Individuals with idic(15) have a total of four copies of this chromosome 15 region instead of the typical two copies (1 copy each on the maternal and paternal chromosomes and 2 copies on the idic(15)).

Researchers think that approximately 1 in 5,000 babies are born with idic(15). In comparison, Down syndrome (caused by an extra chromosome 21) occurs in 1 in 800 babies.

Generally, people with idic(15) do not have family members with the chromosome abnormality. The idic(15) usually forms by chance in one person in the family.

Since chromosomes carry genes that determine how our bodies grow and develop, having extra chromosomal material can alter a person’s physical and mental development. Many individuals with idic(15) have delays in language development and motor skills such as walking or sitting up. Other traits may include low muscle tone (hypotonia), seizures, short stature, and mental retardation. Distinctive facial features associated with idic(15) may include epicanthal folds (skin folds at the inner corners of the eyes), a flattened nasal bridge, button nose, and a high arched palate (roof of the mouth). Some individuals with idic(15) also have features of autism, such as problems with communication, social interactions, and repetitive and stereotyped behaviors (e.g., lining up toys, playing with a toy in the same manner over and over again, hand flapping, rocking back and forth).

Isodicentric Chromosome 15 and Autism
For more than 12 years, scientists have noticed that some individuals with autism also have idic(15). In fact, idic(15) is the most frequently identified chromosome problem in individuals with autism. (A chromosome anomaly involves extra or missing chromosomal material, not changes within the genes such as Fragile X syndrome). There are now over 20 reports in the literature of individuals with both autism and idic(15). The frequency of these reports suggest that the co-occurrence of autism and idic(15) is not by chance. There may be a gene or genes in the 15q11-q13 region that is/are related to the development of autism in some individuals.

Genetic research studies of individuals without chromosome anomalies also support this idea that an autism-related gene may be present in 15q11-q13. Specifically, research studies found that certain DNA markers from the 15q11-q13 region were found more often in individuals with autism than in individuals without autism. Although these DNA markers are too small to be genes, they suggest that researchers may be getting close to finding an autism gene in this region.

Candidate Genes in 15q11-q13
Researchers are currently focusing on genes in this region called the GABA receptor genes, known as GABRB3, GABRA5, and GABRG3. They are good candidates for being related to autism not just because of their location, but also because of their function. The GABA genes make proteins that carry messages between nerve cells. Several studies have found associations between GABRB3 and autism (Buxbaum et al., 2002; Shao et al., 2003; Cook et al., 1998), but further study is needed.

Asperger Disorder
Asperger disorder, sometimes referred to as Asperger syndrome, is a neurobiological disorder. In contrast to autistic disorder, individuals with Asperger disorder do not have a delay in spoken language development. However, they can have serious deficits in social and communication skills. They often have obsessive, repetitive routines and preoccupations with a particular subject matter.

Pervasive Developmental Disorder Not Otherwise Specified (PDD/NOS)
PDD/NOS is a diagnosis often considered for children who show some signs of autistic disorder, but who do not meet the specific diagnostic criteria for the other PDDs.

Rett Disorder
Rett disorder is a complex neurological disorder that affects mainly girls, but there are reports of males who this disorder. Rett disorder is genetic in origin, and is among the most common genetic cause of profound intellectual and physical disability in girls, occurring more commonly than 1 in 10,000 female births. Individuals with Rett disorder develop normally until 6 to 18 months of age followed by a developmental regression. This regression is followed by a deceleration of head growth, loss of purposeful hand movements and followed by the appearance of midline, stereotypic hand movements. A gene associated with Rett disorder was identified in 1999.

Childhood Disintegrative Disorder
Children with childhood disintegrative disorder develop normally for a relatively prolonged period (usually 2 to 4 years) before developing a condition that resembles autistic disorder. Typically language, interest in the social environment, and often toileting and self-care abilities are lost, and there may be a general loss of interest in the environment.