FUNCTION OF THE HUMAN MNB/DYRK1A GENE ON THE "DOWN SYNDROME CRITICAL REGION" OF CHROMOSOME 21

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Down syndrome(trisomy 21) is the most frequent birth defect and is a major cause of mental retardation and congenital heart disease. Studies of cases with partial trisomy of chromosome 21 have suggested that the region around locus D21S55 is particularly important in the etiology of the syndrome. This subchromosomal region is called the "Down syndrome critical region". Earlier we performed exon trapping experiments using a series of cosmid clones isolated from this chromosomal region, and identified the genomic structure and cDNA sequence of the human MNB/DYRK1A gene in this region. This gene encodes a dual specificity protein kinase that catalyzes its autophosphorylation on serine/threonine and tyrosine residues. In this research, we examined HeLa cells transfected with cDNA encoding a green fluorescent protein(GFP)-MNB/DYRK1A fusion protein and found 2 patterns of expression : In one group of transfected cells, GFP-MNB/DYRK1A was localized as dots within the nucleus ; and in the other group, it was overexpressed and had accumulated all over the nucleus. In the cells overexpressing GFP-MNB/DYRK1A, multinucleation was clearly observed ; whereas in those with the nuclear dots, such aberrant nuclei were not found. Furthermore, in the latter cells essential processes such as mitosis and cytokinesis occurred normally. Multinucleation was dependent on the kinase activity of MNB/DYRK1A, because it was not observed in cells overexpressing kinase activity-negative mutants, GFP-MNB/DYHK1A(K179R) and GFP-MNB/DYRK1A(Y310F/Y312F). Immunostainihg of GFP-MNB/DYRK1A-overexpressing cells. with specific antibodies against α-and γ-tubulin revealed that multiple copies of centrosomes and aberrant multipolar spindles were generated in these cells. These results indicate that overexpression of MNB/DYRK1A induces multinucleation in HeLa cells through overduplication of the centrosome during interphase and production of aberrant spindles and missegregation of chromosomes during mitosis.