# The common murine retroviral integration site activating Hhex marks a distal regulatory enhancer co-opted in human early T-cell precursor leukemia

## TL;DR

This study identifies a regulatory region near the Hhex gene that is activated by retroviral integration and plays a role in a type of human leukemia.

## Contribution

The discovery of a new enhancer region near HHEX that is co-opted in human ETP-ALL pathogenesis.

## Key findings

- Retroviral integration 57-61kb 3′ of Hhex activates Hhex gene expression in mouse T-ALL models.
- The +65kb CRE interacts with the HHEX gene in human ETP-ALL and normal progenitor cells.
- The LMO2/LDB1 complex binds to the CRE and contributes to HHEX activation in leukemia.

## Abstract

The Hhex gene encodes a transcription factor that is important for both embryonic and post-natal development, especially of hematopoietic tissues. Hhex is one of the most common sites of retroviral integration in mouse models. We found the most common integrations in AKXD (recombinant inbred strains) T-ALLs occur 57-61kb 3′ of Hhex and activate Hhex gene expression. The genomic region of murine leukemia virus (MLV) integrations has features of a developmental stage-specific cis regulatory element (CRE), as evidenced by ATAC-seq in murine progenitor cells and high H3K27 acetylation at the syntenic CRE in human hematopoietic cell lines. With ChIP-exonuclease, we describe occupancy of LIM domain binding protein 1 (LDB1), the constitutive partner of the LIM Only-2 (LMO2), GATA1, and TAL1 transcription factors at GATA sites and at a composite GATA-E box within the CRE. With virtual 4C analysis, we observed looping between this +65kb CRE and the proximal intron one enhancer of HHEX in primary human ETP-ALLs and in normal progenitor cells. Our results show that retroviral integrations at intergenic sites can mark and take advantage of CREs. Specifically, in the case of HHEX activation, this newly described +65kb CRE is co-opted in the pathogenesis of ETP-ALL by the LMO2/LDB1 complex.