Science for Health
Trisomy of human chromosome 21 (Hsa21) occurs in ∼1 in 750 live births, and the resulting gene dosage imbalance gives rise to Down syndrome (DS), the most common known genetic form of mental retardation. DS is a constellation of different phenotypes: while all people with DS have hypotonia, mental retardation and neurodegeneration, most also present with a spectrum of other disorders such as heart defects, leukaemias, autoimmune disorders, etc.
In collaboration with Prof. Elizabeth Fisher (Institute of Neurology, UCL) we are interested in identifying dosage sensitive genes on Hsa21 which, when present in three copies, cause the many different phenotypes seen in DS. We are addressing this using mouse models. We have created a novel mouse strain, termed Tc1, which carries a freely segregating copy of Hsa21. Tc1 mice show defects in memory, synaptic plasticity, locomotor function, heart development and in the craniofacial skeleton (O’Doherty et al, 2005; Morice et al, 2008; Galante et al 2009).
Human DS people have increased rates of mekaryoblastic leukaemia, but lower rates of solid tumours. We have shown that Tc1 mice have perturbed megakaryopoiesis, a pathology that may contribute to the increased rates of leukaemia in human DS (Alford et al, 2010). In contrast, studies of Tc1 mice have shown that the reduction in solid tumours may be due to defective angiogenesis (Reynolds et al, 2010).
We are now breeding these mice to strains carrying chromosomal deletions to identify dosage sensitive genes. We are focusing in particular on identifying dosage sensitive genes that contribute in DS to the increased frequency of leukaemias, to defects in brain and heart development, defective angiogenesis and alterations in behaviour and synaptic plasticity.
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Using gene targeting we inserted a neomycin resistance gene into human chromosome 21 (Hsa21) in a human cell line. These cells carrying a tagged Hsa21 were arrested in metaphase and then centrifuged to isolate microcells carrying one or just a few chromosomes. The microcells were fused to murine embryonic stem (ES) cells, which were then selected with G418 for uptake of the neomycin resistance gene, and screened for lines carrying Hsa21. These ES lines were injected into mouse blastocysts to generate chimeric mice, which were bred to establish the Tc1 mouse strain carrying a freely segregating Hsa21.
Fluorescence in situ hybridization (FISH) of a spread of metaphase chromosomes from a splenocyte taken from a Tc1 mouse. The blue colour indicates DAPI staining which shows all chromosomes, green is a probe specific for human chromosome 21 (Hsa21) and red is a probe specific for mouse chromosome X (MmuX).
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