Differential expression of desmosomal glycoproteins in keratoacanthoma and squamous cell carcinoma of the skin: an immunohistochemical aid to diagnosis.

The distinction between keratoacanthoma and squamous cell carcinoma is a common dermatopathological dilemma. Although the mainstay of the diagnosis is still clinico-pathological correlation, many dermatopathologists now include keratoacanthomas in the spectrum of squamous cell carcinomas. Recent reports, however, have pointed out that keratoacanthoma is "deficient squamous cell carcinoma" since it loses the expression of bcl-2 antigen, consistent with initiation of apoptosis, i.e. its involution. Electron microscope studies performed in keratoacanthomas and squamous cell carcinomas also revealed significantly reduced desmosomes in squamous cell carcinoma, but not in keratoacanthoma. A series of 38 keratoacanthomas and 62 squamous cell carcinomas of the skin (28 well-differentiated, 21 moderately differentiated and 13 poorly differentiated) were stained immunohistochemically with the monoclonal antibody 32-2B to desmosomal glycoproteins desmoglein 1 and desmoglein 3. Thirty-five keratoacanthomas showed extensive pericellular desmoglein expression. Three keratoacanthomas and 20 squamous cell carcinomas (19 well-differentiated, 1 moderately differentiated) showed focal staining, and in 11 squamous cell carcinomas (2 moderately differentiated, 9 poorly differentiated) the staining was negative. The remaining 31 squamous cell carcinomas (9 well differentiated, 18 moderately differentiated, 4 poorly differentiated) showed juxtanuclear staining. None of the squamous cell carcinomas exhibited the extensive pericellular pattern found in keratoacanthomas. Assessment of staining intensity, by 3 independent examiners, revealed a strong negative correlation between desmoglein expression and degree of dysplasia in the squamous cell carcinomas (p < 0.01). This antibody therefore clearly distinguishes these tumours and may be of value in the differential diagnosis of keratoacanthoma and squamous cell carcinomas in routine histopathology.