**13. Conclusion**

across many genes and are incompletely penetrant. The results support polygenic models in which spontaneous coding mutations in any of a large number of genes increases risk by 5-

Iossifov et al. (2012) did not find significantly greater numbers of *de novo* missense muta‐ tions in ASD children versus unaffected, but gene-disrupting mutations (nonsense, splice site and frameshifts) were twice as frequent in the first group. Based on this differential and the number of recurrent and total targets of gene disruption they estimated between 350 and 400 autism susceptibility genes. Many of the genes are associated to the FMRP protein, rein‐ forcing links between autism and synaptic plasticity. They suggested that genes associated

Another aspect of exomes should also be considered. Mitochondria are cellular organelles that function to control energy production necessary for brain development and activity. Al‐ though each individual is typically characterized by a single mitochondrial DNA type, the fact is that each individual is a population of mitocondrial DNA genomes, and the presence of multiple types within an individual is termed heteroplasmy. Although each individual is typically characterized by a single mitocondrial DNA type, in fact to date, more than 400 mi‐ tochondrial mutations have been associated with human disease and most were observed in heteroplasmic states, with pathogenic mutations coexisting with normal mitochondrial ge‐ nomes. This suggests that the heteroplasmic level is of particular interest, as the disease phe‐ notype becomes evident only when the percentage of mutant molecules exceeds a critical threshold value. Although this value differs for different mutations and in different tissues, it is usually in the range of 70%~90%. However, all the various techniques that have been employed to detect heteroplasmy have disadvantages. WES allows rapid detection of not only nuclear mutations but also mitochondrial mutations that also seem to be involved in the etiology of ASD. In this context, Li et al. (2012) sequenced the mitochondrial genome of 131 healthy individuals of European ancestry. In 32 individuals they identified 37 hetero‐ plasmies at frequencies of 10% or higher at 34 different sites in the mitochondrial DNA indi‐ cating that variations commonly occur in mitochondrial DNA. 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 variation or heteroplasmy useful

All these results show that there are a lot of regions/genes being identified by very advanced methods, but no common etiology can be proposed. It is clear that whatever the proposed model to explain ASD, all aspects such as environmental, oligogenic, *de novo* mutations, polygenic, multifactorial, pleiotropic effects, combination of locus heterogeneity, heteroplas‐ my, among others, do not apply to all cases. Perhaps ASD emerge due to highly specific and individual biological patterns. The possibility of distinguishing primary and secondary ef‐ fects will require a better understanding of the underlying biology and identification of the association between genetic and environmental factors within the phenotypic context of

each family. The bottom line is that you must have a systemic view of the problem.

to *FMRP* are especially targets of cognitive disorders that are dosage-sensitive.

in the study of human disorders including autism.

to 20-fold.

230 Recent Advances in Autism Spectrum Disorders - Volume I

Knowledge about the biological mechanisms involved in the etiology of ASD has increased significantly over the past three years. A genetic etiology of these disorders is certain, as cer‐ tain as is their complexity. An understanding of the genetic factors involved is crucial to es‐ tablish future intervention strategies. Although the current emphasis on deciphering ASD has demonstrated the necessity of multidisciplinary approaches, clinical geneticists have an important role in diagnosis and research of autism. The interpretation of this new genetic data requires a set of skills. It is important to know how to get and to interpret genetic tests, family pedigrees, to analyze dysmorphic, neurologic, and medical phenotypes, to interpret heterogeneity, develop rational genetic models, and to design researchs.

Despite the numerous known or, at least, allegedly involved causes of predisposition for ASD, the etiology is identified in a few cases (~ 10%) thereby highlighting the importance of genetic testing in affected individuals. The discovery of an etiological agent in a given case will, very probably, not interfere in treatment. However, this will reduce the distress of pa‐ rents by explaining the cause of the problem and clarify about the possibility of familial re‐ currence. On identifying the etiologic agent, genetic counseling can be better targeted. Thus, a clinical-genetic evaluation of the patient is important as are the karyotypic analysis, molec‐ ular test for FRAXA, the investigation of inborn errors of metabolism, performing imaging tests and multiplex ligation-dependent probe amplification (MLPA) for at least three hot spots in ASD (15q11-13, 16p11.2 and 22q11.2). These are strategies available to better assess the etiology ASD. Certainly, in the not too distant future, other more sophisticated genetic research tools will be commercially available. The question that remains is whether the in‐ terpretation of results will accompany the speed of technical advances.
