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Heterozygous carriers of germ-line loss-of-function variants in the DNA repair gene PALB2 are at a highly increased lifetime risk for developing breast cancer. While truncating variants in PALB2 are known to increase cancer risk, the interpretation of missense variants of uncertain significance (VUS) is in its infancy. Here we describe the development of a relatively fast and easy cDNA-based system for the semi high-throughput functional analysis of 48 VUS in human PALB2 . By assessing the ability of PALB2 VUS to rescue the DNA repair and checkpoint defects in Palb2 knockout mouse embryonic stem (mES) cells, we identify various VUS in PALB2 that impair its function. Three VUS in the coiled-coil domain of PALB2 abrogate the interaction with BRCA1, whereas several VUS in the WD40 domain dramatically reduce protein stability. Thus, our functional assays identify damaging VUS in PALB2 that may increase cancer risk. PALB2 is an established breast cancer risk gene but the pathogenicity of many variants remains uncharacterised. Here, the authors present a cDNA-based system for the functional analysis of PALB2 variants of unknown significance.

The authors show that tau can be released by neurons and transferred to other neurons via the extracellular space. Moreover, they show that enhancing neuronal activity accelerates transneuronal tau propagation and exacerbates tau pathology. Tau protein can transfer between neurons transneuronally and trans-synaptically, which is thought to explain the progressive spread of tauopathy observed in the brain of patients with Alzheimer's disease. Here we show that physiological tau released from donor cells can transfer to recipient cells via the medium, suggesting that at least one mechanism by which tau can transfer is via the extracellular space. Neuronal activity has been shown to regulate tau secretion, but its effect on tau pathology is unknown. Using optogenetic and chemogenetic approaches, we found that increased neuronal activity stimulates the release of tau in vitro and enhances tau pathology in vivo . These data have implications for disease pathogenesis and therapeutic strategies for Alzheimer's disease and other tauopathies.

DNA double-strand breaks (DSB) elicit a ubiquitylation cascade that controls DNA repair pathway choice. This cascade involves the ubiquitylation of histone H2A by the RNF168 ligase and the subsequent recruitment of RIF1, which suppresses homologous recombination (HR) in G1 cells. The RIF1-dependent suppression is relieved in S/G2 cells, allowing PALB2-driven HR to occur. With the inhibitory impact of RIF1 relieved, it remains unclear how RNF168-induced ubiquitylation influences HR. Here, we uncover that RNF168 links the HR machinery to H2A ubiquitylation in S/G2 cells. We show that PALB2 indirectly recognizes histone ubiquitylation by physically associating with ubiquitin-bound RNF168. This direct interaction is mediated by the newly identified PALB2-interacting domain (PID) in RNF168 and the WD40 domain in PALB2, and drives DNA repair by facilitating the assembly of PALB2-containing HR complexes at DSBs. Our findings demonstrate that RNF168 couples PALB2-dependent HR to H2A ubiquitylation to promote DNA repair and preserve genome integrity.

Loss-of-function variants in PALB2 give rise to defects in DNA damage repair by homologous recombination (HR), increasing the risk of breast cancer in female carriers. However, genetic testing frequently reveals missense variants of uncertain significance (VUS) for which the impact on protein function and cancer risk are unclear. Here we assay 84% of all possible missense variants in 11 out of 13 PALB2 exons using site-saturation functional screens with PARP inhibitor sensitivity as a readout for HR. These exons encode the coiled-coil and WD40 domains, which we identify as the minimal regions required for HR. Furthermore, we reveal the functional impact of 6718 missense variants, classifying 3904 variants as functional (58%), 2422 as intermediate (36%), and 392 as damaging (6%). A burden-type analysis shows that damaging missense variants in PALB2 are associated with a significantly increased risk of breast cancer, similar to that observed for truncating variants. These results will be valuable for the classification of PALB2 missense VUS and clinical management of carriers. The impact of variants of uncertain significance (VUS) on protein function and cancer risk remain unclear. Here, the authors focus on the functional impact of VUS of the PALB2 gene and identify defects in DNA damage repair by homologous recombination associated with increased risk of breast cancer.

