Functional Analysis of Genes Implicated in Down Syndrome: 1. Cognitive Abilities in Mice Transpolygenic for Down Syndrome Chromosomal Region-1 (DCR-1)

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Abstract
Down syndrome occurs every 1/1000 births and is the most frequent genetic cause of mental retardation. The genetic substrate of Down syndrome, an extra chromosome 21, was discovered by Lejeune, half-a-century ago, and the chromosome has been fully sequenced, although the gene(s) implicated in the mental retardation observed with the syndrome are still unknown. Observations of patients with partial trisomy of the 21q22.2 fragment suggest that most of the signs of the syndrome, including mental retardation, could be influenced by the region referred to as the Down Minimal Chromosomal Region-1 (DCR-1) for that reason. Using the extensive syntenies between human chromosome 21 and murine chromosome 16, Smith et al. (1995, 1997) developed transpolygenic mice with human chromosome 21 fragments covering the DCR-1. Here, we explored cognitive performances in mice over-expressing the genes carried by these fragments with the Morris water-maze and fear-conditioning procedures. The 152F7 transpolygenic mice had lower performance levels, compared to non-transgenic and other transgenic mice on most measurements in the water-maze. In fear-conditioning, all transgenic mice recorded lower performance levels compared to controls in the altered context stage. The 230E8, 141G6 and 285E6 mice failed to learn or react when the sound used as the conditional stimulus was added. These results showed that the 152F7 region played a crucial role in cognitive impairment, supporting the hypothesis of DYRK-1A gene involvement. However, the data presented here also suggest that other chromosomal regions within the DCR-1 may be involved in specific cognitive functions.