Outcome of publicly funded nationwide first-tier noninvasive prenatal screening

PURPOSE

Noninvasive prenatal screening (NIPS) using cell-free DNA has transformed prenatal care. Belgium was the first country to implement and fully reimburse NIPS as a first-tier screening test offered to all pregnant women. A consortium consisting of all Belgian genetic centers report the outcome of two years genome-wide NIPS implementation.

METHODS

The performance for the common trisomies and for secondary findings was evaluated based on 153,575 genome-wide NIP tests. Furthermore, the evolution of the number of invasive tests and the incidence of Down syndrome live births was registered.

RESULTS

Trisomies 21, 18, and 13 were detected in respectively 0.32%, 0.07%, and 0.06% of cases, with overall positive predictive values (PPVs) of 92.4%, 84.6%, and 43.9%. Rare autosomal trisomies and fetal segmental imbalances were detected in respectively 0.23% and 0.07% of cases with PPVs of 4.1% and 47%. The number of invasive obstetric procedures decreased by 52%. The number of trisomy 21 live births dropped to 0.04%.

CONCLUSION

Expanding the scope of NIPS beyond trisomy 21 fetal screening allows the implementation of personalized genomic medicine for the obstetric population. This genome-wide NIPS approach has been embedded successfully in prenatal genetic care in Belgium and might serve as a framework for other countries offering NIPS.

Polycystin-1 but not polycystin-2 deficiency causes upregulation of the mTOR pathway and can be synergistically targeted with rapamycin and metformin

Autosomal dominant polycystic kidney disease (ADPKD) is caused by loss-of-function mutations in either PKD1 or PKD2 genes, which encode polycystin-1 (TRPP1) and polycystin-2 (TRPP2), respectively. Increased activity of the mammalian target of rapamycin (mTOR) pathway has been shown in PKD1 mutants but is less documented for PKD2 mutants. Clinical trials using mTOR inhibitors were disappointing, while the AMP-activated kinase (AMPK) activator, metformin is not yet tested in patients. Here, we studied the mTOR activity and its upstream pathways in several human and mouse renal cell models with either siRNA or stable knockdown and with overexpression of TRPP2. Our data reveal for the first time differences between TRPP1 and TRPP2 deficiency. In contrast to TRPP1 deficiency, TRPP2-deficient cells did neither display excessive activation of the mTOR-kinase complex nor inhibition of AMPK activity, while ERK1/2 and Akt activity were similarly affected among TRPP1- and TRPP2-deficient cells. Furthermore, cell proliferation was more pronounced in TRPP1 than in TRPP2-deficient cells. Interestingly, combining low concentrations of rapamycin and metformin was more effective for inhibiting mTOR complex 1 activity in TRPP1-deficient cells than either drug alone. Our results demonstrate a synergistic effect of a combination of low concentrations of drugs suppressing the increased mTOR activity in TRPP1-deficient cells. This novel insight can be exploited in future clinical trials to optimize the efficiency and avoiding side effects of drugs in the treatment of ADPKD patients with PKD1 mutations. Furthermore, as TRPP2 deficiency by itself did not affect mTOR signaling, this may underlie the differences in phenotype, and genetic testing has to be considered for selecting patients for the ongoing trials.

Polycystin-1 and polycystin-2 are both required to amplify inositol-trisphosphate-induced Ca2+ release

Autosomal dominant polycystic kidney disease is caused by loss-of-function mutations in the PKD1 or PKD2 genes encoding respectively polycystin-1 and polycystin-2. Polycystin-2 stimulates the inositol trisphosphate (IP(3)) receptor (IP(3)R), a Ca(2+)-release channel in the endoplasmic reticulum (ER). The effect of ER-located polycystin-1 is less clear. Polycystin-1 has been reported both to stimulate and to inhibit the IP(3)R. We now studied the effect of polycystin-1 and of polycystin-2 on the IP(3)R activity under conditions where the cytosolic Ca(2+) concentration was kept constant and the reuptake of released Ca(2+) was prevented. We also studied the interdependence of the interaction of polycystin-1 and polycystin-2 with the IP(3)R. The experiments were done in conditionally immortalized human proximal-tubule epithelial cells in which one or both polycystins were knocked down using lentiviral vectors containing miRNA-based short hairpins. The Ca(2+) release was induced in plasma membrane-permeabilized cells by various IP(3) concentrations at a fixed Ca(2+) concentration under unidirectional (45)Ca(2+)-efflux conditions. We now report that knock down of polycystin-1 or of polycystin-2 inhibited the IP(3)-induced Ca(2+) release. The simultaneous presence of the two polycystins was required to fully amplify the IP(3)-induced Ca(2+) release, since the presence of polycystin-1 alone or of polycystin-2 alone did not result in an increased Ca(2+) release. These novel findings indicate that ER-located polycystin-1 and polycystin-2 operate as a functional complex. They are compatible with the view that loss-of-function mutations in PKD1 and in PKD2 both cause autosomal dominant polycystic kidney disease.