BackgroundRare protein-truncating variants (PTVs) in partner and localiser of BRCA2 (PALB2) confer increased risk to breast cancer, but relatively few studies have reported the prevalence in South-East Asian populations. Here, we describe the prevalence of rare variants in PALB2 in a population-based study of 7840 breast cancer cases and 7928 healthy Chinese, Malay and Indian women from Malaysia and Singapore, and describe the functional impact of germline missense variants identified in this population.MethodsMutation testing was performed on germline DNA (n=15 768) using targeted sequencing panels. The functional impact of missense variants was tested in mouse embryonic stem cell based functional assays.ResultsPTVs in PALB2 were found in 0.73% of breast cancer patients and 0.14% of healthy individuals (OR=5.44; 95% CI 2.85 to 10.39, p<0.0001). In contrast, rare missense variants in PALB2 were not associated with increased risk of breast cancer. Whereas PTVs were associated with later stage of presentation and higher-grade tumours, no significant association was observed with missense variants in PALB2. However, two novel rare missense variants (p.L1027R and p.G1043V) produced unstable proteins and resulted in a decrease in homologous recombination-mediated repair of DNA double-strand breaks.ConclusionDespite genetic and lifestyle differences between Asian and other populations, the population prevalence of PALB2 PTVs and associated relative risk of breast cancer, are similar to those reported in European populations.

The tumour suppressor CHEK2 encodes the serine/threonine protein kinase CHK2 which, upon DNA damage, is important for pausing the cell cycle, initiating DNA repair and inducing apoptosis. CHK2 phosphorylation of the tumour suppressor BRCA1 is also important for mitotic spindle assembly and chromosomal stability. Consistent with its cell cycle checkpoint role, both germline and somatic variants in CHEK2 have been linked to breast and multiple other cancer types. Over 90% of clinical germline CHEK2 missense variants are classified as variants of uncertain significance, complicating diagnosis of CHK2-dependent cancer. We therefore sought to test the functional impact of all possible missense variants in CHK2. Using a scalable multiplexed assay based on the ability of human CHK2 to complement DNA sensitivity of a S. cerevisiae lacking its ortholog RAD53, we generated a systematic missense variant effect map for CHEK2 missense variation. Map scores reflect known biochemical features of CHK2 and exhibit good performance in separating pathogenic from benign clinical missense variants. Thus, the missense variant effect map for CHK2 offers value in understanding both known and yet-to-be-observed CHK2 variants.Competing Interest StatementF.P.R. is a shareholder and advisor for SeqWell, Constantiam, BioSymetrics, and a shareholder of Ranomics, and his lab has received research support from Alnylam, Deep Genomics, Beam Therapeutics and Biogen, Inc.

Deviation from proper muscle development or homeostasis results in various myopathic conditions. Employing genetic as well as chemical intervention, we provide evidence that a tight regulation of Wnt/beta-catenin signaling is essential for muscle fiber growth and maintenance. In zebrafish embryos, gain-of-Wnt/beta-catenin function results in unscheduled muscle progenitor proliferation, leading to slow and fast muscle hypertrophy accompanied by fast muscle degeneration. The effects of Wnt/beta-catenin signaling on fast muscle hypertrophy were rescued by misexpression of Myostatin or p21(CIP/WAF), establishing an in vivo regulation of myofibrillogenesis by Wnt/beta-catenin signaling and Myostatin. Epistatic analyses suggest a possible genetic interaction between Wnt/beta-catenin and Myostatin in regulation of slow and fast twitch muscle myofibrillogenesis.

Deviation from proper muscle development or homeostasis results in various myopathic conditions. Employing genetic as well as chemical intervention, we provide evidence that a tight regulation of Wnt/[beta]-catenin signaling is essential for muscle fiber growth and maintenance. In zebrafish embryos, gain-of-Wnt/[beta]-catenin function results in unscheduled muscle progenitor proliferation, leading to slow and fast muscle hypertrophy accompanied by fast muscle degeneration. The effects of Wnt/[beta]-catenin signaling on fast muscle hypertrophy were rescued by misexpression of Myostatin or p21.sup.CIP/WAF, establishing an in vivo regulation of myofibrillogenesis by Wnt/[beta]-catenin signaling and Myostatin. Epistatic analyses suggest a possible genetic interaction between Wnt/[beta]-catenin and Myostatin in regulation of slow and fast twitch muscle myofibrillogenesis.