Transcriptional complexity at the Hoxa5 locus

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The development of a multicellular organism from the fertilized egg is a complex process involving numerous genes expressed precisely in space and time. Hox genes encode proteins that control the expression of other genes. They occupy a key position in the genetic hierarchy dictating the formation of the embryo. Mutations in Hox genes result in a panoply of anomalies affecting the embryo and causing diseases in the adult, indicative of the large range of action of Hox genes throughout life. A complex array of different modes of regulation governs the specific expression of Hox genes indicative of the importance of correct Hox expression for the normal development of the organism. One of these modes involves the long non-coding RNAs found throughout the Hox clusters. Our laboratory is interested in elucidating the molecular mechanisms responsible for the spatio-temporal pattern of Hox gene expression during mammalian development. We use the Hoxa5 gene as a model. In the mouse, the lack of Hoxa5 function results in defects affecting the formation of several structures including the skeleton, the lung, the gut, the thyroid and mammary glands, revealing its crucial role for normal embryonic development. We have shown that the Hoxa5 specific regional expression in the embryo is under the control of several DNA sequences and transcriptional factors. Multiple RNAs containing Hoxa5 sequences are produced but only one encodes the HOXA5 protein. The other Hoxa5 transcripts possess a developmentally-regulated expression but how they are integrated in the developmental program remains unknown. We propose to pursue our study of the control mechanisms of Hoxa5 expression by characterizing the Hoxa5 regulatory elements and by analyzing the role that the long non-coding RNAs encompassing Hoxa5 sequences may play in the spatially and temporally-restricted expression of Hox genes. This will be addressed by using molecular and genetic approaches. Our research program will lead to a better understanding of the precise regulation of Hox expression, which is essential for Hox gene function and hence for correct embryo patterning.