Functional Consequences of CHRNA7 Copy-Number Alterations in Induced Pluripotent Stem Cells and Neural Progenitor Cells

Abstract

Copy-number variants (CNVs) of chromosome 15q13.3 manifest clinically as neuropsychiatric disorders with variable expressivity. CHRNA7, encoding for the α7 nicotinic acetylcholine receptor (nAChR), has been suggested as a candidate gene for the phenotypes observed. Here, we used induced pluripotent stem cells (iPSCs) and neural progenitor cells (NPCs) derived from individuals with heterozygous 15q13.3 deletions and heterozygous 15q13.3 duplications to investigate the CHRNA7-dependent molecular consequences of the respective CNVs. Unexpectedly, both deletions and duplications lead to decreased α7 nAChR-associated calcium flux. For deletions, this decrease in α7 nAChR-dependent calcium flux is expected due to haploinsufficiency of CHRNA7. For duplications, we found that increased expression of CHRNA7 mRNA is associated with higher expression of nAChR-specific and resident ER chaperones, indicating increased ER stress. This is likely a consequence of inefficient chaperoning and accumulation of α7 subunits in the ER, as opposed to being incorporated into functional α7 nAChRs at the cell membrane. Here, we showed that α7 nAChR-dependent calcium signal cascades are downregulated in both 15q13.3 deletion and duplication NPCs. While it may seem surprising that genomic changes in opposite direction have consequences on downstream pathways that are in similar direction, it aligns with clinical data, which suggest that both individuals with deletions and duplications of 15q13.3 manifest neuropsychiatric disease and cognitive deficits.

Keywords: 15q13.3 CNVs; CHRNA7; NPCs; iPSCs; neurodevelopmental disorders; neuropsychiatric disorders.

Figure 1

15q13.3 Region and CNVs in Probands Region from breakpoint (BP) 3 to BP5 shown, including BP4 and the proximal (P) and distal (D-CHRNA7-LCR) CHRNA7-LCRs. Red/blue regions indicate recurrent CNVs observed. Probands’ CNVs in the study are shown underneath their appropriate recurrent CNV in blue (duplications) or red (deletions), labeled with their identifier. CHRNA7 is highlighted by a red box. Adapted from UCSC Genome Browser.

Figure 2

Characterization of iPSCs and NPCs (A and B) iPSCs stain positive for SOX2 (A) and OCT4-AF488 (B), both pluripotency markers. Figure S1 shows additional characterization of iPSCs using flow cytometry and karyotypes. (C and D) NPCs stain positive for neuronal markers FOXG1 (C), SOX2 (see D for duplications and deletions), and PAX6 (D).

Figure 3

15q13.3 CNV NPCs Have Correlative Gene Expression by qPCR Control NPCs are shown in gray, duplication NPCs are in blue, and deletion NPCs are in red. All groups were compared using a one-way ANOVA. (A) CHRNA7 is fully duplicated for all sizes of 15q13.3 gains (n = 3 clones, 3 individuals). (B) The expression of OTUD7A in small D-CHRNA7-LCR/BP5 gains (n = 1 clone, 1 individual) is not altered as compared to controls (n = 3 clones, 3 individuals), while its expression is appropriately up- or downregulated in larger duplications (n = 2 clones, 2 individuals) and deletions (n = 3 clones, 3 individuals), respectively. ^∗∗∗∗p < 0.0001; ns, not significant; BP, breakpoint. Error bars indicate standard error.

