Cell: Chinese scientists led the analysis of the three-dimensional structure of human MCM2-7 complex, which is expected to develop safer anti-cancer drugs

In a new study, researchers from the University of Hong Kong, the Hong Kong University of Science and Technology, the Hong Kong Polytechnic University, and the Institut Curie in France have discovered a novel mechanism
by which the human MCM2-7 complex regulates replication initiation.
The discovery could help develop a new effective anti-cancer strategy that has the potential to selectively kill cancer cells
.
The results of the study were published in the January 5, 2023 issue of the journal Cell under the title "The human pre-replication complex is an open complex"
.

Human life begins with a fertilized egg
in the mother's womb.
This fertilized egg multiplies through cell division and develops into our multicellular body
.
During each cell division, our genomic DNA, the blueprint of genetic information, is accurately replicated
.
Each cell carries about 2 meters of DNA, assembled into 23 pairs of chromosomes
.
Over the course of our lifetime (about 70 years), our bodies will synthesize DNA that is more than a light-year--- light travels --- long distances in a year, about 1,016 meters
.
This replication process requires the DNA duplex to first be dissolved and then separated into two single-stranded templates for DNA polymerase synthesis into complementary strands
.
Any misregulation of this process can lead to dire consequences, such as tumorigenesis and inherited genetic diseases
.

Dr Yuanliang Zhai, co-corresponding author and Assistant Professor in the School of Biological Sciences at the University of Hong Kong, said, "Uncovering the secrets of DNA replication is key to
understanding the mysteries of life.
Interpreting the structure of a DNA replication machine is key to informing its molecular function, after all, seeing is believing
.
”

Since James Watson and Francis Crick determined the structure of DNA in 1953, how the DNA double strands were first untied has been a perennial question
for biologists 。 In eukaryotes, the enzyme responsible for untying the double strands of DNA during replication was first identified by Professor Bik-Kwoon Tye of Cornell University in 1983 as a microchromosome maintenance protein complex (MCM) gene
from Saccharomyces cerevisiae 。 The products of the six MCM genes --- MCM2 to MCM7 (MCM2-7) --- form a cyclic complex of six subunits that serve as the catalytic core
of the DNA melting machine --- DNA replication helinase ---.

In cells, in order to initiate DNA replication, the MCM2-7 complex must first assemble into a head-to-head double hexamer (DH)
that surrounds double-stranded DNA on thousands of sites on each chromosome.
Of the massively assembled MCM2-7 DH, only one of the subsets is finally selected and converted into powerful replicating helicase for DNA unwinding
.
It is thought that MCM2-7 DH can directly destabilize DNA to trigger the initial opening
of double-stranded DNA.
However, its underlying mechanisms remain largely unknown
.

To solve this problem, the authors sought to use a cutting-edge technique --- cryo-electron microscopy (cryo-EM) to observe the atomic details
of MCM2-7 DH.

In 2015, they first resolved the 3.
8 angstrom cryo-EM structure of MCM2-7 DH isolated from yeast (Nature, 2015, doi:10.
1038/nature14685).

Unfortunately, the captured DNA was unstable and failed to inform MCM2-7 DH of the double-stranded status of the DNA
.

In the new study, they successfully purified MCM2-7 DH from human cells cultured in vitro and determined its structure
at a resolution of 2.
59 angstroms.
This high-resolution structure clearly demonstrates how the MCM2-7 complex disrupts DNA stability, resulting in the initial opening
of the DNA double strand at the junction of two conjugated MCM2-7 hexamers.

They also found that MCM2-7 DH binds to DNA at tens of thousands of sites throughout the human genome that are mutually repellent
with active transcription sites.
In addition, when this initial open structure is disturbed, MCM2-7 DH can no longer be assembled onto DNA, resulting in complete inhibition
of initiation of DNA replication.

Image from Cell, 2023, doi:10.
1016/j.
cell.
2022.
12.
008
.

Dr Shangyu Dang, co-corresponding author and Assistant Professor in the School of Biological Sciences at the University of Hong Kong, said, "This atomic-resolution cryo-EM structure allows us to directly observe the initial DNA melts, which is critical
for our understanding of the molecular mechanisms underlying DNA replication.
The new study also shows the importance of
collaboration.
The efforts of research teams with complementary expertise are needed to answer fundamental biological questions
.
”

DNA replication has been the target of several drugs to treat cancer
.
However, existing drugs kill all dividing cells indiscriminately, as both normal and cancer cells must copy their DNA to proliferate
.
Therefore, the specificity of these drugs raises serious concerns
about these anti-cancer chemotherapy drugs.
A more ideal alternative is to inhibit the initiation of DNA replication, allowing normal cells to stagnate in the G1 phase (first growth phase) or exit the cell cycle into the G0 state (quiescent phase); But cancer cells will experience apoptosis
.
Therefore, inhibition of replication initiation can serve as a new, effective anti-cancer strategy with the potential to
selectively kill cancer cells.

The results of this new study provide high-resolution structural and mechanistic information on the --- of human pre-initiation complexes ---MCM2-7 complexes, which could be used in the future to develop non-toxic anti-cancer drugs
targeting the MCM2-7 complex.
(Biovalley Bioon.
com)

Resources:

Jian Li et al.
 The human pre-replication complex is an open complex.
Cell, 2023, doi:10.
1016/j.
cell.
2022.
12.
008.