TSC2, cathepsin K, and β-catenin as immunohistochemical markers for lymphangioleiomyomatosis.
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The diagnosis of lymphangioleiomyomatosis (LAM) can be straightforward when LAM cells are abundant, but may be difficult with subtle lesions mimicking emphysema or with lung architecture obscured by parenchymal collapse. A variety of markers have been proposed for immunohistochemical confirmation of LAM. The is increasing evidence from sequencing studies that tuberous sclerosis 2 (TSC2) is frequently mutated in sporadic LAM, and TSC2 expression/loss can now be demonstrated by immunohistochemistry but TSC2 immunohistochemistry has not been reported in LAM. Here we examined the immunohistochemical performance of TSC2, β-catenin, cathepsin K, and HMB-45 in 10 cases of pulmonary LAM (4 with florid lesions, 6 with only subtle lesions), 6 cases of pelvic nodal LAM, and two cases of micronodular pneumocyte hyperplasia (MNPH), the latter a common finding in TSC LAM cases. TSC2 loss was observed in all cases in both florid and subtle lesions, nodal LAM, and MNPH. HMB45 was focally positive in 5/10 pulmonary and 3/6 nodal cases. β-catenin was variably, but sometimes weakly, expressed in 6/6 nodal cases and 10/10 pulmonary case but was often difficult to interpret in subtle lesions. Cathepsin K showed consistent strong positivity in all pulmonary and nodal cases. We conclude that, in well inflated lung with obvious LAM lesions, β-catenin, cathepsin K, and TSC2 are easy to interpret. In subtle cases Cathepsin K staining is the easiest to interpret but TSC2 staining also picks up subtle cases and offers the advantage of confirming the genetic change that implies a potential response to mTOR inhibitor therapy.