Improvement of hypoventilation in Phox2b mutant mice modelling the congenital central hypoventilation syndrome by etonogestrel, a potent progestin. Perspectives for a possible potentiation by serotoninergic drugs

Introduction Congenital central hypoventilation syndrome (CCHS) is a life-threatening disorder characterized by hypoventilation during sleep and absence of the ventilatory response to CO2/H+. The disease-causing mutations are generally polyalanine repeat expansion mutations of PHOX2B. Knock-in mice bearing the 7-alanine expanded allele of PHOX2B (the most frequent mutation in patients) exhibit massive loss of retrotrapezoid nucleus neurons, lack CO2/H + chemosensitivity, and die within hours after birth. No pharmacological treatment is available. Clinical observations revealed a recovery of CO2/H+ chemosensitivity in two adult female patients with CCHS, both under desogestrel, a potent progestin used for contraceptive purpose. We previously showed that in mice, etonogestrel (ETO), the active metabolite of desogestrel, increased baseline respiratory frequency (fR) by a medullary action involving the serotoninergic (5-HT) systems. Accordingly, we hypothesized that ETO may stimulate fR on Phox2b mutant mice modelling CCHS and that 5-HT drugs would improve this respiratory effect of ETO. Methods Experiments were made on ex vivo medullary-spinal cord preparations from conditional RTN Phox2b mutant (Egr2cre/+;Phox2b27Alaki) and OF1 WT newborn mice (P0-P2). fR was assessed by recording the 4th spinal ventral root. Effect of ETO on fR under normopH and metabolic acidosis (MA) conditions was evaluated in presence or not of 5-HT reuptake inhibitor. Results Without ETO, the increase in fR induced by MA observed in WT mice was absent in Egr2cre/+;Phox2b27Alaki mutant mice. ETO restored a respiratory response to MA in Egr2cre/+;Phox2b27Alaki mutant mice. In OF1 WT mice, when ETO was associated with 5-HT reuptake inhibitor, the increase in fR induced by MA was enhanced. Surprisingly, this potentiation was not retrieved in Egr2cre/+;Phox2b27Alaki mutant mice at concentrations used in OF1 WT. Conclusion ETO was able to alleviate hypoventilation in Phox2b mutant mice modelling CCHS by a medullary action. Modulation of 5-HT systems may improve the respiratory effect of ETO in WT mice, but further studies are necessary to determine whether this increase of ETO effect by 5-HT extends to mutant mice.