Vitamin D has been implicated as an important environmental factor related to the development of ASD. This study examined the correlation between VDR SNPs and serum vitamin D in children from the Chinese Han population, and then determined their roles in predicting childhood ASD. Our data showed that children with ASD had significantly lower serum levels of vitamin D and a higher rate of vitamin D deficiency. The examined SNPs were not associated with serum vitamin D levels or vitamin D deficiency. SNP rs731236, or low serum vitamin D, or vitamin deficiency was correlated with the risk of childhood ASD. Children with both T/C genotype of rs731236 and vitamin D deficiency had a higher risk of ASD.
This study indicated that there was no correlation between all examined VDR SNPs and serum D levels or vitamin D deficiency in children with ASD, or healthy controls, or all children. Notably, up to 269 children with ASD and 320 healthy controls from the Chinese Han population were included in this study. Similar to this finding, a previous study examined rs731236, rs7975232, rs1544410, rs2228570, and rs11568820 as well as serum vitamin D in 106 overweight/obese and 86 healthy (control) Chinese Han children, from a nearby district. No association was observed between these VDR SNPs and serum vitamin D [34]. A Poland study reported no statistical significant differences according to VDR SNPs (rs731236, rs7975232, rs1544410 and rs2228570) and serum vitamin D concentrations in children with ASD [33]. Conversely, Coskun et al discovered that the T/T genotype of rs2228570 was significantly associated with an increased risk of childhood ASD and with higher serum vitamin D in Turkish children with ASD [25]. An Italian study revealed that rs2228570, but not rs1544410, rs731236 and rs7975232, was significantly correlated with serum vitamin D in multiple sclerosis patients [32]. A Mexican study evaluated the correlation between the vitamin D deficiency and genetic variants on VDR and the vitamin D binding protein (GC) genes in 689 unrelated postmenopausal women. Their results showed that the SNPs rs4516035 in VDR and rs2282679 in GC were associated with vitamin D deficiency [36]. All these findings suggest that the association between VDR SNPs and serum vitamin D is region and race/ethnicity specific.
Our study confirmed that children with T/C genotype, or C allele of the SNP rs731236 had a significantly increased risk of childhood ASD, whereas other SNPs examined in this study were not associated with the risk of childhood ASD [26]. The Poland study enrolled 108 children with ASD, and 196 non-ASD children. Their result indicated that rs2228570 and rs7975232, but not rs1544410 and rs731236, of the VDR gene were correlated with ASD [33]. Another Turkish study revealed that rs731236, rs2228570, and rs1544410 were significantly associated with childhood ASD [25]. Others reported an association between the VDR gene polymorphisms and the risk of ASD [37–39].
Lower levels of serum vitamin D in patients with ASD have been reported previous studies. A study determined serum levels of vitamins and minerals in 274 Chinese children diagnosed with ASD and 97 age-matched healthy children from the Han population from another Chinese region. Serum levels of vitamin D, calcium, magnesium, iron, and zinc in children with ASD were significantly lower than those of healthy controls [14]. Another Chinese study examined the serum vitamin D levels in 215 children with ASD and 285 healthy control children. Their study revealed that serum vitamin D concentrations were negatively correlated with ABC total scores and language subscale scores [40]. Other studies from various countries have indicated lower serum vitamin D in patients with ASD [10–13, 15, 41, 42]. This study corroborated that children with ASD had lower serum vitamin D and a higher prevalence rate of vitamin D deficiency. Consistently, several clinical trials showed the efficacy of vitamin D supplementation in the improvement of symptoms of ASD [12, 40, 43]. However, several other studies with smaller sample sizes reported no significant difference in serum vitamin D between children with ASD and healthy controls [27–29, 44].
Our study also found that serum vitamin D had a fair performance in distinguishing children with ASD from healthy controls with the area under the ROC curve of 0.7285. VDR SNPs did not further improve performance of serum vitamin D in predicting the risk of ASD. However, our results indicate that T/C genotype of rs731236 and deficiency of vitamin D indicated the highest risk of childhood ASD compared with another genotype with or without vitamin D deficiency. Since serum vitamin D can be measured at any time, it has the potential to become a marker in diagnosing ASD at younger ages. It is of clinical significance to identify other biomarkers which improve its predicting performance in the future studies.
Limitations of this study include a non-random selection of cases and controls. Only children with ASD treated in our hospitals were enrolled in this study. The sample size is still relatively moderate. Serum levels of vitamin D were measured only at the baseline. Information regarding dietary intake of vitamin D and sunlight exposure time were not recorded in these subjects. The cause of low serum vitamin D was therefore not able to be assessed in this study.