Unraveling the role of polycystin-2/inositol 1,4,5-trisphosphate receptor interaction in Ca signaling

Autosomal dominant polycystic kidney disease (ADPKD) arises as a consequence of mutations of the genes PKD1 and PKD2, encoding respectively the integral membrane proteins polycystin-1 and polycystin-2 (TRPP2), resulting in a disturbance in intracellular Ca(2+) signaling. Previously we investigated the interaction between TRPP2 and the inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R), an intracellular Ca(2+) channel in the endoplasmic reticulum (ER). We identified the molecular determinants of this interaction and observed an enhanced IP(3)-induced Ca(2+) release (IICR). Since we found that TRPP2 strongly bound to a cluster of positively charged amino acids in the N-terminal ligand-binding domain (LBD) of the IP(3)R, we now investigated whether TRPP2 would interfere with the binding of IP(3) to the IP(3)R. In in vitro experiments we observed that TRPP2 partially inhibited the binding of IP(3) to the LBD of the IP(3)R with an IC(50) of ∼350 nM. The suppressor domain, i.e., the N-terminal 225 amino acids of the LBD of the IP(3)R, mediated this inhibitory effect of TRPP2 on IP(3) binding. The observation that the interaction between the IP(3)R and TRPP2 decreased IP(3) binding is in apparent contrast to the increased IICR. The data can be explained however by a subsequent activation of Ca(2+)-induced Ca(2+) release (CICR) via TRPP2. Implications of this mechanism for cellular Ca(2+) signaling are discussed in this addendum.

Polycystin-2 activation by inositol 1,4,5-trisphosphate-induced Ca2+ release requires its direct association with the inositol 1,4,5-trisphosphate receptor in a signaling microdomain

Autosomal dominant polycystic kidney disease is characterized by the loss-of-function of a signaling complex involving polycystin-1 and polycystin-2 (TRPP2, an ion channel of the TRP superfamily), resulting in a disturbance in intracellular Ca(2+) signaling. Here, we identified the molecular determinants of the interaction between TRPP2 and the inositol 1,4,5-trisphosphate receptor (IP(3)R), an intracellular Ca(2+) channel in the endoplasmic reticulum. Glutathione S-transferase pulldown experiments combined with mutational analysis led to the identification of an acidic cluster in the C-terminal cytoplasmic tail of TRPP2 and a cluster of positively charged residues in the N-terminal ligand-binding domain of the IP(3)R as directly responsible for the interaction. To investigate the functional relevance of TRPP2 in the endoplasmic reticulum, we re-introduced the protein in TRPP2(-/-) mouse renal epithelial cells using an adenoviral expression system. The presence of TRPP2 resulted in an increased agonist-induced intracellular Ca(2+) release in intact cells and IP(3)-induced Ca(2+) release in permeabilized cells. Using pathological mutants of TRPP2, R740X and D509V, and competing peptides, we demonstrated that TRPP2 amplified the Ca(2+) signal by a local Ca(2+)-induced Ca(2+)-release mechanism, which only occurred in the presence of the TRPP2-IP(3)R interaction, and not via altered IP(3)R channel activity. Moreover, our results indicate that this interaction was instrumental in the formation of Ca(2+) microdomains necessary for initiating Ca(2+)-induced Ca(2+) release. The data strongly suggest that defects in this mechanism may account for the altered Ca(2+) signaling associated with pathological TRPP2 mutations and therefore contribute to the development of autosomal dominant polycystic kidney disease.