Figure 4

15q13.3 Deletion and Duplication NPCs Have Decreased α7 nAChR-Dependent Calcium Flux (A and B) Controls are shown with a gray solid line, duplications are shown as blue diamonds, and deletions are shown as red circles. Error bars are not shown for simplicity in the figure but are shown in Figure S4. (A) With application of 1 μM epibatidine, an orthosteric nAChR agonist, NPCs have a response that is decreased for both 15q13.3 deletion and duplication NPCs (two-way ANOVA p < 0.0001; s: seconds). (B) With co-application of 1 μM epibatidine and 3 μM PNU-120596, an α7 nAChR-specific type II positive allosteric modulator (PAM), decreased α7 nAChR-dependent calcium flux is decreased for both 15q13.3 deletion and duplication NPCs (two-way ANOVA p < 0.0001). (C) Peak fluorescence, as measured by normalized relative light units, for each genotype, with co-application of 1 μM epibatidine and 3 μM PNU-120596. Deletions have significantly decreased peak fluorescence, while duplications have trending decreased peak fluorescence, indicating less calcium flux. Data are of three replicates for each condition for each individual (one NPC line each) in two experiments, and averages of each genotype (controls n = 3, duplications n = 3, deletions n = 3); p < 0.05; ns, not significant. Data are normalized to baseline values. (D) 15q13.3 deletion and duplication NPCs have decreased α7 nAChR-dependent calcium signaling via the JAK2-PI3K pathway, as determined by qPCR (p < 0.05, ^∗∗∗∗p < 0.0001, one-way ANOVA). Error bars indicate standard error.

Figure 5

15q13.3 Duplications Have Increased Expression of nAChR-Specific and Resident ER Chaperones All groups were compared using a one-way ANVOA. (A) RIC3, a nAChR-specific ER chaperone involved in folding, assembly, and trafficking of nAChRs, is upregulated in duplication NPCs (n = 3 clones, 3 individuals). The gene is downregulated in deletion NPCs (n = 3 clones, 3 individuals), which may be due to fewer nAChR subunits. NACHO, a nAChR-specific ER chaperone involved in folding, assembly, trafficking, and cell surface expression, is upregulated in duplication NPCs (n = 3 clones, 3 individuals) and downregulated in deletion NPCs (n = 3 clones, 3 individuals). (B) ER stress marker XBP1(s), the spliced mRNA of XBP1, is increased in duplication NPCs (n = 3 clones, 3 individuals) and is unchanged in deletion NPCs (3 clones, 3 individuals). (C) Downstream targets of XBP1 have differential changes in expression. ERDJ4, an ER-resident chaperone important in folding misfolded or unfolded proteins, as well as those that fold slowly, is upregulated by 26.5%, although not significantly due to small sample size, in duplication NPCs (n = 3 clones, 3 individuals). ERO1LB, an ER-resident chaperone and ERAD protein, is similarly upregulated in duplication NPCs (22.5%) (n = 3 clones, 3 individuals) and downregulated in deletion NPCs (n = 3 individuals, 3 clones). EDEM, an ER-associated degradation factor, has no changes in expression in either 15q13.3 duplication or deletion lines (n = 3 clones, 3 individuals for each), suggesting that the ER stress is mediated. (D) ER stress factors downstream of PERK dimerization and phosphorylation CHOP, GADD34, and BCL2, known to lead to cell death with changes in expression, are unchanged in both duplication and deletion NPCs (3 clones, 3 individuals for both), supporting that the ER stress is mediated in 15q13.3 duplication cells. ^∗p < 0.05, ^∗∗p < 0.01, ns, not significant. Error bars indicate standard error.

Figure 6

Proposed Mechanism of Pathogenesis of 15q13.3 Duplications We propose that CHRNA7 duplications result in increased gene expression. This results in increased α7 nAChR subunits (green) in the ER, which overwhelm the ER nAChR-specific chaperones RIC3 (purple) and NACHO (yellow). This causes adaptive ER stress, as indicated by increased ER stress marker expression of select pathways. A portion of these subunits are folded, assembled, and trafficked by RIC3 and NACHO, ending up at the cell membrane. However, there are fewer than in controls, and some of those at the membrane may have inappropriate stoichiometry, including incorporation of CHRFAM7A-encoded subunits (dark blue). This likely causes decreased depolarization at the cell membrane and does result in decreased α7 nAChR-dependent calcium flux. The decreased calcium flux downregulates calcium effectors, which likely has impacts on gene expression. These changes in calcium effectors and gene expression not only explain the phenotypes observed in probands, but may also contribute to the incomplete penetrance and variable expressivity of 15q13.3 CNVs.