Down syndrome (DS) is a complex genetic condition arising from an altered dosage of wild-type genes on human chromosome 21 (Hsa21). One approach to the molecular genetics and pathology of DS has been to model the aberrant gene dosage of trisomy 21 in the mouse by transgenesis with single Hsa21 genes or yeast artificial chromosomes. This approach has highlighted potential loci of interest (1
). Alternatively, mouse aneuploidies have been used to model DS. Approximately two thirds of the orthologs of the 243 known Hsa21 genes (current gene estimate, ENSEMBL database) lie on mouse chromosome (Mmu) 16, whereas the remainder are distributed between Mmu10 and Mmu17 (3
). Thus, trisomies of Mmu16 have been studied as potential models of DS. Mice with full trisomy Mmu16 are not viable after birth, and because Mmu16 carries genes with orthologs on Hsa21 and at least three other chromosomes, mouse trisomy 16 is equivalent to partial trisomy of four human chromosomes (Hsa3, 16, 21, and 22). Therefore, the most widely used models of DS are the partial, or segmental, trisomy strains Ts65Dn and Tc1Cje, which are trisomic for portions of Mmu16 containing only Hsa21 orthologs (5–7
An alternative model is provided by “transchromosomic” (trans-species aneuploid) mouse strains in which mice carry an extra human chromosome and are thus trisomic only for the genes on this chromosome. Such a transchromosomic strain for Hsa21 has several advantages for modelling DS. In contrast to transgenic methods to place Hsa21 genes into mice, this approach potentially reflects more closely the 3:2 dosage difference present between trisomic and disomic individuals through the introduction of only one extra copy of each Hsa21 gene. Additionally, the complete genomic sequence can be included, including upstream and downstream regulatory elements of unusually large genes (8
) or those with complex regulatory elements and multiple transcripts (9
). Unlike other methods of stable gene transfer, the transchromosomic approach should not interrupt endogenous mouse sequences. Furthermore, because individual human chromosomes have orthologs on more than one mouse chromosome, aneuploidy of an individual mouse chromosome is not equivalent to the human situation and only partially represents it, whereas placing an entire human chromosome into mice would model a full human trisomy.
The first transchromosomic mice were created by Oshimura and colleagues, who placed freely segregating portions of Hsa2, 14, or 22 into mouse embryonic stem (ES) cells using microcell-mediated chromosome transfer (MMCT) (10
). The ES cells were used to make chimeras, and germline transmission was achieved with a ~2-Mb Hsa2 fragment and a 1.5-Mb Hsa14 fragment (10
). Using a similar process, irradiation MMCT (XMMCT), to construct a mouse model for human trisomy 21, we generated a panel of transchromosomic male mouse ES cell lines, each carrying a freely segregating Hsa21 or portions thereof (12
). When injected into mouse blastocysts, these cell lines gave high percentage contributions in the resultant chimeras; however, they failed to achieve germline transmission of Hsa21. This is consistent with previous findings that an aneuploid chromosome often will not transmit through the male germline (12
). Oshimura and colleagues later reported stable germline transmission of an Hsa21 fragment of ~5 Mb that carried an internal deletion and contained genes with homology to Mmu16 only (14
). Here, we take this technology forward and report the germline transmission of an almost complete Hsa21 and analysis of the resulting mouse strain, Tc1, which models aspects of human DS.