Knockdown of the type 3 iodothyronine deiodinase (D3) interacting protein peroxiredoxin 3 decreases D3-mediated deiodination in intact cells

The type 3 iodothyronine deiodinase (D3) is the primary deiodinase that inactivates thyroid hormone. Immunoprecipitation of D3, followed by fluorescent two-dimensional difference gel electrophoresis and mass spectrometry, identified peroxiredoxin 3 (Prx3) as a D3-associated protein. This interaction was confirmed using reverse coimmunoprecipitation, in which pull-down of Prx3 resulted in D3 isolation, and by fluorescence resonance energy transfer between cyan fluorescent protein-D3 and yellow fluorescent protein-Prx3. Prx3 overexpression did not change D3 activity in transfected HEK 293 cells; however, Prx3 knockdown resulted in a 50% decrease in D3-mediated whole-cell deiodination. Notably, D3 activity of cell lysates with dithiothreitol as an exogenous reducing factor and D3 protein levels were not decreased with Prx3 knockdown, indicating that the observed reduction in whole-cell deiodination was not simply due to a decrease in D3 enzyme levels. Prx3 knockdown did not change D3's affinity for T3 because saturation of D3-mediated whole-cell deiodination occurred between 20 and 200 nm T3 both with and without Prx3. Furthermore, the decrease in D3 activity in whole cells was not attributable to nonspecific oxidative stress because pretreatment with the antioxidant N-acetyl cysteine did not reverse the effects of Prx3 knockdown. Thioredoxin, the cofactor needed for Prx3 regeneration, supported D3 microsomal activity; however, Prx3 knockdown did not change D3 activity in this system. In conclusion, knockdown of Prx3 decreases D3 activity in whole cells, whereas absolute levels of D3 are unchanged, consistent with Prx3 playing a rate-limiting role in the regeneration of the D3 enzyme.

The IRBIT domain adds new functions to the AHCY family

During the past few years, the IRBIT domain has emerged as an important add-on of S-adenosyl-L-homocystein hydrolase (AHCY), thereby creating the new family of AHCY-like proteins. In this review, we discuss the currently available data on this new family of proteins. We describe the IRBIT domain as a unique part of these proteins and give an overview of its regulation via (de)phosphorylation and proteolysis. The second part of this review is focused on the potential functions of the AHCY-like proteins. We propose that the IRBIT domain serves as an anchor for targeting AHCY-like proteins towards cytoplasmic targets. This leads to regulation of (i) intracellular Ca2+ via the inositol 1,4,5-trisphosphate receptor (IP3R), (ii) intracellular pH via the Na+/HCO3 - cotransporters (NBCs); whereas inactivation of the IRBIT domain induces (iii) nuclear translocation and regulation of AHCY activity. Dysfunction of AHCY-like proteins will disturb these three important functions, with various biological implications.

Protein phosphatase-1 is a novel regulator of the interaction between IRBIT and the inositol 1,4,5-trisphosphate receptor

IRBIT is an IP3R [IP3 (inositol 1,4,5-trisphosphate) receptor]-binding protein that competes with IP3 for binding to the IP3R. Phosphorylation of IRBIT is essential for the interaction with the IP3R. The unique N-terminal region of IRBIT, residues 1-104 for mouse IRBIT, contains a PEST (Pro-Glu-Ser-Thr) domain with many putative phosphorylation sites. In the present study, we have identified a well-conserved PP1 (protein phosphatase-1)-binding site preceeding this PEST domain which enabled the binding of PP1 to IRBIT both in vitro and in vivo. IRBIT emerged as a mediator of its own dephosphorylation by associated PP1 and, hence, as a novel substrate specifier for PP1. Moreover, IRBIT-associated PP1 specifically dephosphorylated Ser68 of IRBIT. Phosphorylation of Ser68 was required for subsequent phosphorylation of Ser71 and Ser74, but the latter two sites were not targeted by PP1. We found that phosphorylation of Ser71 and Ser74 were sufficient to enable inhibition of IP3 binding to the IP3R by IRBIT. Finally, we have shown that mutational inactivation of the docking site for PP1 on IRBIT increased the affinity of IRBIT for the IP3R. This pinpoints PP1 as a key player in the regulation of IP3R-controlled Ca2+ signals.