The Multi‐Organ Activity of an Lmx1b‐Associated cis‐Regulatory Module Suggests a Global Role in Regulating Lmx1b‐Mediated Development

Abstract

Lmx1b is a LIM homeodomain transcription factor important for normal development of many organ systems including the cerebellum, eyes, limbs, and kidneys. In humans, haploinsufficiency of Lmx1b results in a Nail‐Patella syndrome, a disorder characterized by hypoplastic nails, bone deformities, glaucoma, and even kidney failure in severe cases. Despite its importance in developing these organ systems, the induction and regulation of Lmx1b is poorly understood. Previously using ChIP‐seq analysis, we identified two upstream, highly conserved Lmx1b‐associated regulatory modules (LARM1/2) bound by Lmx1b that were found to be associated with limb‐specific auto‐amplification of Lmx1b expression vital for limb dorsalization. We have subsequently identified another highly conserved region 10 kb upstream to these limb‐specific cis‐regulatory modules (CRMs), although it is not bound by Lmx1b. Yet, due to its active chromatin marks and close proximity to the Lmx1b locus, we hypothesized that this third regulatory module (LARM3) may also play a part in modulating Lmx1b expression. We generated a GFP‐reporter construct containing the isolated LARM3 sequence from mouse genomic DNA to test this hypothesis. To determine limb activity, this LARM3 construct was electoporated into the presumptive limb of Hamilton‐Hamburger stage 14 (HH14) chicken embryos. To determine activity in other organ systems such as the kidney and eye, the LARM3 construct was electroporated into a whole embryo (HH4) and in the presumptive lens placode (HH10), respectively. These embryos were viewed by fluorescent microscopy a few days after transfection to determine enhancer activity. LARM3 was found to be active in the limb mesoderm without a dorsoventral bias, showed punctate activity in the intermediate mesoderm that was consistent with mesonephric development (kidney), and was also active in the presumptive lens placode. The multi‐organ enhancer activity of LARM3 suggests a global role in regulating Lmx1b‐mediated development. Support or Funding Information Funded in part by the National Organization for Rare Diseases