One of the more common things that you will hear about autism is that it is a genetic condition or that it has a "strong genetic component". I think that most people hear this and assume this means that there is some gene or mutation that runs in families that "causes" autism.
Unfortunately, autism is not so simple.
Researchers have spent a lot of time and effort searching for genetic causes for autism, and with the exception of Rett's Syndrome, have come up empty handed. Sure, there have been some small successes here and there but most of the promising leads don't pan out. A study will find a small group with a possible genetic commonality only to have the next study will come along and fail to reproduce the finding.
This leads to the question of why do we think that autism is genetic? If numerous large studies have failed to find anything that people with autism have in common at the genetic level, why exactly do we think that autism is genetic?
The answer is twin studies. There have been several studies done in the past that show that identical twins almost always both have autism while fraternal twins don't. This is taken to mean that autism must be genetic because, if identical twins both have autism and fraternal don't, it must be the shared genetics that makes the difference.
With that in mind I would like to point out the most recent study of autism in twins that was just published in Archives of Pediatrics & Adolescent Medicine -
In this study, researchers from the Kennedy Krieger Institute and Johns Hopkins looked at 227 twin pairs from The Interactive Autism Network (IAN) where at least one of the twins had a form of autism. IAN is an Internet based resource where parents can contribute information about their children with autism which can then be used by researchers. As a result of utilizing IAN, the researchers were able to look at more twins than any study has done before (previous studies had less than 50 sets of twins).
A total of 227 sets of twins took part in this study - 67 pairs of identical twins (58 male, 9 female) and 210 pairs of fraternal twins (97 male-male, 23 female-female, 90 female-male). For those of you who are not familiar with twins, the simple rule here is that fraternal twins are basically the same as siblings that happen to be born at the same time while identical twins share the same genes.
The results show that identical twins were about 88% likely to both have autism as well as (~ 90%) to have the same form and severity. However, what was not expected was that in all in male identical twins this rate was 86% while in female twins the rate was 100% and none had aspergers. Unfortunately, there were not enough female identicals for this finding to reach statistical significance but it was still interesting nonetheless.
Prior studies have shown that identical twins will both have autism somewhere between 60% and 95% percent of the time, so this finding was in line with prior results.
For fraternal twins the overall concordance was 31% which, while higher than was though in the past, is not too badly out of line. The interesting part here is that male-male fraternal twins had a 40% chance of both having autism, which is significantly higher than other types of fraternal twins.
Most other studies studies have placed the rate for fraternal twins to be the same as non-twin siblings, although a recent study out of Japan also showed a higher rate for fraternal twins.
For reference, in non-twin siblings, the rate is thought to be somewhere from 5% up to as high as 20%.
So what does this all mean for autism being genetic?
Well, because the majority of identical twins will both have the same type and severity of autism there clearly is something going on at the genetic level that can cause or predispose someone to autism. But, in at least 10% of cases, identical twins do not share their autism and in another 10% beyond that the severity level can differ.
This tells me that while genetics does play a strong role, it is not the entire story. There is something other than genetics that can determine whether someone will develop autism.
If you look at fraternal twins, they are more likely to both have autism than normal siblings. In the case of male-male fraternal twins that risk can be almost double. Remember, the only difference between fraternal twins and regular siblings is that they share the same prenatal environment and grow up in the same environment at the same time.
If autism was solely genetic there would not be a difference between normal siblings and fraternal twins, and yet there is. And in the case of male-male fraternal twins the risk is much higher than in normal siblings, and that shouldn't be the case either.
If you put these two parts together it becomes clear that there is likely something other than genetics that can cause autism. Or in other words, it is likely that there is a genetic predisposition that is triggered by some sort of environmental influence.
Obviously, I am simplifying things things a bit here and there could very well be other factors at work here. This is one study and, interesting though it is, it does not prove that there are environmental influences. But at the same time it does strongly point in that direction.
Maybe it is time to stop spending so much time looking for the autism's smoking gene and start spending more time looking at what environmental influences could be triggering it? Or here is crazy thought, how about if we devote some time to looking for the physical changes that autism causes in a person. Perhaps if we know what autism is and how it effects the body we might have a better shot at treating it?
Just food for thought.