Clear cell trichoblastoma in association with a nevus sebaceus

Clear Cell Proliferations of the Skin: A Histopathologic Review

ABSTRACT

Cutaneous clear cell proliferations encompass a heterogenous group of several primary cutaneous neoplasms and metastatic tumors with different histogenesis. Many of these clear cell proliferations may seem strikingly similar under the microscope resulting in challenging diagnosis. In many of these clear cell lesions, the reason for the clear or pale appearance of proliferating cells is unknown, whereas in other ones, this clear cell appearance is due to intracytoplasmic accumulation of glycogen, mucin, or lipid. Artifacts of tissue processing and degenerative phenomenon may also be responsible for the clear cell appearance of proliferating cells. Awareness of the histopathologic findings as well as histochemical and immunohistochemical techniques are crucial to the accurate diagnosis. This review details the histopathologic features of clear cell cutaneous proliferations, classifying them according their type of differentiation and paying special attention to the histopathologic differential diagnosis among them.

Basal Cell Carcinoma With Matrical Differentiation: Clinicopathologic, Immunohistochemical, and Molecular Biological Study of 22 Cases

Basal cell carcinoma (BCC) with matrical differentiation is a fairly rare neoplasm, with about 30 cases documented mainly as isolated case reports. We studied a series of this neoplasm, including cases with an atypical matrical component, a hitherto unreported feature. Lesions coded as BCC with matrical differentiation were reviewed; 22 cases were included. Immunohistochemical studies were performed using antibodies against BerEp4, β-catenin, and epithelial membrane antigen (EMA). Molecular genetic studies using Ion AmpliSeq Cancer Hotspot Panel v2 by massively parallel sequencing on Ion Torrent PGM were performed in 2 cases with an atypical matrical component (1 was previously subjected to microdissection to sample the matrical and BCC areas separately). There were 13 male and 9 female patients, ranging in age from 41 to 89 years. Microscopically, all lesions manifested at least 2 components, a BCC area (follicular germinative differentiation) and areas with matrical differentiation. A BCC component dominated in 14 cases, whereas a matrical component dominated in 4 cases. Matrical differentiation was recognized as matrical/supramatrical cells (n=21), shadow cells (n=21), bright red trichohyaline granules (n=18), and blue-gray corneocytes (n=18). In 2 cases, matrical areas manifested cytologic atypia, and a third case exhibited an infiltrative growth pattern, with the tumor metastasizing to a lymph node. BerEP4 labeled the follicular germinative cells, whereas it was markedly reduced or negative in matrical areas. The reverse pattern was seen with β-catenin. EMA was negative in BCC areas but stained a proportion of matrical/supramatrical cells. Genetic studies revealed mutations of the following genes: CTNNB1, KIT, CDKN2A, TP53, SMAD4, ERBB4, and PTCH1, with some differences between the matrical and BCC components. It is concluded that matrical differentiation in BCC in most cases occurs as multiple foci. Rare neoplasms manifest atypia in the matrical areas. Immunohistochemical analysis for BerEP4, EMA, and β-catenin can be helpful in limited biopsy specimens. From a molecular biological prospective, BCC and matrical components appear to share some of the gene mutations but have differences in others, but this observation must be validated in a large series.

A cutaneous mixed tumor in a dog

The atypical cutaneous tumor of a 9-year-old mixed breed female dog was examined. The tumor was well-demarcated and histologically composed of a trichoblastic area, tricholemmal area and apocrine glandular area. Neoplastic cells in trichoblastic area and tricholemmal area had PAS-positive granules in the cytoplasm and were positive for pan-cytokeratin, cytokeratin 5/6, 14 and 19 and p63. Neoplastic cells in trichoblastic area were also positive for cytokeratin 15 and CD34. Neoplastic cells in apocrine glandular area were positive for pan-cytokeratin and cytokeratin 7, 18 and 19. Myoepithelial cell proliferation with osteocartilaginous metaplasia was observed in this area. Since neoplastic cells showed multiphenotypic differentiation for hair follicles and apocrine glands, the present case was diagnosed as a cutaneous mixed tumor.

