NEUROBIOLOGICAL CAUSES OF MILD MENTAL RETARDATION: THE ROLE OF ADENOSINE DEAMINASE POLYMORPHISM

Objectives: Mental retardation (MR) affects from 1 to 3% of the population and results from a wide range of aetiologic factors. Although determinants of moderate/severe MR may be identified in the 80% of the cases, they remain unknown in 60% of mild MR. The evidence of genetic involvement in mild MR is increasing with improvement of diagnostic techniques: several categories of genes, such as enzymes, mediators of signal transduction, transcription regulation, binding proteins and transporters have been identified. To improve our understanding of the genetic and biochemical basis of mild MR, we explored the role of adenosine deaminase (ADA), which catalyzes the irreversible deamination of adenosine to inosine.

Methods: We conducted a case-control study recruiting 338 Italian Caucasian children from the Pediatric Neurology Units of the Tor Vergata University of Rome and the University of Messina, and the Pediatric Unit of the University of Catania. Cases were represented by 80 individuals with mild MR of unknown cause, and controls by two groups of participants: 153 apparently healthy children and 105 children with moderate or severe MR of 'known aetiology'. The ADA1, ADA2, and ADA6 polymorphisms were evaluated in cases and controls using the RFLPPCR method. Genotype and allele frequency were compared between cases and controls, and the statistical significance of the associations was tested by Chi Square test and Odds Ratio with its 95%CI. Results: The finding of our study showed that cases were more likely than controls to have the low activity ADA-Asn8 (ADA1*2) polymorphism (p<0.05) and to present the ADA1*2/ADA2*1 haplotype.

Conclusion: In conclusion, our observation suggests a possible role for a low-activity genotype (ADA-8Asn) (ADA1*2) of ADA in the pathogenesis of mild MR. Further studies are needed to confirm these results and to improve knowledge of the neurobiogical basis of mild MR.