C21orf5, a Novel Human Chromosome 21 Gene, Has a Caenorhabditis elegansOrtholog (pad-1) Required for Embryonic Patterning

To contribute to the development of the transcription map of human chromosome 21 (HC21), we isolated a new transcript, C21orf5 (chromosome 21 open reading frame 5), encoding a predicted 2298-amino-acid protein. Analysis of the genomic DNA sequence revealed that C21orf5 consists of 37 exons that extend over 130 kb and maps between the CBR3 (carbonyl reductase 3) and the KIAA0136 genes. Northern blot analyses showed a ubiquitously expressed RNA species of 8.5 kb. RNA in situhybridization on brain sections of normal human embryos revealed a strong labeling in restricted areas of the cerebral cortex. In silicoanalysis of the deduced C21orf5 protein revealed several highly probable transmembrane segments but no known protein domains or homology with known proteins. However, there were significant homologies to several hypothetical Caenorhabditis elegansproteins and Drosophila melanogastergenomic sequences. To investigate the function of C21orf5, we isolated the cDNA of the C. elegansortholog and performed double-stranded RNA-mediated genetic interference experiments. The major phenotype observed in the progeny of injected animals was embryonic lethality. Most of the tissues of the embryo failed to undergo proper patterning during gastrulation, and morphogenesis did not occur; thus we termed the ortholog pad-1,for patterning defective 1. These results indicated that pad-1is essential for the development and the survival of C. elegans.This study provides the first example of the use of C. elegansas a model to study the function of genes on human chromosome 21 that might be involved in Down syndrome.