Clear cell proliferations of the skin

Clear cell proliferations of the skin consist of a diverse group of lesions characterized by the presence of cells with abundant clear cytoplasm. They may arise from primary, metastatic, or non-neoplastic origins, with their distinctive cytoplasmic clearing attributable to causes ranging from artifact to accumulation of materials such as glycogen, mucin, or lipids. Despite the heterogeneity of these lesions, their distinguishing characteristics may be subtle, especially at high power. As such, histologic diagnosis can be challenging, and proper consideration relies on an understanding of the scope of possible origins, etiologies, and key immunohistochemical staining patterns of the various entities. This review categorizes cutaneous clear cell neoplasms by histogenesis, and offers differential diagnostic strategies to aid in their clinicopathologic evaluation.

PHLDA1, a follicular stem cell marker, differentiates clear-cell/granular-cell trichoblastoma and clear-cell/granular cell basal cell carcinoma: a case-control study, with first description of granular-cell trichoblastoma

BACKGROUND

Granular-cell or clear-cell basal cell carcinomas (BCCs), and clear-cell trichoblastomas have rarely been reported in the literature. PHLDA1 is a follicular stem cell marker, the expression of which has been reported to differentiate trichoepithelioma from BCCs. We wondered whether (1) granular-cell trichoblastoma exists and (2) PHLDA1 expression can differentiate a clear- and/or granular-cell trichoblastoma and a BCC.

METHODS

In the records of 2 pathology departments, 12 clear- and/or granular-cell trichoblastomas and BCCs were found. Epidemiological, histopathological, and immunohistochemical data for PHLDA1, Ki67, and AE1-AE3; and cytokeratins 17, 19, 20, and 6hf were collected.

RESULTS

Trichoblastomas and clear- and/or granular-cell BCCs occurred in 7 patients (36-62 years; mean: 49) and in 5 patients (63-91 years; mean: 77), respectively. Three granular-cell trichoblastomas were identified. Both granular and clear-cell changes were present in 4 BCCs. The patterns of expression for AE1/AE3, CK17, CK19, CK6hf, and CK20 were not helpful in differentiating trichoblastoma from BCC. PHLDA1 expression was diffusely or partially present in all trichoblastomas (5/5) and absent in BCCs (0/4) (P = 0.008).

CONCLUSIONS

Granular- and/or clear-cell changes are rare findings in trichoblastoma and BCC. First, we show that granular-cell trichoblastomas do occur. Second, PHLDA1 expression is a useful addition in differentiating trichoblastoma and BCC.

[Giant trichoblastoma]

Trichoblastoma is a benign tumour that differentiates towards the hair germ epithelium with a dense fibrous stroma that gives a hair bulb-like appearance. Currently it is being used the classification proposed by Ackerman and colleagues. The lesion usually presents as a solitary nodule, 1-2 cm in diameter, more frequently located on the scalp and face, that is easily removed during surgery. We describe the case of a large trichoblastoma located on the buttock.

Spiradenoma and spiradenocylindroma with an adenomatous or atypical adenomatous component: a clinicopathological study of 6 cases

We report 5 cases of spiradenoma and 1 case of spiradenocylindroma, which, in addition to areas of conventional growth, manifested an adenomatous component. This consisted of compactly situated, well-developed glands having small round lumens lined by inner pale to eosinophilic cells and surrounded by an outer well-formed peripheral layer of myoepithelial cells, which reacted with a variety of myoepithelial cell markers. In 1 case, apocrine secretion was evident in the glandular part of the lesion. In 4 of the 6 cases, the adenomatous component was a minor but significant portion of the tumors, but in 2 cases it was extensive, comprising approximately 20% of the tumor area. In 1 of these 2 cases, the luminal epithelium showed atypia including rare, atypical mitotic figures. In addition, there were foci of glands showing myoepithelial cell loss. As this alteration was limited and fairly well circumscribed within the tumor bulk, we regard it as an "atypical adenomatous component," but we cannot exclude the possibility that this may represent an incipient apocrine carcinoma, despite uneventful follow-up. Three cases also manifested clear cell areas. Immunohistochemical studies demonstrated that myoepithelial proliferation and overgrowth accounted for the clear cell change in some of the lesions.

