The structure and functions of the transcription factor PHOX2B : new insights in the molecular pathogenesis of congenital central hypoventilation syndrome

Congenital Central Hypoventilation Syndrome (CCHS) is a very rare neonatal neurological disorder characterized by abnormal ventilatory response to hypoxia and hypercapnia, owing to failure of autonomic respiratory control: affected children hypoventilate during sleep, with possible very severe neurological damages. Frameshift mutations (5%) and poly-alanine triplet expansions (95%) have been detected in the coding region of the homeobox gene PHOX2B in about 90% of CCHS patients. A correlation between length of the expansion and severity of the respiratory phenotype has been reported. Since some of the mutations alter the sub-cellular localisation, the DNA-binding affinity and the transcriptional activity of the protein, and that mutated PHOX2B proteins can interfere with the activity of the wild-type protein by sequestering it into aggregates, one crucial question concerns the identification of the functional domains of the protein, the role of the poly-alanine tract and the effects of its expansion on the general architecture and function of the protein. We have performed a deletion analysis of PHOX2B and identified two nuclear localization signals in the homeodomain, both required for the complete import of the protein in the nucleus, corresponding to residues necessary for the binding to DNA, and partially blocked by the expanded poly-alanine tract. By using mammalian two-hybrid system we have also demonstrated that PHOX2B can form homo-dimers and we are currently investigating if the mutations alter the dimerization properties of the protein and the potential contribution to the range of phenotypes and pathogenesis in CCHS patients.