Objective To clarify the molecular mechanism of pathological skin hyperplasia in macrodactyly induced by PIK3CA activating mutations， and to provide a basis for clinical targeted therapy. Methods Lesional skin tissues of
macrodactyly and polydactyly controls were collected. The dermal structure and collagen fiber morphology were observed through histological staining， and the collagen structure was observed and the diameter of collagen fibers was measured by transmission electron microscopy （TEM）. RNA-sequencing （RNA-seq） combined with bioinformatics analysis was used to screen the differential pathways related to collagen synthesis and regulation. Primary fibroblasts were isolated and cultured， and cell functional experiments such as CCK-8， RT-qPCR， and Western blot were conducted to detect the cell proliferation ability and the expression levels of molecules related to collagen formation. Results The histological examination revealed that compared with the control group， the thickness of the dermal layer of the affected skin in patients with macrodactyly was significantly increased （P<0.000 1）， and the diameter of collagen fibers was significantly enlarged （P<0.000 1）. Under the microscope， it was observed that the arrangement of collagen fibers was disordered， elastic fibers were fragmented， and there was infiltration of adipose tissue in the dermal layer. RNA-Seq revealed aberrant activation of collagen synthesis and extracellular matrix remodeling pathways. Cellular experiments confirmed that fibroblasts from patients with macrodactyly had activating mutations in the PIK3CA gene （c. 3140 A>C， p. H1047R）， resulting in abnormal activation of the downstream PI3K-Akt pathway， and the proliferation ability and collagen formation ability of these cells were significantly higher than those of the control group. Conclusion This study suggests that activating PIK3CA mutations in fibroblasts may be involved in the pathological skin hyperplasia of macrodactyly and provides new insights for PI3K-targeted plastic therapy.
