the statistically significant common insertion sites (CISs) we used the TAPDANCE software developed by A. Sarver. In total, 159 CISs were identified in WT mice and 365 CISs were identified in Egr1^+/- mice. Several of these CIS-associated genes, such as Sox4, Pim1 and Myb have been previously identified in other genetic screens according to the Retroviral Tagged Cancer Gene Database (RTCGD). As the main goal of this study is to identify mutations that cooperate with Egr1 haploinsufficiency, we were particularly interested in CISs that were identified exclusively or more frequently in Egr1^+/- mice with myeloid neoplasms. The TAPDANCE software automatically identifies associations between phenotypes and CISs using Fisher’s exact test with multiple testing correction. Using this analysis, we identified six CISs that were statistically associated with myeloid neoplasms in Egr1^+/- mice, but not WT mice. The candidate cancer genes proximal to these proviral insertions included Evi1, Gfi1, Evi5, and Cd47. Analysis of transcript levels revealed elevated expression of Evi1, but not Gfi1, Evi5, or Cd47 in myeloid leukemias with proviral insertions proximal to these genes.

Of interest was a CIS associated with Egr1^+/- mice that mapped to a region of mouse chromosome 18 that is syntenic to human 5q31.2, proximal to the Dnajc18, Ecscr, Tmem173, Cxxc5 and Psd2 genes and adjacent to the commonly deleted region. Moreover, an analysis of co-occurring CISs revealed that this CIS co-occurred with a CIS that mapped to a region of mouse chromosome 13 that is syntenic to human 5q31.1, also deleted in t-MN patients and proximal to the Tifab, and H2afy genes. Of the genes in these two CISs, CXXC5, TMEM173, TIFAB, and H2AFY each show significantly decreased expression in bone marrow cells from t-MN del(5q) patients, consistent with haploinsufficiency. Identification of the relevant del(5q) genes in t-MN continues to be a challenge for developing therapeutic targets. Loss of expression of the tumor suppressor gene, EGR1, which is expressed at haploinsufficient levels in t-MN patients with a del(5q), on its own is not sufficient for the development of myeloid leukemia. Here we performed a forward genetic screen with Egr1^+/- mice and have identified several candidate del(5q) genes, including CXXC5, TMEM173, TIFAB, and H2AFY, that should now be evaluated as candidate genes that cooperate with EGR1 haploinsufficiency in the pathogenesis of t-MN. The identification of aberrant pathways resulting from haploinsufficiency of EGR1 in cooperation with these del(5q) genes may potentially lead to the new therapeutic targets for t-MNs with chromosome 5 abnormalities.