The multi-faceted nature of 15 CFTR exonic variations: Impact on their functional classification and perspectives for therapy

BACKGROUND

The majority of variants of unknown clinical significance (VUCS) in the CFTR gene are missense variants. While change on the CFTR protein structure or function is often suspected, impact on splicing may be neglected. Such undetected splicing default of variants may complicate the interpretation of genetic analyses and the use of an appropriate pharmacotherapy.

METHODS

We selected 15 variants suspected to impact CFTR splicing after in silico predictions on 319 missense variants (214 VUCS), reported in the CFTR-France database. Six specialized laboratories assessed the impact of nucleotide substitutions on splicing (minigenes), mRNA expression levels (quantitative PCR), synthesis and maturation (western blot), cellular localization (immunofluorescence) and channel function (patch clamp) of the CFTR protein. We also studied maturation and function of the truncated protein, consecutive to in-frame aberrant splicing, on additional plasmid constructs.

RESULTS

Six of the 15 variants had a major impact on CFTR splicing by in-frame (n = 3) or out-of-frame (n = 3) exon skipping. We reclassified variants into: splicing variants; variants causing a splicing defect and the impairment of CFTR folding and/or function related to the amino acid substitution; deleterious missense variants that impair CFTR folding and/or function; and variants with no consequence on the different processes tested.

CONCLUSION

The 15 variants have been reclassified by our comprehensive approach of in vitro experiments that should be used to properly interpret very rare exonic variants of the CFTR gene. Targeted therapies may thus be adapted to the molecular defects regarding the results of laboratory experiments.

Receptor Tyrosine Kinases ror1/2 and ryk Are Co-expressed with Multiple Wnt Signaling Components During Early Development of Sea Urchin Embryos

AbstractA combination of receptors, co-receptors, and secreted Wnt modulators form protein complexes at the cell surface that activate one or more of the three different Wnt signaling pathways (Wnt/β-catenin, Wnt/JNK, and Wnt/Ca²⁺). Two or more of these pathways are often active in the same cellular territories, forming Wnt signaling networks; however, the molecular mechanisms necessary to integrate information from these pathways in these situations are unclear in any in vivo model system. Recent studies have implicated two Wnt binding receptor tyrosine kinases, receptor tyrosine kinase-like orphan receptor (Ror) and related-to-receptor tyrosine kinase (Ryk), in the regulation of canonical and non-canonical Wnt signaling pathways, depending on the context; however, the spatiotemporal expression of these genes in relation to Wnt signaling components has not been well characterized in most deuterostome model systems. Here we use a combination of phylogenetic and spatiotemporal gene expression analyses to characterize Ror and Ryk orthologs in sea urchin embryos. Our phylogenetic analysis indicates that both ror1/2 and ryk originated as single genes from the metazoan ancestor. Expression analyses indicate that ror1/2 and ryk are expressed in the same domains of many Wnt ligands and Frizzled receptors essential for the specification and patterning of germ layers along the early anterior-posterior axis. In addition, both genes are co-expressed with Wnt signaling components in the gut, ventral ectoderm, and anterior neuroectoderm territories later in development. Together, our results indicate that Ror and Ryk have a complex evolutionary history and that their spatiotemporal expression suggests that they could contribute to the complexity of Wnt signaling in early sea urchin embryogenesis.

Deciphering the molecular basis of ferroportin resistance to hepcidin: Structure/function analysis of rare SLC40A1 missense mutations found in suspected hemochromatosis type 4 patients

Genetic medicine applied to the study of hemochromatosis has identified the systemic loop controlling iron homeostasis, centered on hepcidin-ferroportin interaction. Current challenges are to dissect the molecular pathways underlying liver hepcidin synthesis in response to circulatory iron, HFE, TFR2, HJV, TMPRSS6 and BMP6 functions, and to define the major structural elements of hepcidin-ferroportin interaction. We built a first 3D model of human ferroportin structure, using the crystal structure of EmrD, a bacterial drug efflux transporter of the Major Facilitator Superfamily, as template. The model enabled study of disease-associated mutations, and guided mutagenesis experiments to determine the role of conserved residues in protein stability and iron transport. Results revealed novel amino acids that are critical for the iron export function and the hepcidin-mediated inhibition mechanism: for example, tryptophan 42, localized in the extracellular end of the ferroportin pore and involved in both biological functions. Here, we propose a strategy that is not limited to structure analysis, but integrates information from different sources, including human disease-associated mutations and functional in vitro assays. The first major hypothesis of this PhD thesis is that ferroportin resistance to hepcidin relies on different molecular mechanisms that are critical for ferroportin endocytosis, and include at least three fundamental steps: (i) hepcidin binding to ferroportin, (ii) structural reorganization of the N- and C-ter ferroportin lobes, and (iii) ferroportin ubiquitination.