Douglas M. Fowler

Variation alters inherent protein properties like thermodynamic stability and catalytic activity, and these intrinsic effects are shaped by the cellular environment. The RAS–MAPK signaling pathway typifies this interplay, with hundreds of proteins acting in concert through regulated states and interactions to transduce signals. To explore how cell context influences variant effects, we measured the abundance and pathway activity of 186,676 variants across 17 MAPK proteins in human cells under diverse conditions, for a total of 503,099 measurements. We discovered 14,412 dominant negative variants, nearly all previously unknown, that disrupt specific domains or interaction interfaces while preserving other functions. MAPK dominant negatives occur at conserved positions and are depleted from the human population relative to other activity-altering variants, representing a distinct constraint on the human genome. Modulating chaperone activity exposed previously hidden effects of genetic variation on protein abundance in human cells, revealing the HSP90 dependence of 67% of missense variants across the pathway. Combining hundreds of experimental and predicted protein interaction structures with variant effect data distinguished functional from non-functional interaction interfaces and revealed how their disruption affected function. We resolved the overlapping KRAS-LZTR1-SOS1 interface into partner-specific and shared binding determinants, identified allosteric networks supporting each interaction, and uncovered distinct mechanisms by which variants promote LZTR1-mediated degradation. Thus, we measure variant effects at pathway scale in human cells, illuminating fundamental MAPK pathway signaling mechanisms and demonstrating that cellular context profoundly shapes the effect of most variants.

Summary: High-resolution measurements of pathway activity make it possible to characterize how signaling responds to changing cellular conditions, to evaluate pathway-targeted inhibitors, and to dissect how sequence variation shapes signaling output. Here, we describe a sequencing-based assay that uses engineered, phospho-state-selective affinity reagents to quantify ERK activation state as a high-resolution, ratiometric readout of RAS/ERK MAPK pathway activity. Profiling ∼1,600 BRAF variants at basal and elevated RAS-GTP levels allowed us to assign activity scores to somatic cancer and germline RASopathy variants and to distinguish residues that stabilize autoinhibition from those required for active dimer formation. We identified a large class of dominant negative variants that suppress signaling below endogenous levels through enhanced RAS engagement and an inability to form productive RAF dimers. Measuring the intracellular potency of the RAF inhibitor LXH254 against ∼1,000 variants in parallel revealed hinge and activation-loop positions conferring resistance and showed that disrupting the BRAF-MEK interface is broadly sensitizing.

Variants of uncertain significance (VUS) have an unknown relationship with disease, undermining genetic medicine implementation. While multiplexed assays of variant effect (MAVEs) can help resolve VUS, major barriers prevent their routine clinical use. We present MaveMD (MAVEs for MeDicine; https://www.mavedb.org/mavemd ), a new MaveDB database interface that displays calibrated evidence strength, provides intuitive visualizations, integrates with established resources, and exports ready-to-use clinical evidence. MaveMD currently contains 476,076 variant effect measurements spanning 39 disease-associated genes, enabling classification of 75% of VUS. MaveMD supports future data generation efforts and the use of MAVE evidence in clinical practice, reducing VUS burden and improving outcomes.

Nucleoli, nuclear speckles and other compartments regulate transcription, RNA processing, and chromatin organization within the nucleus, yet the relationship of their morphology to developmental gene expression programs in vivo is poorly understood. Here, we develop a high-throughput Visual Cell Sorting (VCS) workflow for fixed cells and nuclei that combines antibody-based photoconversion; GPU-accelerated, real-time image analysis; and three-level single-cell combinatorial indexing RNA-seq (sci-RNA-seq3) to link nuclear compartment morphology to single-nucleus transcriptomes at embryo scale. We use VCS to analyze and sort over 1 million mouse embryo-derived nuclei by nucleolar, nuclear speckle, or nuclear size and construct a transcriptional atlas annotated with nuclear compartment phenotypes. Nuclear compartment size varies both between and within lineages and is shaped by proliferation and differentiation. In extracellular matrix protein-producing cell types such as fibroblasts, chondrocytes, and osteoblasts, nucleolar enlargement is uncoupled from cell cycle, and in erythroid cells exhibit a sharp nucleolar contraction preceding cell-cycle exit. We identify a 41-gene transcriptional signature whose expression tracks nucleolar size, enriched for ribosome biogenesis, mitochondrial metabolism, unfolded protein response, stress granule, and ubiquitin–proteasome pathway components. We used this nucleolar transcriptional signature to annotate mouse, zebrafish and human developmental atlases with nucleolar size information, revealing a conserved coupling between nucleolar activity and proteostasis programs. Our work establishes Visual Cell Sorting as a scalable platform for mapping image-based phenotypes to molecular programs; details the relationship between nuclear compartment phenotypes and development; and provides a transcriptional signature to estimate nucleolar size from existing single-cell datasets.

