Handbook of Genetic Counseling/Autism-1

Autisim

What is autism?

• Disorder of brain function that appears early in life -- before the age of 3
• Autism belongs to a family of related brain conditions affecting behavior early in life, which are often referred to as pervasive developmental disorder
• Cause of autism is unknown

Traits associated with autism

• social detachment, e.g., failure to smile at parents and an unawareness of events around them
• abnormal language development, such as repeating phrases in a mechanical voice
• unusual repetitive movements, e.g., rocking and flicking fingers
• mental retardation (usually in more severe cases)
• Traits vary in severity and persist into adulthood
• Some individuals with autism have amazing skills (e.g. Rainman)

Incidence

• Savant syndrome is quite rare (only about 200 cases ever recorded)
• childhood autism is more common, affecting about one in 700 children
• Boys are affected four times more often than girls

Can autism "run in families"?

• If a child has autism, each of the parent's later children has a 3% to 9% chance of having autism.
• This is 100 to 200 times greater than in the general population.
• Believed to be multifactorial inheritance
• Genetic component supported by twin studies

Type of twin pair	Number of twin pairs	Twin pairs in which both twins had autism ("concordance rate")
Identical twins	25	60% (15 of 25 pairs)
Non-identical twins	20	0% (0 of 20 pairs)

• The second study used the same twins, but changed the definition of autism
• new definition was looser -- people with milder behavioral problems were counted as autistic

Type of twin pair	Number of twin pairs	Twin pairs in which both twins had "expanded" autism ("concordance rate")
Identical twins	25	92% (23 of 25 pairs)
Non-identical twins	20	10% (2 of 20 pairs)

Genes involved in autism

• no gene found yet
• Several candidate regions on chromosomes have been identified through linkage studies

Differential Diagnosis

(known genetic conditions that may share some symptoms of autism)

• Fragile X syndrome (the FMR1 gene on the X chromosome).
• Tuberous sclerosis (the TSC1 gene on chromosome 9).
• Duplications of part of the long arm of chromosome 15.
• Untreated phenylketonuria (the PAH gene on chromosome 12)

Environmental factors

• UNKNOWN
• Autism was thought to result from certain events before or shortly after birth
• Infection of the brain with herpes virus and infection of the pregnant mother with rubella virus
• Problems with brain structure are associated with autism, including hydrocephalus and, very rarely, a brain tumor
• 1993 British study was reviewed by both the Medical Research Council in Great Britain and the Centers for Disease Control in the United States
• The Centers for Disease Control concluded that: "to date there is no conclusive evidence that any vaccine can cause autism"

Web sites resources

• The autism page at the National Institute for Neurological Disorders and Stroke:

[1]

• The MedlinePlus web site for autism: [2]
• The February 2000 issue of Scientific American has an article entitled "The Early Origins of Autism" (full text not on-line). [3]
• More about autism from the National Immunization Program at the Centers for Disease Control: [4]
• Diagnosis, assessment and interventions for Autistic Spectrum Disorders: [5]

Notes

The information in this outline was last updated in 2002.