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Transcript

Ataxia Telangiectasia

by: Chet Lin & Karnpreet Gill

Symptoms of the Disease

Normal Signal Transduction Pathways in the Human Body

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

  • Ataxia: difficulty with control of movement
  • Telangiectasia: dilated blood vessels over the whites of the eyes
  • Oculomotor apraxia: difficulty with coordination of head and eye movement when shifting gaze from one place to the next
  • abnormal swaying of the head/tremors
  • inability to walk
  • discoloration of the face
  • nosebleeds
  • abnormal development of adenoids, tonsils and peripheral lymph nodes
  • visible diluted blood vessels in the eyes
  • Slowed rate of growth
  • Slurred speech
  • Easily infected in the ear, sinuses and lungs
  • Excessive drooling

Comparing this process to a generic cell pathway

Cause of the disease

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.

This means that both parents have to carry the defective gene to produce a child with the disease. The chance of two carriers having a child with AT is 25%; for carrying AT, 50%; and having normal genes, 25%. "Autosomal" means the gene is on a chromosome that is not the sex chromosome.

What happens in an patient with Ataxia Telangiectasia?

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.

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