The etiology of ASD is complex and encompasses the roles of genes, the environment (epigenetics) and the mitochondria. Mitochondria are cellular organelles that function to control energy production necessary for brain development and activity. Researchers are increasingly identifying mitochondrial abnormalities in young children with ASD since the most severe cases present early with features of ASD. Better awareness and more accurate and detailed genetic and biochemical testing are now available for the younger patient presenting with developmental delay or behavioral problems.
Epidemiologic and family studies suggest that genetic risk factors are present. Monogenic causes are identifiable in less than 20 percent of subjects with ASD. The remaining subjects have other genetic or multigenic causes and/or epigenetic influences which are environmental factors altering gene expression without changing the DNA sequence [7
]. Epigenetic factors in ASD have been reviewed by Grafodatskaya et al.
]. The recurrence risk for ASD varies by gender for the second child to be affected (4% if the first child affected is female and 7% if a male). The recurrence rate increases to 25-30% if the second child is also diagnosed with ASD. Studies have shown that among identical twins, if one child has ASD, then the other has a 60 to 95% chance of being affected.
Fragile X syndrome and tuberous sclerosis are the most common single gene conditions associated with ASD. The commonest chromosomal abnormality in non-syndromal autism is duplication of the 15q11-q13 region, accounting for 5% of patients with autism. Large microdeletions in chromosome 16p11.2 and 22q regions account for another 1% of cases [12
]. The rapid rise in the incidence of ASD in the past 30 years, apart from improved identification, points to environmental factors acting on essentially unchanged genetic predispositions involving nuclear and mitochondrial DNA since de novo
changes in genes are unlikely to occur so quickly. Specific genetic and cytogenetic conditions associated with ASD are summarized in a recent review [13
The role and importance of genetic testing for individuals with ASD is well recognized [14
] with various studies showing yields of 6% to 40% with newer testing methods [15
]. Early studies by Miles and Hillman [15
] tested 94 children clinically diagnosed with ASD and found that 6 of 94 (6%) had identifiable genetic disorders. Herman et al.
] later found genetic causes in 7 of 71 (10%) subjects with ASD. Schaefer and Lutz [16
] used a three- tier clinical genetic approach to identify causes in 32 clinically diagnosed children with ASD, and reported positive genetic findings in 13 subjects (40%). These included 5% with a high resolution chromosomal abnormality, 5% with fragile X syndrome, 5% with Rett syndrome (MECP2
gene defects in females), 3% with PTEN
gene mutations in those with a head circumference > 2.5 SD, approximately 10% with other genetic syndromes (e.g., tuberous sclerosis) and 10% with small deletions or duplications identified using chromosomal microarrays.
High resolution chromosome analysis detects 3 to 5 megabase-sized abnormalities; however, new technology using DNA or chromosomal microarrays can identify abnormalities 100 times smaller. Therefore, microdeletions and duplications may now be identified with microarrays in individuals with ASD who previously had normal cytogenetic testing. Children with ASD show a higher prevalence of microdeletions and duplications, particularly involving chromosome regions 1q24.2, 3p26.2, 4q34.2, 6q24.3 and 7q35 including those with non-syndromal ASD [18
]. Therefore, if cytogenetic analysis is negative in clinically diagnosed ASD, testing for microdeletions and duplications with newer techniques is warranted.
Shen and coworkers [19
] studied genetic testing results from a cohort of 933 patients with ASD including G-banded karyotypes, fragile X testing and chromosomal microarrays. They reported abnormal karyotypes in 19 of 852 patients (2.2.%), abnormal fragile X testing in 4 of 861 patients (0.5%) and microarray identified deletions or duplications in 154 of 848 patients (18.2%). Fifty-nine of 154 subjects (38.3%) were associated with known genomic disorder variants with possible significance to ASD. With the exception of recurrent deletions or duplications of chromosome 16p11.2 [20
] and chromosome 15q13.2q13.3 [21
], most copy number changes were unique.
Wang et al.
] reported the results from a genome-wide association study of 3,101 subjects representing 780 families with affected children and a second cohort of 1,204 affected subjects along with 6,491 controls. All of their subjects were of European ancestry. They found a strong association with six nucleotide polymorphisms between cadherin 10 (CDH10
) and cadherin 9 (CDH9
) genes. The latter two genes encode neuronal cell-adhesion molecules. These findings were replicated in two independent cohorts and demonstrate an association with susceptibility to ASD.
Sebat et al.
] studied 165 individuals with autism grouped into 118 simplex families without a family history of autism and 47 multiplex families with multiple affected siblings and compared them with control groups without autism. They reported that 10.2% (12 of 118) of simplex families showed copy number variants (CNVs), 2.6% (2 of 77) with CNVs in individuals with autism from multiplex families and 1% (2 of 196) with CNVs among normally developing children. The majority of CNVs were of the deletion type. Thus, CNVs were significantly more common in the sporadic form of autism than in those with a family history due to single gene mutations not detectable by DNA deletion or duplication analysis.
As a result of research into nucleotide sequences, microdeletions and duplications in children with ASD can now be identified including syntaxin binding protein 5 (STXBP5
) and neuronal leucine rich repeat 1 (NLRR1
) genes. Syntaxin 5 regulates synaptic transmission at the presynaptic cleft and is known to inhibit synapse formation. Syntaxin 1 protein is increased in those with high functioning autism. The role of NLRR1
at the synaptic level is unknown but is thought to be related to neuronal growth [24
An autism genome-wide copy number variation study reported by Glessner et al.
] in a large cohort of ASD cases compared with controls showed that NRXN1
genes play a role. They also described new susceptibility genes, NLGN1
which encode neuronal cell-adhesion molecules and other genes involved in the ubiquitin pathways (UBE3A, PARK2, RFWD2
), two important gene networks expressed in the central nervous system.
Next generation DNA sequencing is currently underway, allowing for rapid and efficient detection of mutations at the nuclear and mitochondrial DNA (mtDNA) level in human investigations and becoming part of clinical workup. Heteroplasmy or the existence of multiple mtDNA types within cells of an individual, is detectable using standard molecular genetic techniques which focus on hypervariable regions of the mitochondrial genome. With high-throughput next generation sequencing of the complete human mtDNA, which is faster and more powerful, accurate detection of heteroplasmy can be made throughout the mitochondrial genome not just in the hypervariable regions located in the cytoplasm of the cell enabling the study of correlation with diseases.
Recently, Li et al.
] sequenced the mitochondrial genome of 131 healthy individuals of European ancestry. They identified 37 heteroplasmies at 10% frequency or higher in 32 individuals and located at 34 different sites of the mtDNA indicating that variation commonly occurs in mtDNA. These variations may impact on energy levels and influence brain development and function. Next generation sequencing should provide novel insights into genome-wide aspects of mtDNA variation or heteroplasmy useful in the study of human disorders including autism.
With a significant percentage of children with autism presenting with metabolic abnormalities (e.g., high lactate levels) and other biochemical disturbances, identification of mitochondrial disorders is critical for early treatment. Long standing mitochondrial dysfunction can lead to major health complications and damage. If identified early, mitochondrial disorders can be managed with improved longevity and quality of life. Medical intervention and therapies are now available to specifically target the biochemical defect in the mitochondria and to improve function and bioenergy utilization and diminish the neurological insults that would occur if left untreated.