Types of A-T
A-T is a complex and very variable condition. No two people will have exactly the same symptoms or experience, even within the same famiily. To some extent, the way the condition develops can be affected by the type of mutations the individual has and where they are on the gene. But even siblings who inherit exactly the same mutations may vary in how they are affected.
Recently, however, experts have begun to be able to identify a small subgroup of people who have a form of A-T in which the symptoms tend to be milder and develop more slowly. Genetic studies have shown that individuals with this form of A-T have one of a small group of particular mutations. This form of A-T does not have a formal name yet, but tends to be known as 'mild-variant A-T' or sometimes 'mild A-T'. However this is not an appropriate name as the symptoms are effectively the same as those of A-T. While the condition may be slower to develop, it is only in this way that it is 'milder' than classic A-T.
In addition to the two forms of A-T there are a number of other rare but distinct conditions which share some of the symptoms of A-T. These are also genetic but involve mutations on different genes. However it seems that the proteins that these genes produce are ones which interact with ATM in the cell, and as a result some of the same processes are disrupted when these proteins are not produced. Most of these conditions are extremely rare, but as they share symptoms with A-T they are listed here.
Types of A-T
This is the condition that is usually described as "A-T". In the case of classic A-T, the mutations on the ATM gene mean that no active ATM protein is produced. As a result, people with classic A-T are prone to developing all or most of the classic symptoms. These are likely to start developing in early childhood and children are likely to be quite severely disabled by the time they reach puberty.
Some people with A-T have a particular kind of mutation which enables them to produce a small amount of functioning ATM protein, or else they produce a form of ATM that while not how it should be, is partially functional. The effect of this seems to be that the symptoms are milder or they develop more slowly. A few people with this form of the condition are diagnosed in adulthood, though this is often a question of not finding the right diagnosis for problems they may have known about for some time.
Other related conditions
AOA1 stands for Ataxia-Oculomotor Apraxia Type 1. It is a an extremely rare condition which similar to A-T in many of the physical and neurological symptoms. It is caused by a defect on a different gene, the APTX gene, which results in the lack of production of a protein called Aprataxin. Although different to ATM, this protein is involved in some of the same processes. Like A-T AOA1 usually develops in childhood, and causes a similar range of movement and visual problems. However there are usually no associated immunological problems and telangiectasias do not develop.
AOA2, or Ataxia-Oculomotor Apraxia Type 2, has a range of symptoms similar to those of AOA1, however it tends to develop later, typically in late adolescence or early teens. It is the result of mutations on yet another gene, the SETX gene. AOA2 seems to be a little more common than AOA1.