# Metabolism of Epigenetic Ribonucleosides Leads to Nucleolar Stress and Cytotoxicity

## TL;DR

This study investigates how modified RNA building blocks affect cell health and metabolism, revealing that some modifications cause cell stress and damage.

## Contribution

The paper reveals how bulky N6-modified adenosines cause cytotoxicity and RNA misincorporation, while smaller modifications are restricted by cellular mechanisms.

## Key findings

- N6-modified adenosines like i6A cause high cytotoxicity and RNA misincorporation.
- Small N6-modified adenosines like m6A are restricted by enzymatic deamination.
- Ribopyrimidines show cytotoxicity dependent on UCK2 at higher concentrations.

## Abstract

Post-transcriptional RNA modifications are ubiquitous
in biology,
but the fate of epigenetic ribonucleotides after RNA turnover and
the consequences of their metabolism and misincorporation into nucleic
acids are largely unknown. Here, we explore epigenetic ribonucleoside
metabolism in human cells by studying effects on cell growth, quantifying
RNA misincorporation and identifying metabolic regulators, and exploring
phenotypes associated with cytotoxicity. We find that bulky N6-modified adenosines (i.e., i6A) exhibit high levels
of cytotoxicity and RNA misincorporation, whereas cells dramatically
restrict the misincorporation of small N6-modified adenosines
(i.e., m6A), partly through sanitization by enzymatic deamination,
consistent with a recent report. Epigenetic ribopyrimidines also exhibit
cytotoxicity, dependent on nucleoside kinase UCK2, but only at much
higher concentrations than ribopurines. We further characterize the
effects of cytotoxic ribonucleoside metabolism on nucleolar morphology
and protein translation. Taken together, our work provides new insights
into the metabolism of epigenetic ribonucleosides and mechanisms underlying
their cytotoxicity to cells.