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Impairment of the ATM gene is responsible for ataxia telangiectasia. This section describes its essential role in the body.

order helvetica, look sans-serif;">The first description of patients with ataxia-telangiectasia syndrome was published in 1926. Denise Louis-Bar has also reported a case in 1941 and gave its name to the disease.

The hereditary and familial nature of the disease has been raised in 1958. The fact that ataxia-telangiectasia is transmitted in an autosomal recessive manner (seeTransmission) assumed that a single gene was involved in the disease. Genome analysis of patients helped in locating a gene involved in the 11th human chromosome (see ATM features). These studies have led to identify and sequence the gene mutated in ataxia-telangiectasia in 1995. It has naturally been called ATM for Ataxia Telangiectasia Mutated.

Since then, many projects are being undertaken by scientists, directly or indirectly, to understand the action of this gene and the protein associated with it.

Indeed, the malfunction of this single gene causes a diverse range of symptoms, making it a highly relevant research topic in genetics course, but also in the process of cancer development.

order helvetica, click sans-serif; font-size: 1em; background-color: #92d2dd; border: medium solid #333333; text-align: center;">DNA is our "individual program of manufacture."

help helvetica,sans-serif; font-size: 1em; background-color: #92d2dd; border: medium solid #333333; text-align: center;">A chromosome is a DNA strand temporarily ordered in some way for cell division.

A chromosome consists of genes.

Each gene encodes a protein that, itself, has a function.

There are 25,000 genes in the human genome.

 

DNA

Thumbnail image

It is sort of recipe melting countless of chemical operations that will lead to the establishment of a unique individual. It is the internal memory of every living being. DNA molecules (1) are very long: they measure several meters! (3). They are located in the nucleus of cells as disorderly "balls" (2) in normal times. They do compact (4) and arrange themselves into chromosomes only at the time of cell division (5).


CHROMOSOME

Thumbnail imageThere are 23 pairs of chromosomes in a human being with 22 pairs of autosomes and one pair of chromosomes which determines the sex (XX for females, XY for males). In a pair, each chromosome has the same functions as the other, but one comes from the mother and one from the father.

Thumbnail imageAll chromosomes are in the same shape: a long arm (denoted "q") and a short arm (denoted "p") separated by a central point: the centromere.


GENE

Thumbnail imageEach chromosome consists of a DNA molecule whose parts, genes, will allow the construction and development of a tissue or a body function. There are 25,000 in humans, but this is not the best provided in nature.

Note that they do not act directly as a gene itself is not active: it is simply used as a code to produce a protein that will be active and have a specific and indispensable action.

The ATM gene is located on chromosome 11.

It is a recessive gene.

ATM is one of the most complex genes.

In Ataxia telangiectasia, mutations are different from one patient to another.

 

ATM

Thumbnail image The chromosome is the only ordered form of the DNA molecule. So scientists have used it to create a scale of localization of genes: each chromosome arm is divided arbitrarily into equal parts and subparts numbered from the centromere to the outside.

Thus, the ATM gene, functional or not, is located in 11q22.3, ie:
  • On the long arm (q) of the 11th chromosome (an autosome)
  • On the 22nd section of the arm
  • On the third sub-portion of this section (see diagram).
For information, the ATM gene has 1425 neighbours just on that chromosome 11. This gene is one of the longest and most complex of the organism. Disadvantage: the more a molecule is long and complex, the more sites where a change is possible are numerous.

Beside, it has been established in AT patients that no mutation was identical. Some mutations allow the production of a protein altered ATM, but not the majority.



Recessive

Like all genes, the ATM gene is present in two copies (one on chromosome 11 from the mother and the other one from the father). Each copy is named "allele".

In the case of alteration of one allele, the other is capable of producing the ATM protein, which explains that parents and siblings of AT patients don't have the symptoms of ataxia telangiectasia. However, scientists suspect that this lack can cause a number of other conditions.

The gene is so called "recessive" , as opposed to "dominant": to be suffering from AT, man must have a mutation on each of the two alleles that are inherited. AT is also referred to as an autosomal recessive disease: the chromosome 11 is not the chromosome determining the gender, so the transmission of the disease is not related to the gender.

Unfortunately, as we have seen, mutations (changes) of the ATM gene on each allele of an individual are different and they differ from one individual to another: it makes it even more difficult to develop genetic treatments.

buy helvetica,sans-serif; font-size: 1em; background-color: #92d2dd; border: medium solid #333333; text-align: center;">The cell cycle is the set of steps that will lead to the division of one cell into two identical ones: it is called mitosis

Many compliance checks are conducted during this process.

The ATM gene is one of these elements.

 

Cell cycle

The vast majority of cells constituting an adult body is at rest and perform tasks for which it was programmed. Some tissues are constantly and actively proliferating (our skin or blood cells for example). Sometimes, some cells have to be repaired or replaced as they die naturally or as a result of aggression.

The cell cycle is the set of steps that will lead a cell to divide into two daughter cells containing the same genetic heritage. Spurred on signals received from its environment (growth factors), the cell begins the cycle that will lead to the mitosis..

