Connect the dots

It has been known for roughly three decades that dot-like paranuclear (or, alternatively and reasonably equivalently, perinuclear) keratin immunopositivity can be utilized as a tool for the immunoperoxidase confirmation of a diagnosis of Merkel cell carcinoma.1 Utilized widely as a diagnostic tactic in the early 1990s, the significance of dots of paranuclear keratin expression has been largely superseded in the collective analytical mindset by the exceedingly popular use of CK20 (cytokeratin 20) immunohistochemistry, which also commonly manifests with dots of juxtanuclear positivity in the context of MCC.2,3 (Editor’s note: it is helpful to recall the past steady reliability of generic ‘dot’ keratin positivity as a tool to recognize MCC when dealing with the occasional case in which more modern staining techniques, discussed in the sentences that follow, prove disappointing or confusing.) Since 1993 or so, it has been reasonably clear that neurofilament immunohistochemistry provides a highly effective means to demonstrate cutaneous neuroendocrine lineage, as cytoplasmic dots of neurofilament positivity are both sensitive and specific for the detection of MCC.4 In head-to-head comparison of CK20 and neurofilament for recognition of MCC, neurofilament is superior,5,6 and the combination of the two reagents provides an extremely sensitive detection method.5 Neurofilament is also less prone than CK20 to exhibit the potentially misleading punctate paranuclear positivity that can be occasionally observed in association with selected non-cutaneous neuroendocrine carcinomas, such as ovarian or salivary gland small cell carcinoma.7 Sadly, however, neurofilament immunohistochemistry has never achieved acclaim as a diagnostic approach. CK20 remains the most popular kid on the schoolyard. When the question of is this Merkel cell carcinoma? is broached in the evaluation of a given case, CK20 staining is commonly combined with the evaluation of multiple other epitopes associated with neuroendocrine lineage, such as neuron specific enolase (NSE), CD56, chromogranin, and synaptophysin. Such a panel staining approach is probably inefficient and occasionally can prove to be profoundly detrimental or confusing, diagnostically speaking, as false positive labeling of non-neuroendocrine carcinomas, including basal cell carcinoma, can be found. Additionally, the four determinants mentioned in the last clause of the first sentence of this paragraph suffer from poor specificity, as they label both cutaneous and non-cutaneous neuroendocrine carcinomas. If a staining algorithm intended to identify a cutaneous metastasis of extracutaneous neuroendocrine carcinoma is desired, probably the deployment of thyroid transcription factor-1 (TTF1) in conjunction with neurofilament and CK20 is most advantageous, although vigilance against the rare pitfall of of MCC with a TTF1-positive immunophenotype remains a necessity.8 Intriguingly, MCC cells exhibiting punctate paranuclear dots of chromogen can be spotted (pun intended) utilizing a variety of immunoreagents, and the phenomenon is not restricted to stains that detect intermediate filaments. Juxtanuclear dot-like deposition of CD23, insulin-like growth factor-I receptor, and CD99 has been reported,9– 11 and dot-like paranuclear CK19 expression can be found routinely as well.12 Whether or not immunohistochemistry is identifying authentic juxtanuclear deposition of multiple epitopes – or whether instead there is merely a sticky filamentous ball in the cytoplasm of MCC cells that merely binds multiple epitopes, nonspecifically – has not been addressed in any meaningful way to the knowledge of this observer, but the latter explanation seems most likely (and, if this hypothesis is true, then it should be anticipated that additional diverse reagents exhibiting dot-like labeling of MCC will be found). In a similar vein, although CK19 has been put forth as a stem cell marker, given the hodgepodge of immunoreagents that exhibit paranuclear positivity, it seems likely that dot-like CK19 expression cannot be taken as trustworthy confirmation of a stem cell role in the pathogenesis of MCC.12 Rather, dots of CK19 expression are probably exactly analogous