Potential therapeutic targeting of ALCLs with MSC^E116K. (A) Immunohistochemistry for MYC in 83 T-NHLs with and without MSC^E116K, as indicated. Additional case details are given in supplemental Table 8. (B) Representative MYC expression in ALCLs with and without MSC^E116K. (C) Karpas 299 cells expressing MSC^E116K are more sensitive to JQ1 than are cells expressing MSC^wt. Data represent relative fractions of GFP⁻ cells in 0.2 μM JQ1 compared with vehicle for each of the constructs in competition with Karpas 299 cells overexpressing GFP. Summary of 3 independent experiments. (D) Treatment of Mac-1 cells stably expressing MSC^wt or MSC^E116K with JQ1 revealed increased sensitivity of MSC^E116K to JQ1 compared with MSC^wt (P = .004). (E) Model of interactions among MSC, E2F2, CD30, IRF4, and MYC in ALCL cells with MSC^wt or MSC^E116K. Briefly, MSC^wt transcriptionally regulates E2F2 (Figure 4), and the resultant E2F2 protein represses expression of TNFRSF8 encoding CD30 (Figure 5). MSC^E116K sequesters bHLH proteins but fails to bind to DNA, preventing heterodimer binding to canonical musculin and E2A target sequences (Figure 2) and inhibiting E2F2-mediated repression of CD30 (Figure 5). CD30 and IRF4 augment each other’s expression in a positive-feedback loop that leads to increased MYC expression and proliferation, as described previously.³⁹  In addition, IRF4 transcriptionally regulates MSC to increase expression of MSC^E116K, further augmenting expression of the CD30–IRF4–MYC axis (Figure 5). Although transcriptional regulation of MSC by IRF4 may also increase expression of MSC^wt, this arrow is not indicated in the model because IRF4-MSC^wt is not implicated in the same positive-feedback mechanism as IRF4-MSC^E116K, MSC^wt function is inhibited by the presence of dominant-negative MSC^E116K, and MSC^wt protein is less stable than MSC^E116K and is present at lower levels. NS, not significant.