O-6-alkylguanine-DNA alkyltransferase in cutaneous T-cell lymphoma: Implications for treatment with alkylating agents

Mycosis fungoides is a low-grade cutaneous T-cell lymphoma. Early treatment often involves the use of topical chemotherapy such as mechlorethamine or carmustine although single-agent oral chemotherapy with alkylators is commo n for advanced disease. Recently, in a Phase I study of the new alkylating agent temozolomide, two mycosis fungoides patients experienced a complete r esponse. The mechanism of resistance to alkylating drugs such as temozolomi de is thought to be due to the presence in tumor cells of the DNA repair pr otein, O-6-alkylguanine-DNA alkyltransferase (AGT), The protein mediates a reaction with the O-6-position of guanine in DNA, removing the lesion and l eaving guanine intact. We, therefore, examined the levels of AGT in CD4+ T lymphocytes obtained by negative antibody selection from the blood of nonca ncerous individuals and mycosis fungoides patients, and in paraffin-embedde d sections from mycosis fungoides patch, plaque, or tumor lesions and cells from involved lymph nodes. AGT protein levels were measured by quantitativ e immunofluorescence microscopy using a monoclonal antibody against human A GT, Using this approach, the mean level of our positive control (AGT-expres sing cells) was 84,807 molecules/nucleus; values below 5,000 molecules/nucl eus are considered very low. The mean AGT level in CD4+ T lymphocytes from noncancerous and cancerous individuals was 18,618 (n = 12) and 8,593 (n = 5 ), respectively. The mean fraction of outliers in circulating CD4+ T lympho cytes from mycosis fungoides patients was statistically significantly lower than T cells in lymph nodes. AGT molecules/nucleus from lymph node biopsie s from 8 of 10 patients showed low (<10,000 molecules/nucleus) or undetecta ble levels (n = 5) of AGT. The mean AGT level from samples of mycosis fungo ides patch/plaque and tumor was also low at 221 (n = 4) and 2,363 (n = 6), respectively. Surprisingly, Hut78, a mycosis fungoides T-cell lymphoma cell line, was positive for AGT activity (median: 77,700 molecules/nucleus), an d Hut102-another mycosis fungoides cell line-was low (median: 5,990 molecul es/nucleus). Because AGT is a primary means of cell resistance to alkylatin g agents, the low level of AGT in neoplastic T lymphocytes from patients wi th mycosis fungoides suggests that treatment with alkylating agents produci ng O-6-alkylguanine adducts, such as carmustine or temozolomide, may produc e improved clinical outcomes.