New Factor DDIAS Identified as Protecting DNA During Mitosis, Raising Potential as a Target in BRCA-Deficient Cancers

A new protein factor that protects damaged DNA during cell division has been identified. The factor also plays a role in safeguarding neural development and is expected to become a new target for attacking cancer cells with BRCA gene defects.

According to the international journal Cell on the 28th, Japanese researchers said they had identified the function of a protein called 'DDIAS' that protects sites of DNA damage during mitosis (somatic cell division).

DDIAS is the third core component of the TOPBP1-CIP2A complex. The complex is known to preserve chromosome integrity by holding damaged chromosome fragments together during cell division.

The key feature is DDIAS's ability to bind single-stranded DNA (ssDNA). Neither TOPBP1 nor CIP2A has a DNA-binding site, leaving the question of what separate factor connects them to sites of damage unresolved.

The researchers explained that DDIAS confers ssDNA-binding activity on the complex through its N-terminal OB-fold structure. This activity is regulated by the enzymes CDK1 and PLK1 and operates specifically during mitosis.

DDIAS protects sites of DNA damage from processing by aberrant nucleases. DNA repair enzymes must be tightly regulated to prevent inappropriate or premature activation, and DDIAS is one part of this regulatory system, the researchers said.

During interphase (the period between cell divisions), the RAD51 protein prevents aberrant degradation of ssDNA, but RAD51 loading is suppressed during mitosis. The researchers believe DDIAS takes over this protective function during mitosis.

This is similar to how the Shieldin complex uses its OB-fold-containing SHLD2 subunit during interphase to prevent DNA end resection.

The researchers also found that DDIAS does not make a major contribution to previously known pathways for maintaining the mitotic genome, including MiDAS, DNA end tethering and POLQ-dependent MMEJ repair. Instead, it has a specialized role in protecting ssDNA damage.

The finding is notable because it is directly linked to the survival of BRCA1- and BRCA2-deficient cancer cells. BRCA-deficient cells are known to accumulate unreplicated regions, ssDNA gaps and DNA double-strand breaks during replication.

Synthetic lethality between BRCA deficiency and DDIAS loss—the phenomenon in which cells die only when both defects are present—suggests that DNA damage arising during S phase may be a target of DDIAS. This could provide a starting point for a new treatment strategy targeting BRCA-deficient cancers.

The researchers also demonstrated DDIAS's role in neural development. DDIAS loss reduced the growth of human brain organoids (mini-organs) and selectively increased DNA damage and chromosomal instability in neural progenitor cells (NPCs).

In zebrafish (Danio rerio), it caused a phenotype resembling microcephaly. Expression of normal human DDIAS rescued the phenotype, but the 3A mutant, which has a mutation affecting TOPBP1 binding, did not.

This is interpreted as evidence that DDIAS's role in neural development depends on its function in the TOPBP1-CIP2A mitotic pathway.

It is not yet clear why neural progenitor cells are particularly vulnerable to DDIAS loss. The researchers considered proliferation rate alone an insufficient explanation. Neural progenitor cells have been reported to be hypersensitive to apoptosis and to have a high endogenous DNA break burden.

Indeed, recurrent DNA double-strand breaks were found to be concentrated in long neural genes in mouse neural progenitor cells. This is thought to result from transcription-replication conflicts. As a result, neural progenitor cells may be more likely than other cells to enter mitosis with unreplicated regions and DNA breaks, making them more dependent on DDIAS.

The study is significant because it provides the first evidence that disruption of the mitotic DNA damage response can cause human developmental disorders. The researchers identified biallelic inactivating mutations in the DDIAS gene in two patients with clinical features resembling a chromosomal instability syndrome.

Many human diseases caused by defects in interphase DNA repair pathways have been reported, but no genetic disease associated with abnormalities in the mitotic DNA damage response had been reported to date.

However, pathogenic DDIAS variants have so far been identified in only one family. The researchers said validation through the identification of more patients is needed.

The researchers cited as limitations the fact that the molecular relationship between DDIAS and DNA end-resection factors such as RPA and DNA2 has not yet been established, and that the oligomerization states of TOPBP1, CIP2A and DDIAS remain unclear.

They also did not rule out the possibility that DDIAS functions in a context-dependent manner in specific cell types, in response to other types of damage or during later stages of mitosis. The researchers explained that because they measured the loss of γH2AX foci only in early metaphase cells, they could not rule out a role for DDIAS in repairing DNA double-strand breaks during later mitotic stages.

The study was conducted by Kaima Tsukada and colleagues and published in Cell.