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The ATM gene normally encodes a protein, called the ATM protein, which regulates cell division in cases of stress or breakage in the DNA strand.
This is an example of functional transduction pathway. Normally, when a DNA strand experiences a double-strand break, the ATM protein is activated and effector kinase CHK2 is phosphorylated. The phosphate group is then passed on to phosphatase CDC25A. When CDC25A is phosphorylated, it degrades, preventing it from dephosphorylating kinase CDK2-Cyclin. Usually, the phosphorylation of this kinase allows the cell cycle to continue. When it cannot reach this state, the cell cycle stops and it is prevented from making new DNA.
More susceptible in infants and early childhood
The ATM gene is located on the 11th chromosome, on the q (longer) arm. It starts at 108,222,484 bp and ends at 108,369,102 bp. It is an inherited autosomal recessive trait.
All of this takes place in a single cell which does not release any extracellular signals, so we couldn't really consider this autocrine signaling. What's happening here is a function entirely localized in the cell body. It was confusing trying to research this because I originally thought the ATM protein was the ligand. It's actually conventionally referred to as a kinase.
In fact, all three proteins that we were mentioned earlier (CHK2, CDC25A, and CDK2-Cyclin) are kinases. In other words, they're all apart of the cell phosphorylation cascade. What replaces the signaling molecule in activating ATM is something called an MRN complex within the cell, that binds to a double-strand break and recruits the ATM molecule to initiate the cascade.
Patients with AT have a mutation in the gene that encodes the ATM protein. As a result, these patients have cells with DNA damage that continue to divide, meaning there's an increase in the risk of cancer. This is also why many cancer patients are found with hypermethylated (methyl groups added to the DNA molecule) ATM genes.
Other disorders that are similar to Ataxia Telangiecta instead have mutations in proteins that encode proteins in the MRN complex.