(395) Absence of Slc26a9 function results in neonatal mortality in mice due to pulmonary complications.

Background: Modifier genes are of interest for their role in altering disease outcomes and revealing biologic relationships that would not be otherwise apparent. From a clinical perspective, modifier gene candidates of cystic fibrosis provide an opportunity to optimize patient treatment regimens and even identify new therapeutic targets. Intestinal manifestations of cystic fibrosis are numerous, yet are an area that requires further research. A previously published genome-wide association study identified modifier gene candidates of one such manifestation, meconium ileus, an ileal obstruction consisting of abnormally viscous secretions. These candidates include the solute carriers SLC26A9 and SLC6A14 as well as the H⁺/K⁺ transporter ATP12A [1]. Slc26a9 homozygous null mice had significantly reduced survival, most expiring within 24 hours after birth, with what appeared to be respiratory challenges, precluding the opportunity to study gastrointestinal manifestations. While there is still significant interest in the role of Slc26a9 in the intestine, we must first address the pulmonary phenotype.

Methods: To empirically evaluate these candidates in vivo in mouse models, temporal and spatial gene and protein expression were examined in neonates and adult animals by RT-PCR and immunostaining, respectively. All mice were on a common background, C57BL/6J. Magnetic resonance imaging (MRI) was used to compare air space in the lungs of one-day old littermates. Single cell gene expression databases were queried for sites of gene expression in lungs.

Results: Homozygous knockout of Slc26a9 in C57BL/6J mice results in early postnatal death, which prevented intestinal study. However, the mortality we observe was not reported in a published study in which the Slc26a9 null allele was bred on a mixed background. MRI scans of C57BL/6J newborn mice suggest little airspace in the lungs of homozygous knockout animals. Preliminary histological analysis also shows reduced air space in mutant neonates. Single cell gene expression data indicates Slc26a9 is expressed in the lungs in alveolar type II cells.

Conclusions: We hypothesize a failure of these pneumocytes to absorb fluid is contributing to the observed neonatal death. The survival differences related to genetic background provide an opportunity to identify genes that affect survival, presumably through fluid absorption. To this end, we are examining the difference in inbred and mixed background mouse lines, the latter of which shows greater survival, and will compare gene expression in the type II cells from the two backgrounds. Ultimately, we believe the modifying effects of modifier genes on a primary disease may be more complex and nuanced than we have previously appreciated, which will have further implications on precision medicine efforts.

Acknowledgements: This work was supported by the Research Institute for Children's Health

References: [1] Gong, J., Wang, F., Xiao, B., Panjwani, N., Lin, F., Keenan, K., … Strug, L. J. (2019). Genetic association and transcriptome integration identify contributing genes and tissues at cystic fibrosis modifier loci. PLoS Genetics, 15(2). https://doi.org/10.1371/journal.pgen.1008007