Exploring Chromosome X
Other genes associated with autism may be located on other chromosomes.
Interestingly, researchers have found evidence that certain genes on the X chromosome may influence social
interaction skills. The X and Y chromosomes determine the sex of an individual. Males have one X
and one Y chromosome and females have two X chromosomes. If there is a gene that does not work
properly on one of a female's X chromosomes, then the spare gene on the second X chromosome will
compensate. This is not true for males because they only have one X chromosome. This lack of a
spare X chromosome explains why some disorders, known as X-linked disorders, are more common in
men than women. Because more males have autism than females, researchers have suspected for a long
time there might be a gene(s) on the X chromosome involved with autism. Although some research
studies have ruled out a gene on the X chromosome as being a major cause of autism, a gene on the
X chromosome affecting social interaction could still contribute to behaviors associated with autism.
The MeCP2 gene and Rett disorder
Autistic disorder is a general term that refers to a spectrum of disorders (Pervasive Developmental
Disorders) which includes Autistic Disorder, Rett Disorder, Asperger Disorder, and several others. In
1999, Baylor College of Medicine researchers reported they had discovered the gene that causes Rett
disorder. This gene is called the MeCP2 gene and is located on the X chromosome. Rett disorder
(sometimes called Rett syndrome) occurs more frequently in females than in males. Children with Rett
disorder usually develop normally until ages 6 to 18 months when they experience a regression. This
regression is followed by the development of a smaller-than-normal head size (microcephaly), loss of
purposeful hand movements the appearance of "hand wringing" (American Psychiatric Association 1994).
NLGN3 and NLGN4
In a recent European study researchers studied two genes on the X chromosome called NLGN3 and NLGN4
to see if these genes might be associated with autism. These genes contain our body’s instructions
for making proteins called neuroligins. Neuroligin proteins play an important role in the central
nervous system, by helping to transmit chemical “messages” from one neuron (nerve cell) to another.
The researchers looked for changes in these two neuroligin genes in 158 individuals (140 males and
18 females) with autism or Asperger disorder. They found gene changes (also called mutations) in
two of the 158 families that were studied. In one family a change was identified in the NLGN4 gene.
In a second family a change was identified in the NLGN3 gene. Both families had two children with
autism spectrum disorders. These gene changes were not found in the remaining 156 families studied
and they were not found in unrelated families with no history of autism. The presence of a mutation
in these genes does not mean that the protein is not being produced and is not necessarily related
to the autism phenotype. These results indicate that mutations in NLGN genes may be involved in the
occurrence of autism in some, but not all, families with autism. Further research on these gene
changes, including testing larger or different groups of families with autism, is necessary to
better understand the potential role for these gene changes in the cause of autism.
Reference
- Jamain, S., Quach, H., Betancur, C., Råstam, M., Colineaux, C., Gillberg,
I.C., Soderstrom, H., Giros, B., Leboyer, M., Gillberg, C., Bourgeron, T., &
Paris Autism Research International Sibpair Study. (2003). Mutations of the
X-linked genes encoding neuroligins NLGN3 and NLGN4 are associated with autism.
Nature Genetics, 34:27-29.
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