Figure 2. Enhanced purine biosynthesis in chemoresistant SCLC.

(A) Enrichment scores reporting transcript abundance between 20 cell lines from relapsed patients and 34 cell lines from treatment-naive patients. Dashed lines demarcate P = 0.05. (B) Relative abundance of GMP, AMP, IMP, and XMP in 22 of the cell lines from A. Individual data points are shown with mean and SD for 3 cultures of each line. *P < 0.05, **P < 0.01, ****P < 0.0001. (C) IMPDH2, GMPS, and MYC abundance from single- cell RNA sequencing of SC68 PDX tumors and their chemoresistant counterpart SC68-CR. (D) Immunoblot analysis of ASCL1, MYC, IMPDH1, IMPDH2, GMPS, and ADSL in 3 pairs of treatment-naive and chemoresistant SCLC cell lines. CR, cisplatin resistant; ECR, etoposide and cisplatin resistant. (E) MYC, IMPDH1, and IMPDH2 expression in DMS53-CR and H1048-ECR cells with CRISPR/Cas9-mediated MYC KO or transfected with nontargeting (NT) guide RNA. (F) Relative GMP, AMP, GTP, and ATP abundance in 2 cell line pairs. *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001. (G and H) Fractional labeling of GMP and AMP in pairs of treatment-naive and chemoresistant cells cultured in medium containing [amide-¹⁵N]glutamine (G) or [U-¹³C]glucose (H) for 1, 3, and 6 hours. **P < 0.01, ***P < 0.001, ****P < 0.0001. (I) Relative abundance of GTP m + 3 and GTP m + 5 isotopologues in pairs of treatment-naive and chemoresistant cells cultured in medium containing [amide-¹⁵N]glutamine or [U-¹³C]glucose for 1, 3, and 6 hours. ***P < 0.001, ****P < 0.0001. Data are shown as mean and SD (F–I). Statistical significance was assessed using a 2-tailed Student’s t test (B and C), 1-way ANOVA with Tukey’s multiple-comparison test (F–I). Metabolomics in B was performed once. All other experiments were repeated twice or more.