br Materials and methods br
Materials and methods
Results As expected, due to matching by age, the mean ages were similar for the two groups of children when stratified by ASD status; children with an ASD had a mean age of 67.0 months, while the mean age for children in the TD control group was 68.0 months. Nearly all of the ASD cases (93.7%) and TD controls (99.1%) were Afro-Caribbean and 85.6% of the ASD cases and TD controls were male. Similarly, 96.4% of mothers and 97.3% of fathers were Afro-Caribbean. Our data indicate frequencies of 27.9% and 20.7% for the GSTM1 and GSTT1 null genotype, respectively, for TD Jamaican children. The allele frequency of Val for the GSTP1 polymorphism was 46% and 49% for the ASD cases and TD controls, respectively. CLR analysis showed that there is no correlation between any two of the three GST AMI-1 (P>0.20). There was no significant deviation from Hardy-Weinberg equilibrium in the TD controls (P=0.99) for the GSTP1 polymorphism. Demographic and other characteristics of the ASD cases and TD controls are reported in Table 1. As shown in Table 2, in the univariable CLR analyses, we did not observe significant associations between ASD status and GSTT1, GSTM1, or GSTP1 genotype in either the co-dominant model or any of the aforementioned reduced models (all P>0.15). An additive model was also used in CLR for assessing the association between GSTP1 and ASD, but no significant association was observed (P=0.49). We then investigated the gene–gene interactions between any two GST genes. For the GSTP1 gene, we considered not only the co-dominant model but also all the aforementioned reduced models. Under the co-dominant model, the genotype frequencies of the GSTT1 null genotype (DD) and GSTP1 Ile/Val was 18.0% in ASD cases compared to 9.8% in TD controls. Under the recessive model, the genotype frequencies of the GSTT1 null genotype (DD) and GSTP1 Ile/Ile or Ile/Val was 24.3% in ASD cases compared to 14.4% in TD controls. Additional information regarding the genotype frequencies of GSTT1 and GSTP1 under other models is provided in Table 3. As shown in Table 4, in children heterozygous for the GSTP1 Ile105Val polymorphism, there was a significantly higher odds of also having the GSTT1 null genotype in ASD cases when compared to TD controls under the co-dominant model [matched odds ratio (MOR)=2.97, 95% CI: (1.09, 8.11); P=0.03]. Though marginally significant, in the recessive models of GSTP1 when GSTP1 is Ile/Val or Ile/Ile, the odds of genotype DD for GSTT1 was higher in the ASD group when compared with the TD control group [MOR=2.14, 95% CI: (1.00, 4.71); P=0.06]. No significant associations were observed when the dominant or additive models of GSTP1 were employed (both P>0.05). As is shown in the last column of Table 4, our permutation tests showed that the empirical P-values were very close to the observed P-values reported earlier. Specifically, the observed significance of the effect of GSTT1 when GSTP1 was heterozygous held true in the permutation analysis, indicating that this finding is unlikely to be due to multiple testing.
Discussion In this study, we reported the frequency of GSTM1 and GSTT1 null genotypes in Jamaican children with and without ASD. In children with ASD, we found a frequency of 29.7% and 28.8% for the null genotypes for GSTM1 and GSTT1, respectively. In TD children, we found frequencies of 27.9% and 20.7% for the null genotypes for GSTM1 and GSTT1, respectively. The frequency of the GSTM1 null genotype from our study is similar to that previously reported (26.2%) for the Jamaican population by Taioli et al. (2011), but the frequency of the GSTT1 null genotype reported earlier (35.2%) appears to be higher than in our study (Taioli et al., 2011). Chen et al. (1996) reported that the frequency of the GSTT1 null genotype was 24.1% in African-Americans in the US population; however, they reported that the frequency of the GSTM1 null genotype in African-Americans (under 10 years of age) was 53.5% (Chen et al., 1996, Chen et al., 1997). In contrast, Li et al. (2000) reported a frequency for the GSTM1 null genotype of 17.5%, and a frequency of 25.9% for the GSTT1 null genotype in middle aged African-Americans (Li et al., 2000). These are within the range of 17–35% for the GSTM1 null genotype and 22–44% for the GSTT1 null genotype reported in Brazilians of African descent (Hiragi et al., 2007). Since over 90% of the population in Jamaica is of African descent, it is not surprising that the frequency of the GSTM1and GSTT1 null genotypes resemble those of the null genotypes in African-Americans in the US.