AXIN1 bi-allelic variants disrupting the C-terminal DIX domain cause craniometadiaphyseal osteosclerosis with hip dysplasia.

Sclerosing skeletal dysplasias result from an imbalance between bone formation and resorption. We identified three homozygous, C-terminally truncating AXIN1 variants in seven individuals from four families affected by macrocephaly, cranial hyperostosis, and vertebral endplate sclerosis. Other frequent findings included hip dysplasia, heart malformations, variable developmental delay, and hematological anomalies. In line with AXIN1 being a central component of the ß-catenin destruction complex, analyses of primary and genome-edited cells harboring the truncating variants revealed enhanced basal canonical Wnt pathway activity. All three AXIN1-truncating variants resulted in reduced protein levels and impaired AXIN1 polymerization mediated by its C-terminal DIX domain but partially retained Wnt-inhibitory function upon overexpression. Addition of a tankyrase inhibitor attenuated Wnt overactivity in the AXIN1-mutant model systems. Our data suggest that AXIN1 coordinates the action of osteoblasts and osteoclasts and that tankyrase inhibitors can attenuate the effects of AXIN1 hypomorphic variants.

CHARGE syndrome and related disorders: a mechanistic link.

CHARGE syndrome is an autosomal dominant malformation disorder caused by pathogenic variants in the chromatin remodeler CHD7. Affected are craniofacial structures, cranial nerves and multiple organ systems. Depending on the combination of malformations present, its distinction from other congenital disorders can be challenging. To gain a better insight into the regulatory disturbances in CHARGE syndrome, we performed RNA-Seq analysis on blood samples of 19 children with CHARGE syndrome and a confirmed disease-causing CHD7 variant in comparison with healthy control children. Our analysis revealed a distinct CHARGE syndrome pattern with downregulation of genes that are linked to disorders described to mimic the CHARGE phenotype, i.e. KMT2D and KDM6A (Kabuki syndrome), EP300 and CREBBP (Rubinstein-Taybi syndrome) and ARID1A and ARID1B (Coffin-Siris syndrome). Furthermore, by performing protein-protein interaction studies using co-immunoprecipitation, direct yeast-two hybrid and in situ proximity ligation assays, we could demonstrate an interplay between CHD7, KMT2D, KDM6A and EP300. In summary, our data demonstrate a mechanistic and regulatory link between the developmental disorders CHARGE-, Kabuki- and Rubinstein Taybi-syndrome providing an explanation for the overlapping phenotypes.

USP22 promotes HER2-driven mammary carcinoma aggressiveness by suppressing the unfolded protein response.

The Ubiquitin-Specific Protease 22 (USP22) is a deubiquitinating subunit of the mammalian SAGA transcriptional co-activating complex. USP22 was identified as a member of the so-called "death-from-cancer" signature predicting therapy failure in cancer patients. However, the importance and functional role of USP22 in different types and subtypes of cancer remain largely unknown. In the present study, we leveraged human cell lines and genetic mouse models to investigate the role of USP22 in HER2-driven breast cancer (HER2+-BC) and demonstrate for the first time that USP22 is required for the tumorigenic properties in murine and human HER2+-BC models. To get insight into the underlying mechanisms, we performed transcriptome-wide gene expression analyses and identified the Unfolded Protein Response (UPR) as a pathway deregulated upon USP22 loss. The UPR is normally induced upon extrinsic or intrinsic stresses that can promote cell survival and recovery if shortly activated or programmed cell death if activated for an extended period. Strikingly, we found that USP22 actively suppresses UPR induction in HER2+-BC cells by stabilizing the major endoplasmic reticulum (ER) chaperone HSPA5. Consistently, loss of USP22 renders tumor cells more sensitive to apoptosis and significantly increases the efficiency of therapies targeting the ER folding capacity. Together, our data suggest that therapeutic strategies targeting USP22 activity may sensitize tumor cells to UPR induction and could provide a novel, effective approach to treat HER2+-BC.