# Chromatin Engagement and Transcriptional Activity of the ZNF217 Exon 4–Skipping Isoform Are Associated with Breast Cancer Aggressiveness and Bone Metastasis

## TL;DR

This study shows that a specific variant of the ZNF217 gene is linked to aggressive breast cancer and bone metastasis, suggesting it could be a new target for treatment.

## Contribution

The study reveals the functional role of the ZNF217 exon 4-splice isoform in promoting breast cancer aggressiveness and bone metastasis.

## Key findings

- ZNF217-ΔE4 retains transcriptional activity and promotes cancer cell aggressiveness.
- High ZNF217-ΔE4 mRNA levels correlate with increased risk of bone metastases in breast cancer patients.
- ZNF217-ΔE4 accelerates bone marrow micrometastases formation in vivo.

## Abstract

Breast cancer remains a significant global health issue, particularly due to the detrimental effects of bone metastases on patient outcome. Generation of aberrant splicing variants with oncogenic potential contribute to tumor development and progression. While the ZNF217 wild-type oncogenic transcription factor has been well studied, the unique role of the ZNF217 exon 4-splice isoform in breast cancer remains elusive. The aim of this study is to elucidate the chromatin engagement and transcriptional activity retained by this isoform, as well as its role in breast cancer aggressiveness and its contribution to the development of bone metastases. Our study highlights this ZNF217 isoform as an emerging oncogenic player, opening new avenues for its potential as a therapeutic target and biomarker for breast cancers prone to developing bone metastases.

Background: Breast cancer remains a major health issue, with bone metastases negatively impacting patient outcomes. The biochemical and biological functions of the exon 4-splice isoform (ZNF217-ΔE4) of the oncogenic transcription factor ZNF217 have been poorly investigated. Methods/Results: This study, for the first time, elucidates through advanced live-cell single-molecule tracking microscopy that the C-terminus of ZNF217 influences chromatin engagement and binding stability. ZNF217-ΔE4 retains its ability to be recruited and to promote positive transcriptional activity. CRISPR/Cas9-mediated silencing of the ZNF217 gene in MDA-MB-231 breast cancer cells impairs cell aggressiveness, while reintroduction of the ZNF217-ΔE4 isoform is sufficient to restore increased cell proliferation, migration, invasion, and stemness features. In vivo, ZNF217 ΔE4—although less potent than the wild-type isoform—accelerates the formation of bone marrow micrometastases. A retrospective analysis of primary breast tumors revealed that patients with high ZNF217-ΔE4 mRNA levels had a higher risk of developing bone metastases. Conclusions: Overall, this study identifies ZNF217-ΔE4 as a novel functional isoform that mediates breast cancer cell aggressiveness and bone marrow homing. It also highlights this isoform as a promising biomarker and potential therapeutic target for breast cancers at elevated risk of bone metastasis.