DNA sequencing reveals how rare cancer spread between twins before birth

by Sanjukta Mondal, Medical Xpress

Sanjukta Mondal

Author

Meet our staff & contributors
Learn about our editorial standards

edited by Sadie Harley, reviewed by Robert Egan

Sadie Harley

Scientific Editor

Meet our editorial team
Behind our editorial process

Robert Egan

Senior Editor

Meet our editorial team
Behind our editorial process Editors' notes

This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility:

fact-checked

peer-reviewed publication

trusted source

proofread

The GIST Add to Preferred Sources

Two identical sisters came into the world carrying the same rare cancer. They lost their lives to an undifferentiated sarcoma, a soft-tissue cancer driven by a unique gene fusion (MN1::ZNF341). The story began before they were born, when pregnancy imaging picked up a tumor on Twin A's face. Then came the birth, when cancer deposits were found in both Twin A and Twin B. Researchers now had to work out how the cancer reached both girls. Did the cancer start on its own in each sister, or did it begin in one and pass to the other while they were still in the womb?

The researchers sequenced the entire genetic code of healthy and tumor tissues, along with placental samples, to trace the cancer's journey between the twins. All tumor samples from both twins carried a distinctive genetic signature belonging to Twin A.

The cancer likely began as a facial tumor in Twin A at the end of the first trimester, which was visible on scans before birth. The cancer then crossed the placenta to reach Twin B late in the second trimester.

Beyond tracing the cancer's spread, the findings challenged the classical model of how identical twins develop from an early embryo. These findings are published in Nature Communications.

Rare twinning cancer evolution

Identical twins share the same starting point, developing from a single fertilized egg, but it is extremely unusual for both to develop the same cancer. This can happen if each twin develops a tumor independently, particularly when both carry an inherited cancer-causing mutation. In rarer cases, cancer cells from one twin can cross the placenta and establish a tumor in the other while they are still in the womb.

As healthy cells divide, they accumulate subtle genetic marks that act like a cellular record of their history. By examining normal tissues from both twins alongside placental samples, researchers can unlock these genetic clues to retrace the earliest cell divisions and uncover how the embryo split to form identical twins.

In this study, the researchers used a genetic method called DNA lineage tracing, which essentially builds an evolutionary family tree for the cells of both twins, their tumors, and the placenta.

The team collected 23 normal tissue samples—six from each twin and 11 from the placenta—and 10 tumor samples from both twins. They then performed high-depth whole-genome sequencing on all samples to map complete DNA sequences, along with RNA sequencing to verify the specific gene fusion driving the sarcoma.

To trace the cancer's evolution, researchers sorted 212 tumor mutations into 13 distinct genetic clusters, revealing the steady pace at which mutations accumulated in the cells.

It spread from A to B

Every tumor sample from both children carried Twin A's distinctive genetic signature, revealing that the soft-tissue sarcoma originated in Twin A before crossing the placenta to Twin B. The genetic evidence pointed to a single transfer of cancer cells, suggesting that this kind of transmission between twins before birth is extremely rare.

In addition, the researchers uncovered evidence of blood-cell exchange during pregnancy, with about 75% of the cells in Twin A's spleen originating from Twin B.

The findings also raised questions about how identical twins begin to form. The traditional view is that twins sharing a placenta develop when an embryo splits around day six after conception, at the blastocyst stage, when it contains roughly 100 cells. The twins, however, shared only one early mutation, while each carried distinct genetic labels. This pointed to a much earlier split, possibly when the embryo contained as few as four cells.

The sequencing also revealed that early embryonic cells contributed unevenly to the placenta and the two twins, with each twin arising almost entirely from a different set of early cell lineages.

The researchers note that this study does more than follow a cancer's path through the womb. It offers a powerful way to reconstruct the earliest moments of life, from a single fertilized egg to the point where two separate twins take shape.

Written for you by our author Sanjukta Mondal, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you.

Publication details

Barbara Walkowiak et al., Embryonic origin of cancer in newborn twins, Nature Communications (2026). DOI: 10.1038/s41467-026-78178-5

Journal information: Nature Communications

Clinical categories

OncologyClinical geneticsObstetrics & gynecology Who's behind this story?

Sanjukta Mondal

Master's in Chemistry. Freelance science journalist and communicator. Published in Chemistry World, BioSpace, and The Hindu. Full profile →

Sadie Harley

BSc Life Sciences & Ecology. Microbiology lab background with pharmaceutical news experience in oil, gas, and renewable industries. Full profile →

Robert Egan

Bachelor's in mathematical biology, Master's in creative writing. Well-traveled with unique perspectives on science and language. Full profile →

© 2026 Science X Network

Citation: DNA sequencing reveals how rare cancer spread between twins before birth (2026, October 16) retrieved 18 October 2026 from https://medicalxpress.com/news/2026-10-dna-sequencing-reveals-rare-cancer.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.