Articles

Mouse lymphoma assay & TK6 (OECD 490): the mammalian gene-mutation test

5 min read

Bacteria are excellent reporters of gene mutation — which is why the Ames test is a battery cornerstone — but they are not human, and they cannot register the larger-scale genetic damage that mammalian chromosomes can suffer. To measure mutation in a mammalian cell, and to catch both small and large genetic changes at once, toxicologists turn to the gene-mutation tests of OECD 490 — most famously the mouse lymphoma assay (MLA).

OECD Test Guideline 490 covers in vitro mammalian cell gene-mutation tests that use the thymidine kinase (TK) gene, in either mouse lymphoma L5178Y cells (the MLA) or human TK6 cells. This article explains what makes the assay distinctive, how it reads two mechanisms in one experiment, and where it fits relative to the Ames and HPRT tests.

Key points

What it measures

Forward mutation at the thymidine kinase (TK) locus in mammalian cells.

Two mechanisms, one test

Colony size distinguishes point mutations (large colonies) from chromosomal-level damage (small colonies).

With and without S9

Metabolic activation is included to detect mutagens that require bioactivation.

MLA or TK6

OECD 490 covers both the mouse lymphoma (L5178Y) and human TK6 versions of the assay.

Reading two mechanisms at once

The defining feature of the TK gene-mutation assay is its broad mutational spectrum. Cells that lose thymidine kinase function survive selection with trifluorothymidine (TFT) and grow into colonies — and crucially, the size of those colonies is informative:

  • Large colonies generally reflect point mutations and small intragenic changes, where the cell keeps growing normally.
  • Small colonies reflect larger-scale damage — deletions, mitotic recombination and chromosomal events affecting the TK locus and neighbouring genes, which slow growth.

This means a single MLA can signal both gene mutation and clastogenic-type events — a breadth that neither the Ames test nor the HPRT assay offers on its own.

At a glance

OECD 490

In vitro mammalian cell gene-mutation test at the TK locus

L5178Y / TK6

The mouse lymphoma and human cell lines the guideline covers

TFT

Trifluorothymidine — the agent that selects for TK-deficient mutants

±S9

Run with and without metabolic activation

How the assay is run

Cultures are exposed to a range of concentrations of the test item, with and without S9, then allowed an expression period so that mutant phenotypes emerge. Cells are plated in the presence of TFT; only TK-deficient mutants form colonies. Mutant frequency is calculated against concurrent cytotoxicity, and — in the MLA — colonies are sized to separate the two mechanistic classes. Robust acceptance criteria (positive and negative controls, cytotoxicity limits, historical control ranges) keep the result defensible.

MLA, Ames and HPRT: how they differ

  • The Ames test (OECD 471) measures gene mutation in bacteria — sensitive and efficient, but prokaryotic and blind to mammalian chromosomal effects.
  • The HPRT assay (OECD 476) measures gene mutation in mammalian cells but, because of the locus involved, detects essentially point mutations only — it misses large deletions.
  • The TK assay / MLA (OECD 490) detects both point mutations and larger deletions and chromosomal events, thanks to the autosomal TK locus and colony sizing.

That breadth is why the MLA is often selected as the mammalian gene-mutation component when a wider mutational net is wanted.

Where it fits in the battery

Within ICH S2(R1), the standard battery relies on the Ames test for bacterial gene mutation and an in vitro test for chromosomal damage. A mammalian cell gene-mutation test such as the MLA is used where additional or mammalian-specific mutation data are needed — for example when bacterial assays are poorly suited to a compound, or when the broad spectrum of the TK locus is advantageous.

Frequently asked questions

What is the mouse lymphoma assay?

It is an in vitro mammalian cell gene-mutation test (OECD 490) that measures forward mutation at the thymidine kinase locus in L5178Y mouse lymphoma cells. Colony size distinguishes point mutations from larger chromosomal-level damage.

How does the MLA differ from the Ames test?

The Ames test measures gene mutation in bacteria; the MLA measures it in mammalian cells and additionally captures larger deletions and chromosomal events. They are complementary rather than interchangeable.

MLA or HPRT — which mammalian gene-mutation test?

The HPRT assay (OECD 476) detects essentially point mutations only. The MLA (OECD 490) detects both point mutations and large deletions, giving a broader mutational spectrum, which is often why it is chosen.

What do small and large colonies mean?

In the MLA, large colonies mostly reflect point mutations, while small colonies reflect larger-scale damage such as deletions and chromosomal events. Sizing colonies therefore adds mechanistic insight.

Design your gene-mutation study with us

GenEvolutioN performs GLP mammalian cell gene-mutation testing and will help you choose between the MLA, HPRT and bacterial approaches based on your compound and regulatory route — as part of a coherent, defensible battery.

See how it complements the Ames test and the wider genotoxicity battery, or talk to our team.