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16p11.2 deletion syndrome is a rare genetic condition caused by microdeletion on the short arm of chromosome 16. Most affected individuals experience global developmental delay and intellectual disability, as well as childhood-onset obesity. [1] 16p11.2 deletion is estimated to account for approximately 1% of autism spectrum disorder cases. [3] [4]
Examples of autism that has arisen from a rare or de novo mutation in a single-gene or locus include neurodevelopmental disorders like fragile X syndrome; metabolic conditions (for example, propionic acidemia); [64] and chromosomal disorders like 22q13 deletion syndrome and 16p11.2 deletion syndrome. [65]
17q12 microdeletion syndrome is an autosomal dominant disorder, where one copy of the relevant mutation is enough to cause the condition. Most cases are de novo, or spontaneous mutations that do not occur in the proband's parents; [10] approximately 75% are de novo, while 25% are inherited. [4]
Fragile X syndrome co-occurs with autism in many cases and is a suspected genetic cause of the autism in these cases. [12] [23] This finding has resulted in screening for FMR1 mutation to be considered mandatory in children diagnosed with autism. [12]
The conclusion of these recent studies of de novo mutation is that the spectrum of autism is breaking up into quanta of individual disorders defined by genetics. [38] One gene that has been linked to autism is SHANK2. [39] Mutations in this gene act in a dominant fashion. Mutations in this gene appear to cause hyperconnectivity between the neurons.
The ASD-linked mutation results in a complete loss of SHANK 3 (like a deletion) and impaired striatal synaptic transmission. The schizophrenia-linked mutation results in a truncated SHANK 3 protein and severe synaptic impairments in the prefrontal cortex. [52] Other studies suggest that SHANK3 knockout mice display behavioral phenotypes of ASD.
Autism is associated with several genetic disorders, [4] perhaps due to an overlap in genetic causes. [5] About 10–15% of autism cases have an identifiable Mendelian (single-gene) condition, chromosome abnormality, or other genetic syndrome, [6] a category referred to as syndromic autism.
Smith–Magenis syndrome (SMS), also known as 17p-microdeletion syndrome, is a microdeletion syndrome characterized by an abnormality in the short (p) arm of chromosome 17. [1] It has features including intellectual disability , facial abnormalities, difficulty sleeping, and numerous behavioral problems such as self-harm.