Comparative transgenic analysis of enhancers from the human SHOX and mouse Shox2 genomic regions

Disruption of presumptive enhancers downstream of the humanSHOX gene (hSHOX) is a frequent cause of the zeugopodal limb defects characteristic of Leri -W eill dyschondrosteosis (LWD). The closely related mouse Shox2 gene (mShox2) is also required for limb development, but in the more proximal stylopodium. In this study, we used transgenic mice in a comparative approach to characterize enhancer sequences in the hSHOX andmShox2 genomic regions. Among conserved noncoding elements (CNEs) that function as enhancers in ver- tebrate genomes, those that are maintained near paralogous genes are of particular interest given their ancient origins. Therefore, we first analyzed the regulatory potential of a genomic region containing one such duplicated CNE (dCNE) downstream of mShox2 and hSHOX. We identified a strong limb enhancer directly adjacent to the mShox2 dCNE that recapitulates the expression pattern of the endogenous gene. Interestingly, this enhancer requires sequences only conserved in the mammalian lineage in order to drive strong limb expression, whereas the more deeply conserved sequences of the dCNE function as a neural enhancer. Similarly, we found that a con- served element downstream ofhSHOX (CNE9) also functions as a neural enhancer in transgenic mice. However, when the CNE9 transgenic construct was enlarged to include adjacent, non-conserved sequences frequently deleted in LWD patients, the transgene drove expression in the zeugopodium of the limbs. Therefore, both hSHOX and mShox2 limb enhancers are coupled to distinct neural enhancers. This is the first report demon- strating the activity of cis-regulatory elements from the hSHOX and mShox2 genomic regions in mammalian embryos.