Functional characterization of SIX3 homeodomain mutations in holoprosencephaly: interaction with the nuclear receptor NR4A3/NOR1.

Holoprosencephaly (HPE) is a relatively common brain malformation resulting in an incomplete separation of the two cerebral hemispheres. A number of mutations in different genes have been linked to this malformation, including three missense mutations in the homeodomain of the transcription factor SIX3. In this study, we investigated the functional consequences of these SIX3 mutations with respect to the ability of the protein to interact with and stimulate the transcriptional activity of the nuclear receptor NOR1 (NR4A3). Using glutathione S-transferase fusion protein pull-down assays and transient cotransfections of Neuro-2a cells with expression and reporter vectors, we found that one mutation, c.676C>G (p.L226V), does not alter the properties of SIX3 toward NOR1. Another mutation, c.749T>C (p.V250A), results in the production of a highly unstable protein in Neuro-2a cells. The third mutation, c.770G>C (p.R257P), results in a mutant SIX3 protein that no longer interacts with NOR1 in vivo. These observations suggest that different SIX3 mutations in HPE2 may affect different signaling pathways, and that one of these pathways may involve the nuclear receptor NOR1.