DNA protection during cell division: the role of DDIAS

DNA protection during cell division involves an unusual trade-off. Just as chromosomes are preparing to separate into two daughter cells, many of the cell's conventional DNA-repair pathways are temporarily suppressed. Any damage that remains at this stage can therefore be particularly dangerous.

A study published in Cell identifies another layer of protection. The protein DDIAS binds exposed stretches of single-stranded DNA and shields them from further damage.

That role turned out to matter far beyond chromosome maintenance. Loss of DDIAS was linked to a severe neurodevelopmental disorder, while cells deficient in the tumour-suppressor genes BRCA1 or BRCA2 became unusually dependent on DDIAS for survival.

Protecting DNA when repair is difficult

Before a cell divides, it copies its genome. Replication does not always finish perfectly, however, and DNA lesions or unresolved replication intermediates may remain.

Once the cell enters mitosis and starts separating its chromosomes, attempting extensive DNA repair can itself interfere with chromosome segregation. Many conventional repair pathways and damage checkpoints are therefore restricted during this phase.

Previous research had identified a mitotic damage-response pathway involving the proteins TOPBP1 and CIP2A. The new work places DDIAS as another core component of this system.

The researchers found that DDIAS accumulates at damaged regions of mitotic chromosomes and binds single-stranded DNA — stretches in which one strand of the usual double helix is exposed.

Its job appears to be less like a repair enzyme and more like a protective cover. DDIAS does not simply repair the lesion. Instead, it prevents exposed DNA from being processed inappropriately by other enzymes, including the nuclease DNA2.

Without this protection, chromosome instability increases and cells form more micronuclei — small DNA-containing structures produced when chromosomes or chromosome fragments fail to enter the main nucleus correctly.

Why developing brain cells are vulnerable

The study was prompted in part by two individuals carrying inactivating variants in both copies of DDIAS. They showed features of a severe neurodevelopmental disorder, including microcephaly, developmental delay and intellectual disability.

Patient-derived cells showed increased chromosome abnormalities. Reintroducing normal DDIAS reduced those abnormalities, supporting a direct role for the protein in maintaining chromosome integrity.

The researchers then tested the mechanism in several experimental models. In human cerebral organoids — three-dimensional cellular models of early brain development — loss of DDIAS impaired growth and increased DNA damage.

Neural progenitor cells, the rapidly dividing cells that give rise to the nervous system, appeared particularly sensitive. The same basic biological importance was also seen in zebrafish, where loss of DDIAS produced a microcephaly-like phenotype.

This does not mean that DDIAS alone determines brain size. The more specific conclusion is that developing neural cells have an unusually strong requirement for protecting unresolved DNA damage while they divide.

A possible weakness in BRCA-deficient cancer cells

The study also points to a potentially important cancer connection.

Cells lacking functional BRCA1 or BRCA2 already have a major defect in homologous recombination, one of the pathways used to accurately repair DNA. Such cells can accumulate more replication-associated lesions and may therefore rely heavily on mechanisms that protect damage carried into mitosis.

That is what the researchers observed. BRCA1- or BRCA2-deficient cells were particularly dependent on DDIAS. Removing both forms of protection produced substantially more chromosome instability and reduced cell survival.

This resembles synthetic lethality: a cell can tolerate disruption of either of two protective mechanisms individually, but losing both becomes lethal.

In principle, that creates an attractive therapeutic idea. If BRCA-deficient tumour cells depend unusually strongly on DDIAS, disrupting the DDIAS pathway might selectively expose a weakness in those cancers.

But this remains a hypothesis for therapy, not a treatment. The study did not test a DDIAS-targeting drug in patients and does not establish clinical efficacy. The cancer experiments reveal a cellular dependency that now needs to be investigated further.

The broader finding is more fundamental. Cells do not always need to repair DNA immediately. During mitosis, when extensive repair can be dangerous, simply shielding a vulnerable DNA structure may be enough to preserve it until the cell is in a safer state. DDIAS appears to provide precisely that kind of temporary protection — and developing brain cells may depend on it especially strongly.