TP53 Abnormalities and MMR Preservation in 5 Cases of Proliferating Trichilemmal Tumours

Malignant Proliferating Pilar Tumor With Sarcomatous Transformation ("Carcinosarcoma"): Case Report With Molecular Profile

ABSTRACT

Malignant proliferating pilar tumors (MPPTs) are rare, unique cutaneous adnexal tumors. Sarcomatous transformation in MPPTs is even rarer (4 previous cases reported). Here, we report an extraordinary case of a MPPT with sarcomatous transformation occurring on the scalp of a 63-year-old man with an in-depth molecular profile along with histologic, immunohistochemical, and follow-up data. Shared mutations in the epithelial and sarcomatous components included a loss-of-function TP53 mutation. An inactivating TP53 mutation was only identified in the epithelial component, and an inactivating CDKN2A mutation was only identified in the sarcomatous component. Copy number variations previously reported in MPPT were also identified, including 6p21.1 loss, 6q arm loss, and 15q21.1-q26.3 gain [epithelial], and 6p22.2-p22.3 loss [sarcoma]. Histologically, the tumor demonstrated juxtaposed areas of proliferating pilar tumor, carcinoma with clear cell change, and sarcomatous areas that did not stain for AE1/AE3, p40, CD34, S100 protein, and smooth muscle actin  by immunohistochemistry. The patient is alive at 2 years without evidence of recurrence or metastasis.

Malignant Proliferating Pilar Tumor: Clinicopathologic, Immunohistochemical, and Molecular Study of 17 Cases

Proliferating pilar tumors are rare neoplasms that differentiate toward the outer sheath near the isthmus and can rarely undergo malignant transformation. We performed histopathologic evaluation on 26 benign proliferating pilar tumor (BPPT) and 17 malignant proliferating pilar tumor (MPPT). Ki-67 and p53 immunostains were performed on 13 BPPT and 10 MPPT. Six MPPT cases were successfully analyzed by a next-generation sequencing platform which surveyed exonic DNA sequences of 447 cancer genes and 191 regions across 60 genes for rearrangement detection. Patient demographics and clinical characteristics were similar between the BPPT and MPPT groups. Follow-up data of 16 of 17 MPPT (median, 25 mo) showed metastasis in 1 MPPT. The histologic features associated with MPPT include size >2.5 cm, adjacent desmoplastic stroma, small nests or cords of atypical epithelium in surrounding stroma, irregular infiltration or borders, abnormal keratinization, large hyperchromatic nuclei, prominent nucleoli, severe cytologic atypia, nuclear pleomorphism, necrosis, and increased mitotic figures. MPPT harbors copy number gains of 15q and losses of 6p and 6q, findings previously reported in BPPT. However, MPPT harbors frequent TP53 mutations as molecular markers of progression. Different from cutaneous squamous cell carcinoma, MPPT more frequently demonstrates low tumor mutational burden and typically lacks a UV signature, suggestive of a different etiologic pathway than squamous cell carcinoma. In summary, with a median follow-up of 25 months, this study shows that MPPT is a biologically indolent carcinoma with rare metastasis. Molecular analyses suggest a non-UV-related pathogenesis with frequent TP53 aberration.

Proliferating Pilar Tumors Are Characterized by Recurrent 15q, 6q, and 6p22.2 Alterations

ABSTRACT

Proliferating pilar tumors (PPTs) are rare neoplasms of external root sheath derivation, which most commonly occur on the scalp of elderly women. Although typically showing classic histologic features such as trichilemmal type keratinization, a lobular architecture and peripheral palisading, squamous cell carcinoma (SCC) remains a common diagnostic pitfall. Therefore, we sought to explore the molecular pathogenesis of PPTs and compare it with that of cutaneous squamous cell carcinoma (cSCC). Herein, we describe the use of a next-generation DNA sequencing platform to provide the most comprehensive molecular genetic analysis to date of a cohort of 5 PPTs and compare them to 5 head and neck cutaneous SCCs. Recurrent broad arm-level gains of 15q and concurrent single-copy losses of 6q and 6p22.2 were observed in 4 of 5 (80%) PPT cases. Other recurrent mutations or alterations of significance were not found in PPTs. Notably, these chromosomal changes were not identified in any of the 5 cutaneous SCCs, which instead showed recurrent alterations in the known SCC driver genes TP53 , CDKN2A , and NOTCH1 . Here, we show for the first time that PPTs are molecularly distinct from cutaneous SCC and provide evidence that recurrent alterations in chromosome 15 and chromosome 6 are central to the pathogenesis of PPTs.