Two important studies – both receiving support from the Autism Consortium – have been published in this month's issue of the Proceedings of the National Academies of Sciences.
In the first study, "Partial Reversal of Rett Syndrome-like Symptoms in MeCP2 Mutant Mice," researchers demonstrated that daily injections of insulin-like growth factor-1 (IGF-1) in a mouse model of Rett-syndrome resulted in significant reduction of associated movement and respiratory symptoms. Rett syndrome, one of the autism spectrum disorders (ASD) seen almost always in girls, arises from mutations of the MeCP2 gene on the X chromosome. In these children, the resulting lack of MeCP2 expression interferes with nerve cell development, and hampers the efficiency of communication through nerve cell processes known as spines. Scientists involved in this study theorized that IGF-1, known to play a role in synapse maturation, might also increase the number of nerve cell spines in the lab's mouse model of Rett syndrome. The positive outcome of the study suggests that IGF-1 could potentially lessen the severity of symptoms of Rett syndrome in humans. This important work is now leading to new human clinical trials for IGF-1 by Autism Consortium members at Children's Hospital Boston.
In the second study, titled "Haploinsufficiency for PTEN and Serotonin Transporter Cooperatively Influences Brain Size and Social Behavior," researchers demonstrate that multiple, interacting genetic risk factors may influence the severity of autistic symptoms. Both the PTEN and serotonin transporter genes have been previously implicated in autism. Study findings demonstrate that mice with a mutation in PTEN alone or in the serotonin transporter gene alone had larger-than-normal brains, while mice with simultaneous mutations in both PTEN and serotonin transporter genes had even larger brains. Approximately 24 percent of humans with autism have macrocephaly – head circumference above the 98th percentile – and increased brain size. Studies in patients with ASD have shown that brain size is correlated with the severity of behavioral problems. Interestingly, in female mice, those with a PTEN mutation showed impaired sociability, while those with a concomitant mutation in serotonin transporter had even more severe symptoms. This important study highlights the possibility that multiple, interacting genetic risk factors may influence the severity of autistic symptoms (e.g. that mutations in the serotonin transporter might be one of many factors to modify the severity of symptoms seen in those who had previously developed PTEN mutations). The work done here could lead to the development of drugs which target the signaling mechanisms between the two interacting genes associated with autism symptoms.