Making Contact With DNA Breaks

[Guest guest]

Making Contact With DNA Breaks

http://www.medicalnewstoday.com/articles/87149.php

When the genetic material inside a cell's nucleus starts to fall

apart, a protein called ATM takes charge and orchestrates the rescue

mission. Surprisingly, for ATM to kick into full gear, the stretches

of DNA flanking a chromosomal break are just as important as the

damaged site itself, report scientists at the Salk Institute for

Biological Studies.

Until now, it had been thought that only already activated ATM could

be recruited to the DNA damage sites, but the Salk's team findings,

which are reported in the Oct. 25 advanced online edition of Nature

Cell Biology, show just the opposite.

" We found that efficient ATM activation occurs only when it has

physical contact with areas flanking the DNA breaks, " says

postdoctoral researcher and lead author Zhongsheng You, Ph.D. " When

we blocked access to the adjoining regions, ATM activation was

severely reduced, " he adds.

" Activating ATM 'on scene' ensures a strong local DNA repair

response, while the extent of the global response will depend on the

number of double strand breaks within the cell, " according to senior

author Tony Hunter, Ph.D., a professor in the Molecular and Cell

Biology Laboratory.

Our genetic material or DNA is constantly damaged by both external

sources such as the sun's ultraviolet rays, and internal sources

such as reactive oxygen species. Fortunately, cells have developed

elegant surveillance systems to detect and repair the damaged DNA.

In the event of the most dangerous form of DNA damage, double-strand

breaks, ATM coordinates the cellular response. ATM functions as a

kinase -- an enzyme that can install phosphate molecules on its

substrates -- and activates a wide variety of DNA repair enzymes and

cell-cycle regulators by phosphorylating them. As a result, the cell

cycle is halted until DNA repair is completed to prevent cells from

passing on damaged genetic material, which could lead to cancer-

causing mutations. If the damage is beyond repair, cells undergo

programmed cell death.

A lack or deficiency of functional ATM (ataxia-teleangiectasia,

mutated) is the underlying cause for a debilitating human genetic

disease called ataxia-teleangiectasia. It is characterized by a wide

spectrum of defects including neurodegeneration leading to

uncoordinated or ataxic movements, immune dysfunction,

radiosensitivity and cancer predisposition.

A lot of work has centered on the downstream targets of ATM since

its discovery more than a decade ago. But the precise mechanism by

which damaged DNA activates ATM had remained unclear. To address

this issue You took advantage of a unique property of cellular

extracts prepared from unfertilized frog eggs. Adding linear DNA

fragments to these extracts mimics DNA double-strand breaks in

cellular DNA: ATM rapidly self-activates and slams the brake on the

cell cycle machinery.

The Salk scientist happened to have DNA fragments of different

lengths (80 bps to 10 kbps) at hand and just added the same number

of molecules, assuming that it was the number of ends or " breaks "

that mattered and not their size. Not so. " The longer, the better, "

says You to describe what he found when he assessed the ability of

the DNA fragments in activating ATM. " Efficient ATM activation

critically depends on both the number of DNA breaks and the total

length of damaged DNA molecules. "

This puzzling observation led him to ask what role the intact DNA

neighborhood played in the activation process. His experiments, he

says, suggest that ATM is cooperatively activated after being

recruited to the regions flanking broken DNA ends. " This mechanism

directly couples ATM activation with damaged DNA and ensures that

ATM is rapidly activated in response to just a few DNA breaks, " he

explains.

Adds Hunter: " Recruiting ATM not only to the break itself but to the

flanking regions as well, amplifies the signal from a small number

of breaks to generate robust cell cycle block and DNA repair

responses. "

DNA is not just floating around inside a cell's nucleus, instead it

is tightly wound around proteins known as histones, which are lined

up along the DNA molecule like beads on a string. The whole assembly

is collectively known as chromatin. " Our findings suggest that an

important signal eliciting the DNA damage response emanates from

modified chromatin flanking the DNA breaks in addition to that

generated by the primary DNA lesions themselves, " says Hunter.