Autism spectrum disorder (ASD) has a strong genetic background with a higher frequency of affected males suggesting involvement of X-linked genes and possibly also other factors causing the unbalanced sex ratio in the etiology of the disorder. Syndrome pathogenesis is associated with abnormal brain development and manifests in several specific brain regions, especially cerebellum, amygdala and hippocampus. Positive linkage findings in genome wide screens and association studies identified susceptibility regions and genes on the X chromosome.
Objectives:
The ribosomal protein L10 gene (RPL10) located in Xq28 was identified as a candidate susceptibility gene for ASD through RNA in situ hybridization experiments. We detected high expression of RPL10 in mouse hippocampus. A first screen in 317 cases with autistic disorder, 21 cases with Asperger syndrome, and 7 cases with PDD-NOS representing 296 families revealed two missense mutations, L206M and H213Q, in two independent male-male affected sib-pair families (Klauck et al. 2006, Mol Psychiatry 11, 1073-84). In the follow-up study presented here, further 175 patients (145 autistic disorder, 27 Asperger syndrome, 3 PDD-NOS) representing 169 independent families have been screened to enlarge the study group. To understand the involvement of RPL10 in the pathogenesis of ASD, the RPL10 mRNA level in patients with ASD and controls was quantified.
Methods:
All seven exons of the RPL10 gene and the intronic snoRNA U70 have been screened by direct sequencing for mutations. RPL10 transcript levels were tested by quantitative RT-PCR on RNA extracted from lymphoblastoid cell lines (LCL) of different probands.
Results:
In the follow-up sample the H213Q mutation inherited from the carrier mother was identified in a male patient from a simplex family. The two different mutations, L206M and H213Q, are both located at the yet uncharacterized C-terminal end of the RPL10 protein, a constituent of the large ribosomal subunit. RPL10 itself is known to have impact on differential gene expression in yeast and man. Functional analyses in yeast revealed that both amino acid substitutions L206M and H213Q, respectively, confer hypomorphism with regard to the alteration of the translation process while keeping the basic translation functions intact. No statistical significant difference in RPL10 expression levels was found by testing 12 autistic patients including 2 patients from the two families carrying the H213Q mutation, 8 relatives of autistic patients and 7 controls. Moreover, the H213Q mutation has no effect on RPL10 expression in this cell system.
Conclusions:
The mutations identified may have a modulating effect on translation processes of synaptogenesis in neuronal development with impact especially on those cognitive functions that are mediated through the limbic system. Alterations due to the inherited mutation at the transcript level may be too subtle to be identified in the LCL system. In future, the functional impact of both mutations in its hemizygous state on the X chromosome will be further analyzed by in vitro and in vivo modeling.