beta-Catenin mutation and its nuclear localization are confirmed to be frequent causes of Wnt signaling pathway activation in pilomatricomas

Background: beta-Catenin has been shown to play an important role in the formation of hair follicle-related tumors, including pilomatricomas. Several investigators have shown that beta-catenin gene mutation is observed in pilomatricomas. However, the relationship between the pattern of beta-catenin localization in the cell and beta-catenin gene mutation is still controversial. Objectives: This work was performed to determine the frequency of beta-catenin nuclear localization in pilomatricoma, the relationship between the pattern of beta-catenin localization and beta-catenin mutation, and the involvement of APC mutation. Methods: Typical 32 pilomatricomas were examined for beta-catenin expression by immunostaining. Genomic DNA was extracted, amplified and sequenced from 23 pilomaticomas with nuclear beta-catenin staining and 4 pilomaticomas without nuclear beta-catenin staining. Mutations of beta-catenin gene were confirmed by subcloning assay and restriction endonuclease assay. Results: Using immunostaining, we found that 81% (26/32) of pilomatricomas displayed nuclear beta-catenin staining in basophilic cells. Sequence analysis revealed that 61% (14/23) contained mutations in exon 3 of beta-catenin. However, no mutations were detected in 4 pilomaticomas without beta-catenin nuclear staining. Detected mutations were adjacent to or abolished well-known regulatory phosphorylation sites of beta-catenin. APC gene mutations were not detected in 27 pilomatricomas with/without beta-catenin nuclear staining. Conclusions: These results confirmed that beta-catenin mutation and its nuclear localization are frequent causes of Wnt signaling pathway activation and suggested that beta-catenin activation mutations contribute to tumorigenesis of pilomatricomas.