Nature Cancer: Chinese scientists discover signs that "cancer king" is sensitive to "synthetic lethality"

RNA，RNA6（m6A）2011，[1]
。RNA m6A[2-3]，METTL16[4-5]
。

，（Mayo Clinic）、，，《·》METTL16[6]
。

，METTL16RNA，，MRE11，DNA
。DNA，METTL16ATM，，RNA，MER11METTL16
。

，，METTL16PARP，
。

FDA[7]
。，10%，
。，METTL16DNA，PARP
。

，、，
。

RNA m6ADNA[8-9]，m6A（METTL3-METTL14，METTL16）DNA
。

DNA（DSB）METTL3-METTL14，METTL16，，METTL16DSB
。，DSB（DR-GFP/EJ5-GFP），HEK293TMETTL16（HDR）DNA
。METTL16DNA，
。

METTL16DSB

METTL16METTL16，DSB
。

，8，，METTL16（KO）γ-H2AX（DSB）
。，4，KODNA
。METTL16，γ-H2AXDNA
。

，METTL16DNA
。

METTL16DNA，DSB

，METTL16ATM/ATR（DNA）（Ser419Ser455）
。，METTL16ATM/ATR
。
。、，ATMMETTL16Ser419，HDR
。

ATMMETTL16HR

HDRDNA，（end resection）、3’-（strand invasion）、Holliday（resolution of Holliday Junction）
。，METTL16HDR？

The researchers found that after 2 hours of radiation treatment, compared with normally expressed METTL16, METTL16 knockout cells had significantly increased levels of proteins RPA32 and RAD51, and reexpression of METTL16 could reverse these changes
.

The protein RPA32 is a single-stranded DNA-binding protein that binds to 3' single-stranded proteins produced during DNA replication or in the event of DNA damage, thereby acting as a stabilizing single-strand, while an increase in RPA32 staining levels means an increase
in 3'-single-stranded strands in cells.
This suggests that METTL16 inhibits end-of-strand excision
.

In addition, the results of co-immunoprecipitation showed that METTL16 interacted with MRE11 in the key complex MRN-CtIP, a key complex of chain end resection, and radiation treatment could reduce this interaction
.

These results suggest that METTL16 is able to inhibit strand-end excision
during HDR repair DNA by interacting with MRE11.

METTL16 inhibits the chain end excision step in HDR by interacting with MRE11

The question then becomes, how does METTL16 interact with MRE11?

Starting with the two DNA-binding domains present on MRE11 and the four RNA-binding motifs present in METTL16, the researchers demonstrated through DNase/RNase treatment and a series of truncated mutation experiments that METTL16 interacts with MRE11 through RNA bound to these two
.
In addition, the results of extracellular nuclease experiments showed that METTL16-RNA complex inhibited MRE11 nuclease activity
after binding to MRE11.

METTL16 interacts with MRE11 via RNA and inhibits the latter's nuclease activity

As mentioned at the beginning, the main function of METTL16 is to modify
RNA with m6A.
However, the researchers found that compared with wild-type METTL16, METTL16 mutants (F187G), which lost methyltransferase activity, could still interact with MRE11 and inhibit chain end resection, thereby inhibiting HDR efficiency
.
That is, the interaction between METTL16 and MRE11 does not depend on the methyltransferase activity
of METTL16.

In addition, the presence or absence of m6A modifications in the RNA that mediates the interaction between METTL16 and MRE11 has no effect
on this interaction.
On the contrary, the phosphorylation of METTL16 by ATM will reduce its ability to bind to RNA, thereby reducing the interaction
between METTL16 and MRE11.

METTL16's binding to RNA and interaction with MRE11 do not depend on METTL16's methyltransferase activity or RNA's m6A modification

So far, the researchers have determined the role of METTL16 in HDR: after DNA damage activates ATM, ATM phosphorylates METTL16 and causes its conformational change, so that RNA is separated from its original bound state on METTL16, which in turn leads to reduced interaction between METTL16 and MRE11, which is released from the complex and participates in HDR to repair DNA damage
.

Considering the inhibitory effect of METTL16 on HDR and the synthetical lethality of PARP inhibitors on HDR-deficient tumors, the researchers analyzed the killing effect of PARP inhibitor olaparib on BRCA1/2 wild-type pancreatic cancer cells expressing different levels of METTL16
.

The results showed that cell lines with high METTL16 expression were more sensitive to olaparib, knocking out METTL16 in the background of high METTL16 expression reduced the sensitivity of cells to olaparib, and overexpression of METTL16 in the background of low expression METTL16 increased the sensitivity
of cells to olaparib.
This positive correlation between METTL16 expression levels and sensitivity to olaparib has also been demonstrated
in mouse pancreatic cancer models.

METTL16 expression levels in pancreatic cancer cells and tumors were positively correlated with sensitivity to olaparib

Finally, the researchers evaluated the effect
of METTL16 expression levels on the survival of pancreatic cancer patients and on the effectiveness of clinical therapy for pancreatic cancer.

The results showed that the expression level of METTL16 in pancreatic cancer patients was positively correlated
with their survival.
In addition, animal model results showed that tumors with high METTL16 expression were more sensitive to gemcitabine monotherapy and olaparib monotherapy, and tumor development was slowed down
.
In tumors with high METTL16 expression, the combination of gemcitabine + olaparib can cause tumor recession and significantly prolong the survival of
mice.

Pancreatic cancer patients with high METTL16 expression have better survival, which also indicates higher sensitivity to pancreatic cancer treatment drugs

Overall, this study identified a novel function of METTL16, namely that METTL16 inhibits homologous recombination repair
of DNA by interacting with MRE11, a key enzyme for DNA homologous recombination.

At the same time, the study revealed that the level of METTL16 is a potential prognostic indicator of pancreatic cancer, and the combination of gemcitabine + olaparib may be an effective strategy
for the treatment of pancreatic cancer with high METTL16 expression.

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