Sc-RNA seq in familiar Gardner syndrome combined left atrial appendage fibroma reveals APC-C-MYC signaling modulates fibrotic subpopulation remodeling

Background Gardner’s syndrome was once considered a subtype of familial adenomatous polyposis (FAP), and their molecular pathological features remain to be clarified. Familiar Gardner’s syndrome complicated by a rare giant left atrial appendage aneurysm (LAAA) is an unreported novel type of FAP syndrome, and exploring its causative cellular subtypes and molecular pathological features will provide new insights into the precise treatment of the syndrome. Methods Whole-exome sequencing was performed in the familial Gardner’s syndrome patients, and single cell sequencing of left atrial dilatation tumor, intestinal polyp and scalp cyst in the proband was performed to explore the cellular and molecular pathological mechanisms. Results Exon sequencing indicated the presence of a rare germline variant (c. 4666 dup A, p. Thr1556fs, rs587783031) in APC (the adenomatous polyposis coli gene), which caused changes in the APC related Wnt pathway. Sc-RNA seq in LAAA revealed an increased proportion of fibroblasts and immune cells. We re-clustered fibroblasts and identified five distinct sub-populations in LAAA: cancer associcated fibroblasts (CAF), cardiomyocyte like fibroblasts (CLF), endothelial like fibroblasts (ELF), T\_cells like fibroblasts (TLF), and nomal like fibroblasts (NLF). Notably, nomal fibroblasts should constitute the main component of normal cardiac fibroblasts in LA tissues, while CAF mainly dominated in LAAA tissues. The trajectory and RNA velocity analysis revealed LAAA fibroblasts made a preferential transition from the immature phase (CLF, ELF, TLF, and NLF) at the apex of the trajectory to mature phase of tumor-like properties (CAF), indicating normal fibroblast was reprogrammed into CAF in LAAA. These results suggested diseased LA tissue with GS patient appears to display tumor-prone properties. Moreover, Sc-RNA seq in intestinal polyp revealed an increased proportion of T\_NK cells, and epithelial cells, while plasma cells, mesenchymal cells, and fibroblasts showed a significantly decreased in this patient. GO terms of intestinal polyp-derived fibroblasts suggested that APC/c-MYC signaling modulates fibrotic subpopulation remodeling in intestinal polyp of GS patient. In addition, Sc-RNA seq in scalp cyst revealed an increased proportion in epithelial cells and T cells in this patient. Furthermore, the expression of APC was lowly expressed in endothelial cell, fibroblasts, melanocytes, luminal epithelial, and T cells and the expression of c-MYC was highly expressed in melanocytes, luminal epithelial, and endothelial cell in scalp cyst. Fibroblasts of three tissues was integrated and re-clustered to evaluate the commone menchanisms of fibroblasts remodeling. We identified three fibroblast subpopulations (FC0, FC1 and FC2), the ratio of FC2 was shown a significantly increased in GS patient, and APC-C-MYC signaling might modulates FC2 subpopulation to proliferate fibroblasts in the occurrence of three GS tissues. Conclusions Using large-scale single cell RNA sequencing, cellular landscape and aetiology-specific alterations were identified in the left atrial dilatation tumor, intestinal polyp and scalp cyst of the proband. APC-C-MYC signaling modulates fibrotic subpopulation remodeling in LAAA and intestinal polyp while epithelial subpopulation remodeling in scalp cyst, indicating that syndrome subtypes caused by the same gene mutation in the same individual may still lead to different cellular and gene expression signatures and heterogeneity. This new approach provides a wealth of novel insights into the molecular changes that underlie the cellular processes relevant for cardiac biology and pathophysiology and also shed light on strategies for cell type- and stage-specific intervention in cardiac diseases. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This work was supported by grants from the Chinese National Natural Science Foundation (No. 81770379, 32171182, 81470521, and 81670290). ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: This study was approved by the ethics committee of the Sichuan Academy of Medical Sciences and the Sichuan Provincial People Hospital. 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Yes I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors.