By: Erin Thomas
While it is suspected that autism has a genetic component, recent research has been minimizing the role of genetics in the onset of autism and maximizing the role of environmental factors. Until now, twin studies have lead us to believe genes are involved in causing autism. One study reported that when one sibling has an ASD, the other sibling also has an ASD twice as often in identical twins than in non-twin siblings (Bohm, 2013). Another study in Denmark using data from 1980 to 2004 reported that the concordance risk of ASDs was 6.9 in siblings, 2.4 in maternal half siblings, and 1.5 in paternal half siblings (Gronborg, 2013). This shows that the risk is higher in full siblings than half siblings, further intimating the genetic component of autism. The risk is also higher in maternal half siblings than paternal half siblings, suggesting that issues during pregnancy may have an effect.
Because these concordance rates in twins and siblings are smaller than previously thought, there has been increasing research into environmental-gene interactions and exposure to harmful chemicals during the first trimester of pregnancy.
Epidemiological studies have pointed to thalidomide and valproic acid as possible teratogens, agents that cause malformation of the embryo (Matsuzaki, 2012). In the 1960s, children of women who used thalidomide during pregnancy were born with developmental disabilities, most notably shortness of the limbs. 5% of these children also had autism. However, autistic children usually only have malformed ears, not shortness of the limbs. This led to the assumption that the developmental issues associated with autism occurred in the first trimester, when the brainstem is formed. In addition, children born to women who took valproic acid (VPA) during pregnancy developed a condition known as fetal VPA syndrome. Children with this syndrome also had a high probability of having autism. A study of the postmortem brain of an autistic patient revealed a decrease in nerve cells in certain areas that led researchers to believe that the onset of autism occurred immediately after the neural tube of the fetus was closed in the first trimester of pregnancy. Valproic acid-exposed animal (rat) models have also been shown to present the symptoms of autism, and they have been used frequently as autistic models of environmental factors (Matsuzaki, 2012).
There is also ongoing research into the effects of pollution on autism onset. One study conducted in Los Angeles, California from 1998 to 2009 revealed that there was a 12-15% increase in odds of being diagnosed with autism after increasing exposure to ozone and particulate matter, and a 3-9% odds increase with increasing exposure to nitric oxide and nitrogen dioxide (Becerra, 2013). The data suggested that there may be an association between autism and exposure to air pollutants during pregnancy, primarily in relation to traffic sources. Most recently, a study into the effects of air pollution was conducted on a national scale. The study looked for correlations between the level of air pollutants at the time and place a woman was pregnant and the presence of an ASD in her child. The results showed that women living in areas that were most polluted, areas with the highest levels of diesel particulates or mercury, were twice as likely to have a child with autism (Roberts, 2013). Women exposed to other pollution, such as lead, manganese, and hard metals also had a higher risk of having a child with autism. However, the researchers of this study warn that it is not conclusive as to what causes the autism. Previous studies have shown that there is no relationship between mercury in vaccines and autism onset. The next step would be to study the blood samples of mothers and children with autism to see which toxin specifically is being transmitted (Roberts, 2013).
So is there something in the air causing autism rates to soar? Not necessarily. As you can see, the research is far from conclusive. However, scientists do have strong reason to believe that environmental factors play a larger role than previously thought, and this knowledge is crucial for finding effective treatments.
Becerra, T et. al. (2013). Ambient air pollution and autism in Los Angeles County, California. Environ Health Perspect. 121(3): 380–386
Bohm, H.V. et. al. (2013). On the autism spectrum disorder concordance rates of twins and non-twin siblings. Med Hypotheses. pii: S0306-9877(13)00408-8.
Gronborg, TK et. al. (2013). Recurrence of Autism Spectrum Disorders in full- and half- siblings and trends over time: A population-based cohort study. JAMA Pediatr. 167(10):947-953.
Roberts, A et al. (2013). Perinatal air pollutant exposures and Autism Spectrum Disorder in the children of Nurses’ Health Study II participants. Environ Health Perspect 121:978–984. http://dx.doi.org/10.1289/ehp.1206187 [Online 18 June 2013]