Background: Clear guidance is lacking regarding how ‘truthset’ variants should be used for clinical validation of functional assays, namely determining the allocatable evidence points (EPs) towards clinical classification. It is argued that assays should be validated using truthsets of missense variants, as this is the variant type for which classification is most impacted by functional data. EPs will be influenced by both the number of available ‘truthset’ variants and their concordance with assay readouts. Methods: We first reviewed 112 sets of ClinGen gene-specific classification specifications (CSPECs) to assess methodologies they applied for truthset assembly and clinical validation of assays. We then proposed differing rules regarding variant type and stringency of classification by which truthsets might be assembled using ClinVar-extracted classifications. We then examined augmentation of ClinVar-classified truthsets with ‘proxy-clinical’ benign-classified missense variants systematically assembled applying ACMG/AMP rules (of differing stringencies). In total, these constituted 70 basic approaches to ClinVar-based truthset assembly, which we applied to VHL, BRCA1, BRCA2 and RAD51C . We additionally analysed the impact on the size of the truthsets of changing the specified phenotypes against which ClinVar classification had been submitted. We then applied these truthsets to quantify concordance and allocatable EPs for five large-scale multiplexed functional assays for VHL , BRCA1 , BRCA2 , and RAD51C . Results: The EPs from clinical validation of each assay varied widely according to which truthset was used across 2,120 permutations of gene-truthset-assay combinations. For example, sequentially applying 700 different ClinVar-based truthsets to 2,268 VHL assay variant readouts (70 basic ClinVar-based approaches, augmented by examining 5 different phenotypes for each basic approach, and separate validation against two defined deleterious zones), the evidence strength allocatable for pathogenicity ranged from nil to strong evidence (0.0 to 5.6 EPs); for benignity it ranged from supporting to strong evidence (-1.5 to -6.4 EPs). Clinical validation using truthsets comprising just ClinVar-classified missense variants typically resulted in lower EPs than truthsets comprising protein truncating (PTV) and synonymous variants; this was more due to paucity of ClinVar-classified missense truthset variants than poorer concordance. Augmentation with larger ‘proxy-clinical’ benign-classified missense truthsets typically improved evidence allocatable for pathogenicity, with improved power negating modest reduction in concordance. Conclusions: EPs can be improved by augmentation with systematically-generated ‘proxy-clinical’ benign-classified missense variants and/or reduction of truthset stringency. Explicit prescriptive clinical guidance is urgently required to improve consistency in clinical validation of functional assays and consequent evidence application for clinical variant classification.

Background: Large-scale functional assays, including multiplex assays of variant effect, have substantial potential to resolve variants of uncertain significance (VUS), particularly for rare missense variants where clinical and population evidence are limited. The ClinGen assay-level clinical validation framework described by Brnich et al provided baseline guidance for the use of functional data for variant classification. However, clear consensus regarding construction of variant ‘truthsets’ by which to clinically validate functional data remains lacking. Methods: CanVIG-UK developed consensus recommendations for truthset construction through an iterative national consultation process involving the CanVIG Steering Advisory Group (CStAG), wider CanVIG-UK membership, and engagement with international functional genomics experts. Consultation was based on previous analyses of 2,120 truthset constructions examining the impact of truthset composition on evidence point allocation within the ClinGen assay-level clinical validation framework. Results: Across several consultations, CanVIG-UK established nine guiding principles and seven best-practice recommendations for assay-level clinical validation, using the assumed context of an assay for a cancer susceptibility gene where loss-of-function is the mechanism of pathogenicity. The principal recommendation stipulates, where assays are intended for use in interpretation of largely missense variants, the truthset used to validate should comprise only missense variants. Rather than mixtures of different variant types which may serve to over-estimate assay performance. Additional recommendations support option for relaxation of truthset stringency to improve power, augmentation of benign missense truthsets with systematically derived ‘proxy-clinical’ benign variants, independent clinical validation separate from assayist-defined validation, and careful evaluation of missense score distributions against that of protein-truncating and synonymous variants. Guidance is also provided for scenarios with limited pathogenic truthset availability and for assays reporting multiple deleterious zones or readouts. Conclusions: The CanVIG-UK principles and recommendations for truthset construction upon the ClinGen assay-level clinical validation framework, while aiming to form a baseline for future discussion regarding other functional and disease contexts and helping to address the gap between publication of new data and routine clinical implementation.