Thumbnail imageThe phases of this cycle are:

  1. G1: phase of cell growth that accumulates material for the division
  2. S: phase of DNA replication so that each daughter cell has the same genetic material
  3. G2: control phase of the genetic material and preparation of molecules for mitosis itself
  4. M: mitosis. Divided into four phases that we will not describe here. Mitosis is the division itself, giving two daughter cells that will have the same course, if the conditions are right, of course.


Checkpoints of the cell cycle

Throughout the cell cycle, mechanisms will rigorously monitor the integrity of genetic material in the cell nucleus. It is indeed excluded that a cell divides when its DNA is the bearer of anomalies as it would transmit them.

Thumbnail imageThese oversight mechanisms are called checkpoints of the cell cycle.
Schematically, a checkpoint is organized with a detection phase, the transmission of information (signaling), then the phase of implementation of the necessary means to DNA repair or cell destruction by apoptosis if the damage is too great.

The following diagram illustrates some of the many control points (shown as stop signs) that exist throughout the cell cycle and that maintain the integrity of our genome.

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  1. in G1 phase, under the influence of growth factors, the cell receives signals allowing it to move forward in a cell cycle which lead to division.
  2. Upon entry into G1 phase, there is a checkpoint which autorises only normal DNA to divide.
  3. Also in S phase, when the cell doubles its amount of DNA, abnormalities in DNA lead to cell cycle arrest. And if they are not repairable the cell progresses to death by apoptosis.
  4. Finally, in late G2 phase, just before the entry into mitosis, the DNA quality is checked one last time before the division process begins.
  5. A final checkpoint in the separation of the two sets of chromosomes between the two daughter cells resulting from the division will ensure the absolute equity of this division.

prescription helvetica,sans-serif; font-size: 1em; background-color: #92d2dd; border: medium solid #333333; text-align: center;">Complete breakage of the DNA molecule is called double-strand break.

The ATM protein is responsible for their detection and "decides" whether to repair them or not, ie of the survival of the cell.

This explains his involvement in the heart of the cancer process.

 

Breaks

Thumbnail imageThe DNA molecule has a shape well known with its two strands linked together in a double helix. But it can undergo various alterations and among these, full of breaks called double-strand breaks (DSB).
Thumbnail image"Non-desirable" DSB may be due to various factors like:
  • Exposure to radiation (radioactivity, X rays, ultraviolet ...)
  • Exposure to chemical agents (chemotherapy, toxins, pollution ...), to certain viruses, deprivation of oxygen or nutrients.
DSBs are also associated with normal "physiological" situations in the life of the cell:
  • The normal function of developing specific immune defenses.
  • Cell division: whether during meiosis, the process of creation of reproductive cells (sperm and ova), or during mitosis (cell division into two identical daughter cells).
The role of the ATM protein is directly linked to the DSB. It is present in every cell (in the nucleus and cytoplasm), ready to intervene if these lesions occur. Under normal circumstances, the ATM proteins are grouped in pairs: dimers. In this state, they cancel each other out, leaving the cell cycle proceed normally.

ATM: DNA controller

Many proteins are involved in these control mechanisms of potential anomalies throughout the cell cycle. The ATM protein is one of them. It is responsible for detecting DNA double-strand breaks. It does this by itself or through other proteins such as 53BP1 seen on the video below being recruited at the breaks where the DNA of cells is damaged by a laser.

Film description: cells genetically engineered to express a fluorescent protein 53BP1 were exposed to laser radiation of a few micrometers in diameter that generates DNA damage in their nuclei (areas where the fluorescence disappears in both nuclei at the beginning of the film). Immediately, the 53BP1 protein which was dispersed in the nucleus of cells come to focus at the level of detected lesions and thus contribute to the development of cellular responses.


Thumbnail image While at rest, the ATM protein were grouped in pairs to inactivate each other, the signal of the DSB separates them and they activate by the addition of a phosphorus atom (procass named "phosphorylation"). Becoming active, proteins ATM trigger cell cycle arrest in S phase (DNA duplication) or G2 (control of genetic material, see Cell cycle).

Then, depending on the "diagnosis" may be engaged:

Thumbnail image- A process of DNA repair involving, among others, the protein complex MRN (MRE11, RAD50, NBS1) and the BRCA1 protein. For information, the lack of functional MRE11 causes the ATL syndrome (AtaxieTélangiectasie-Like) and the absence of BRCA1 leads to a very high risk of breast and ovarian cancer (65% and 45%, compared to 12% and 1.4% in the general population)


Thumbnail imageThumbnail image- A destructive process involving the protein P53, another essential protein since it is found in 50% of cancers in an altered form.

This occurs in most body cells and makes ATM indispensable everywhere. This explains the wide range of symptoms presented by AT patients.

We can also understand that the development of treatments is difficult, as the chemistry of drugs can treat only a portion of these symptoms. The gene therapy, that is to say correcting the ATM gene, seems therefore to be the only way to cure the disease, but it will be difficult and without guarantee of repairing the harm already done.

However, the central role of ATM in the body implies that the research is interested in it.

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