PTCH mutations are not mainly involved in the pathogenesis of sporadic trichoblastomas

Trichoblastomas are rare, benign tumors of the appendix in human skin. The histopathology comprises elements of basal cell carcinoma and trichoepithelioma with a variable degree of follicular differentiation. Both basal cell carcinoma and trichoepithelioma reveal alterations of PTCH, the human homolog of the Drosophila segment polarity patched gene. Furthermore, heterozygous PTCH knockout mice develop trichoblastoma-like tumors. This suggests an involvement of the PTCH gene in the pathogenesis of human trichoblastomas. However, trichoblastomas arising in nevus sebaceus did not show loss of heterozygosity at the PTCH locus (9q22.3) in a previous study. Sequencing of the PTCH gene and analysis of sporadic human trichoblastomas have not been performed yet. We therefore screened 10 sporadic trichoblastomas and 1 trichoblastoma arising within a nevus sebaceus for PTCH mutations. After microdissection of the tumors, single-strand conformational polymorphism (SSCP)/heteroduplex analysis of exons 2 to 23 of PTCH was performed, and polymerase chain reaction products with aberrant band patterns were sequenced. One trichoblastoma revealed a silent mutation at codon 562 in exon 12. Another trichoblastoma showed a somatic C > T single nucleotide substitution at codon 1,315 (exon 23), which was not present in corresponding normal epidermis. This mutation at codon 1,315 represents an already described PTCH germline polymorphism and results in a heterozygous Pro to Leu substitution in the tumor. The Pro/Leu polymorphism in germline is associated with a higher risk for breast cancer, but a potential contribution to the tumorigenesis of trichoblastoma is unknown. We detected no classical PTCH mutations in the investigated trichoblastomas. Our results indicate that PTCH mutations are not mainly involved in the pathogenesis of sporadic trichoblastomas, in contrast to basal cell carcinomas and trichoepitheliomas. The genetic basis of this rare appendageal tumor remains elusive.

Clear cell trichoblastomas in two dogs

Two cases of clear cell trichoblastomas were diagnosed in young dogs. The tumour had ribbons of basaloid cells as seen in ribbon trichoblastomas, as well as differentiation to external root sheath of the hair follicle and few cells with sebaceous differentiation. This is the first report of clear cell trichoblastoma in dogs.

Clear Cell Trichoblastoma

Clear cell change in basal cell carcinomas is a well-recognized phenomenon, but is obviously rare in trichoblastomas. We present two cases of clear cell trichoblastoma in which clear cell change was very much prominent, and the results of an ultrastructural study intended to explore the basis of that feature. Both our patients were women, aged 56 and 77 years, who presented with solitary, slowly growing nodules that measured 3 to 5 cm in largest dimension and were located on the scalp and the flexor aspect of the lower arm. Microscopically, the tumors in both cases were symmetric, non-ulcerated, and composed of variably sized and shaped (cribriform, racemiform, strands, cords, nodules) aggregations of monomorphous basaloid epithelial cells that were associated with a specific trichogenic stroma. Common to both tumors was clear cell cytoplasm evident in the majority of the epithelial cells in one case and almost in the entire epithelial cell population in the other. In most epithelial aggregations the epithelial cells with clear cytoplasm often appeared columnar and were arranged in a palisade along a recognizable basal membrane, thus indicative of outer sheath differentiation at the bulb. There were other signs of follicular differentiation. Ultrastructurally, variably sized clusters of uniform small basaloid epithelial cells were separated from the stroma by a thin discontinuous basement membrane. In addition to the usual organelles, the cytoplasm contained fairly conspicuous tonofilaments and variably sized vacuoles devoid of a limiting membrane, located between the palisaded nuclei and the outer cell membrane. The majority of vacuoles were empty, although clumps of a finely granular substance were occasionally evident. No distinct lipid droplets or glycogen particles were identified. The basaloid cells were joined by scattered small desmosomes. These findings were consistent with trichilemmal differentiation at the bulb.

