# Hypomethylating agents increase L1 retroelement expression without inducing novel insertions in myeloid malignancies

## TL;DR

This study finds that hypomethylating agents used in blood cancers increase retroelement protein expression but do not cause new genetic insertions.

## Contribution

The study is the first to investigate retroelement activity in leukemia and myeloid malignancies under hypomethylating agent treatment.

## Key findings

- Hypomethylating agents increase LINE-1-encoded protein expression in myeloid cell lines.
- No de novo retrotransposition events were detected in cell lines or patient samples after long-term HMA treatment.
- The results suggest that HMAs do not promote new insertional mutagenesis in myeloid malignancies.

## Abstract

Retroelements in the human genome are silenced via multiple mechanisms, including DNA methylation, to prevent their potential mutagenic effect. Retroelement activity, demonstrated by their expression and somatic retrotransposition events, was shown to be deregulated in multiple tumors but not yet in leukemia. We hypothesized that treatment with hypomethylating agents, commonly used in myelodysplastic syndromes and acute myeloid leukemia, could lead to increased retroelement activity and somatic retrotranspositions, thus contributing to disease progression. To address this hypothesis, we induced the expression of ORF1p protein with hypomethylating agents in DAMI and HL‐60 myeloid cell lines. To study whether long‐term hypomethylating agent therapy induces somatic retrotranspositions, we analyzed (i) both cell lines treated for 4 weeks, and (ii) sequential samples from 17 patients with myelodysplastic syndrome treated with hypomethylating agents. Using a sensitive next‐generation sequencing (NGS)‐based method, no retroelement events were identified. To conclude, we show that although hypomethylating agents induce the expression of LINE‐1‐encoded proteins in myeloid cell lines, de novo somatic retrotransposition events do not arise during the long‐term exposure to these agents.

We investigated whether hypomethylating agents (HMAs) used in myeloid malignancies induce somatic retrotransposition. Our findings indicate that HMA treatment increases L1‐encoded protein expression but does not lead to detectable de novo retrotransposition events in either patient samples or cell lines. This suggests HMAs do not promote new insertional mutagenesis.