Lack of association between three genetic markers of brain growth factors and infantile autism

Imbalanced genomic imprinting in brain development: an evolutionary basis for the aetiology of autism

We describe a new hypothesis for the development of autism, that it is driven by imbalances in brain development involving enhanced effects of paternally expressed imprinted genes, deficits of effects from maternally expressed genes, or both. This hypothesis is supported by: (1) the strong genomic-imprinting component to the genetic and developmental mechanisms of autism, Angelman syndrome, Rett syndrome and Turner syndrome; (2) the core behavioural features of autism, such as self-focused behaviour, altered social interactions and language, and enhanced spatial and mechanistic cognition and abilities, and (3) the degree to which relevant brain functions and structures are altered in autism and related disorders. The imprinted brain theory of autism has important implications for understanding the genetic, epigenetic, neurological and cognitive bases of autism, as ultimately due to imbalances in the outcomes of intragenomic conflict between effects of maternally vs. paternally expressed genes.

The genetics of autism

OBJECTIVE

To review systematically the empirical evidence for the involvement of genetic risk factors in infantile autism.

METHOD

We aimed at including all relevant papers written in English. We conducted a Medline search in September 2000. In addition we searched the reference lists of related papers.

RESULTS

A relatively small number of reports including family and twin studies, comorbidity, cytogenetic and molecular genetic studies were reviewed.

CONCLUSION

As well family, twin, cytogenetic and molecular genetic studies supported the importance of genetic risk factors in infantile autism. In most individual cases probably at least a few gene variants simultaneously determine the genetic risk. Presently the most interesting chromosome regions concerning the aetiology of autism are chromosomes 7q31-35, 15q11-13 and 16p13.3 which have been suggested by different lines of genetic research.

Evidence for an association with the serotonin transporter promoter region polymorphism and autism

We have examined three functional polymorphisms, serotonin transporter promoter region polymorphism (5-HTTLPR), dopamine D4 exon III repeat region (DRD4), and catechol-O-methyltransferase (COMT), in a small family-based design toward identifying candidate genes that confer risk for autism. A significant excess of the long/long 5-HTTLPR genotype was observed (likelihood ratio = 7.18; P = 0.027; 2 df; n = 33 families) as well as preferential transmission of the long allele of the 5-HTTLPR (TDT chi-square = 5.44; P<0.025; 1 df). No association was observed between the COMT and DRD4 polymorphisms and autism in this sample. Some previous studies have observed linkage between autism and the 5-HTTLPR polymorphism and the current results are similar to those first reported by Klauck et al. [1997: Hum Genet 100:224-229; 1997: Hum Mol Genet 6:2233-2238]. Additionally, elevated serotonin levels have been consistently found in 30%-50% of autistic patients and may represent a marker for familial autism. Hyperserotonemia in autism appears to be due to enhanced 5-HT uptake, as free 5-HT levels are normal and the current report of an excess of the long/long 5-HTTLPR genotype in autism could provide a partial molecular explanation for high platelet serotonin content in autism.

X chromosome and infantile autism

Family studies and epidemiologic data in autism show the involvement of genetic factors in the etiology of this syndrome. The frequent association of X chromosome with mental retardation and behavior disturbances raises the question of its implication in the etiology of autism. Several markers of X chromosome were tested in autistic and control populations by association study. The autistic population was submitted to an extensive clinical examination. For the DXS287 marker, chi 2 analysis showed a different allele distribution between control and patient groups. This difference was enhanced when children with the most severe autistic behaviors and the least serious cognitive disorders were selected for statistical comparison. To our knowledge, this is the first association study described using markers of X chromosome in infantile autism. These preliminary results encourage our research on this chromosome, which could be considered as a significant genetic component of the multifactorial etiology of autism.