Obtaining a precise genetic tuberous sclerosis complex diagnosis is a challenge as many missense TSC2 variants are variants of uncertain significance. Variants of uncertain significance in TSC2 have been resolved by one-at-a-time functional assays, but these assays cannot scale to the 3634 TSC2 missense variants of uncertain significance observed so far. To address this challenge, we use massively parallel sequencing to measure the steady-state abundance of almost 9000 TSC2 missense variants and develop an mTOR pathway activity assay using genome editing and cell sorting to generate activity scores for 391 missense variants. We observe that 1256 of 8864 (14.17%) missense variants assayed have altered TSC2 abundance, and 69 of 391 (17.65%) missense variants assayed have altered mTOR pathway activity. Calibration and integration of these data into classification of variants identified in a clinical cohort putatively reclassifies 212 of 276 (76.8%) TSC2 missense variants of uncertain significance. These datasets will lead to improved genetic diagnosis of tuberous sclerosis complex with potential positive impacts on the clinical management of patients and their families.

Germline pathogenic variants in RAD51D and XRCC2 , which encode RAD51 paralogs that form a heterodimer within the BCDX2 complex, confer increased cancer risk and homologous recombination deficiency. However, most RAD51D and XRCC2 variants in ClinVar are classified as variants of uncertain significance (VUS), limiting clinical utility. Here, we applied saturation genome editing (SGE) to measure the effects of 5,412 RAD51D and 3,743 XRCC2 variants on cellular fitness and additionally assessed 2,876 and 2,069 variants in these genes for effects on RNA expression. Fitness scores discriminated pathogenic from benign variants with near-perfect accuracy (AUC=0.994 for RAD51D ; AUC=1.000 for XRCC2 ). Integration of RNA expression data revealed RAD51D , but not XRCC2 , is exceptionally sensitive to splice-altering variation, with 24% of RAD51D loss-of-function missense variants acting through RNA-mediated mechanisms compared to only 5% in XRCC2 . These SGE datasets provide strong, splice-resolved functional evidence to support variant classification across both genes.

Background: G6PD deficiency is one of the most common enzyme deficiencies worldwide, and increases the likelihood of adverse reactions to certain drugs and foods. As availability of routine sequencing and biobanking increases, more variants in G6PD have been identified but not connected to phenotypes, since most G6PD-deficient individuals are asymptomatic until they encounter a trigger. This is further complicated since over 60% of 1,559 known genetic variants in G6PD are variants of uncertain significance and thus cannot guide drug prescribing and dosing. Results: To resolve which variants are clinically meaningful and avoid harm from adverse drug reactions, we conducted two high-throughput functional assays: one for G6PD activity, and one for abundance. We measured the function of 9,504 missense, nonsense, and synonymous G6PD variants. The patterns of variant effect on activity and abundance confirmed the importance of structural NADP ⁺ for G6PD activity and abundance, and G6PD dimerization for G6PD activity. Based on the ability of our functional assay scores to accurately classify G6PD variants of known clinical effect, we generated evidence that 4,773 missense variants contribute to G6PD deficiency and 2,295 are unlikely to contribute to G6PD deficiency. Conclusions: Our data can be used to deepen our understanding of G6PD as a protein, and to close the gap in classification for variants of uncertain significance to improve implementation of genetic medicine for G6PD deficiency.