Basal cell carcinoma with tricholemmal (at the lower portion) differentiation within seborrheic keratosis

BACKGROUND

Recent genetic investigations support the idea that basal cell carcinoma (BCC) is trichoblastic carcinoma. However, it is generally thought that clear cell basal cell carcinoma is a result of degeneration rather than tricholemmal differentiation.

METHODS

We report a case of BCC, with clear cell components, that developed within seborrheic keratosis, with histopathological and immunohistochemical findings.

RESULTS

The clear cell components in the present case showed the following four characteristics: (i) at the periphery of the aggregations, columnar clear cells were aligned in a palisade along a well-defined basement membrane; (ii) the nuclei of the columnar clear cells were at the pole opposite the basement membrane; (iii) the clear cells contained glycogen; (iv) in the aggregations with clear cell components, there was diffuse positive staining for cytokeratin 7 (CK7) (OV/TLR/30), but only the inner region stained positive for CK17. These four characteristics are comparable to those of the lower portion of normal outer root sheath. In addition, the BCC in the present case was partly composed of squamous cells that contained glycogen and were selectively positive for CK17 - features similar to those of squamous cells in normal outer root sheath.

CONCLUSIONS

Some clear cell BCCs are simply the result of degenerative change, but other clear cell BCCs may be the result of tricholemmal (at the lower portion) differentiation.

Melanotrichoblastoma: immunohistochemical study of a variant of pigmented trichoblastoma

Trichoblastomas (TBs) are benign skin tumors recapitulating the differentiation of hair follicles. Several pathologic variants have been described, including the rare pigmented TB. We report a deeply pigmented nodular tumor excised on the scalp of a 32-year-old African woman, which was clinically suggestive of blue nevus or melanoma. Histologically, the tumor presented features of TB, remarkable by virtue of heavy melanin deposits found within and around tumor nests. By immunohistochemistry, abundant dendritic melanocytes with features of hair follicle melanocytes (expression of S100 protein, tyrosinase, and, most importantly, gp100/HMB-45) were found within the tumor masses. By analogy with melanoacanthoma, a tumor consisting of a combined proliferation of epidermal keratinocytes and melanocytes, we called this tumor "melanotrichoblastoma" and view it as a peculiar variant of (pigmented) TB.

Clear-cell porocarcinoma in situ: a cytologic variant of porocarcinoma in situ

Poromas are benign neoplasms composed of poroid and cuticular cells. Four histopathologic variants of poromas are accepted, according to the architectural features of the neoplasm: hidroacanthoma simplex or intraepidermal poroma; eccrine poroma, which is a poroma connected to the epidermis that extends to superficial dermis; dermal duct tumor, which develops when the neoplasm is composed of small, solid aggregations of poroid and cuticular cells confined to the dermis with little or no connection with the epidermis; and poroid hidradenoma, which is a solid-cystic, dermal poroma. The malignant counterpart of hidroacanthoma simplex is named malignant hidroacanthoma simplex or porocarcinoma in situ. This report describes an example of clear-cell malignant hidroacanthoma simplex, a cytologic variant of porocarcinoma in situ, which, to our knowledge, has not been previously reported. In contrast with other clear-cell neoplasms, a relation with diabetes mellitus could not be clearly established in this case.