Ekaterina Menis

Response assessment during treatment of multiple myeloma (MM) typically relies on immunofixation and serum electrophoresis. However, low levels of IgG and especially IgA paraprotein are difficult to quantify reliably. The Hevylite Assay quantifies the kappa and lambda fractions of IgG and IgA separately and is useful to determine response to therapy. Using serum samples of 360 evaluable patients from the prospective GMMG‐MM5 trial (EudraCT‐No. 2010–019173‐16) we assessed the normalization of the kappa/lambda ratio with the Hevylite Assay (HLCr) at baseline, after induction, mobilization, autologous blood stem cell transplantation, consolidation and every three months during maintenance or follow‐up within two years after the end of consolidation. We observed a steady increase in the proportion of patients with normalized HLCr over the course of therapyAchieving HLCr normalization any time until the end of consolidation was associated with a trend towards a prolonged progression‐free survival (PFS; hazard ratio (HR) = 0.75, 95% confidence interval (95% CI) = 0.56–1.01, p = 0.06) but not overall survival (OS; HR = 0.94, 95% CI = 0.69–1.26, p = 0.66) in multivariable time‐dependent Cox regression analyses. Using a landmark analysis from the end of consolidation there was again a marginally statistically significant effect of HLCr normalization by the end of consolidation on PFS using a multivariable Cox model on the subset of the two study arms with continuous lenalidomide maintenance (HR 0.61, 95% CI 0.37–1.02, p = 0.06). No such effect was observed in study arms in which maintenance was only applied to patients not in CR at the end of consolidation. In conclusion, our analysis of the Hevylite Assay in patients with IgG or IgA myeloma from the GMMG‐MM5 study did not find evidence to support the general use of HLCr normalization as a response parameter for predicting PFS or OS. However, the differential effects of HLCr normalization depending on the way in which treatment was adapted to response may be of interest for future study designs on response‐adapted therapy. Trial Registration: ISRCTN05622749.

Significance Statement: Inverted formin 2 (INF2) is the key regulator of a stress response—calcium-mediated actin reset, or CaAR—that reorganizes the actin cytoskeleton of mammalian cells in response to calcium influx. INF2 has been linked to the podocytic kidney disease focal segemental glomerulosclerosis (FSGS) and to cases of the neurologic disorder Charcot–Marie–Tooth disease that are accompanied by FSGS. The authors show that >50 disease-associated INF2 variants lead to deregulation of CaAR in cell lines, in Drosophila nephrocytes, and in cells from patient cells with these disorders. Their findings suggest that CaAR can be used as a sensitive assay for INF2 function and for robust evaluation of disease-linked variants of this formin. This work also highlights the use of quantitative cellular assays in assessing effects of disease-associated mutations to better understand complex disease phenotypes. Background: Monogenic diseases provide favorable opportunities to elucidate the molecular mechanisms of disease progression and improve medical diagnostics. However, the complex interplay between genetic and environmental factors in disease etiologies makes it difficult to discern the mechanistic links between different alleles of a single locus and their associated pathophysiologies. Inverted formin 2 (INF2), an actin regulator, mediates a stress response—calcium mediated actin reset, or CaAR—that reorganizes the actin cytoskeleton of mammalian cells in response to calcium influx. It has been linked to the podocytic kidney disease focal segemental glomerulosclerosis (FSGS), as well as to cases of the neurologic disorder Charcot–Marie–Tooth disease that are accompanied by nephropathy, mostly FSGS. Methods: We used a combination of quantitative live cell imaging and validation in primary patient cells and Drosophila nephrocytes to systematically characterize a large panel of >50 autosomal dominant INF2 mutants that have been reported to cause either FSGS alone or with Charcot–Marie–Tooth disease. Results: We found that INF2 mutations lead to deregulated activation of formin and a constitutive stress response in cultured cells, primary patient cells, and Drosophila nephrocytes. We were able to clearly distinguish between INF2 mutations that were linked exclusively to FSGS from those that caused a combination of FSGS and Charcot–Marie–Tooth disease. Furthermore, we were able to identify distinct subsets of INF2 variants that exhibit varying degrees of activation. Conclusions: Our results suggest that CaAR can be used as a sensitive assay for INF2 function and for robust evaluation of diseased-linked variants of formin. More broadly, these findings indicate that cellular profiling of disease-associated mutations has potential to contribute substantially to sequence-based phenotype predictions.

Actin has well established functions in cellular morphogenesis. However, it is not well understood how the various actin assemblies in a cell are kept in a dynamic equilibrium, in particular when cells have to respond to acute signals. Here, we characterize a rapid and transient actin reset in response to increased intracellular calcium levels. Within seconds of calcium influx, the formin INF2 stimulates filament polymerization at the endoplasmic reticulum (ER), while cortical actin is disassembled. The reaction is then reversed within a few minutes. This Calcium-mediated actin reset (CaAR) occurs in a wide range of mammalian cell types and in response to many physiological cues. CaAR leads to transient immobilization of organelles, drives reorganization of actin during cell cortex repair, cell spreading and wound healing, and induces long-lasting changes in gene expression. Our findings suggest that CaAR acts as fundamental facilitator of cellular adaptations in response to acute signals and stress.