Background & Aims: Expression of mucin MUC2, the structural component of the colonic mucus layer, is lowered in inflammatory bowel disease. Our aim was to obtain insight in the role of Muc2 in epithelial protection. Methods: Muc2 knockout (Muc2^−/−) and Muc2 heterozygous (Muc2^+/−) mice were characterized and challenged by a colitis-inducing agent, dextran sulfate sodium (DSS). We monitored clinical symptoms, intestinal morphology, and differences in intestine-specific protein and messenger RNA levels. Results: The Muc2^−/− mice showed clinical signs of colitis (as of 5 weeks), aggravating as the mice aged. Microscopic analysis of the colon of Muc2^−/− mice showed mucosal thickening, increased proliferation, and superficial erosions. Colonic goblet cells in the Muc2^−/− mice were negative for Muc2, but trefoil factor 3 was still detectable. In Muc2^−/− mice, transient de novo expression of Muc6 messenger RNA was observed in the distal colon. On day 2 of DSS treatment, the histologic damage was more severe in Muc2^+/− versus wild-type (Muc2^+/+) mice, but the disease activity index was not yet different. By day 7, the disease activity index and histologic score were significantly elevated in Muc2^+/− versus Muc2^+/+ mice. The disease activity index of the Muc2^−/− mice was higher (versus both Muc2^+/+ and Muc2^+/− mice) throughout DSS treatment. The histologic damage in the DSS-treated Muc2^−/− mice was different compared with Muc2^+/+ and Muc2^+/− mice, with many crypt abscesses instead of mucosal ulcerations. Conclusions: This study shows that Muc2 deficiency leads to inflammation of the colon and contributes to the onset and perpetuation of experimental colitis.

γ-secretase inhibitors (GSIs) can block NOTCH receptor signaling in vitro and therefore offer an attractive targeted therapy for tumors dependent on deregulated NOTCH activity. To clarify the basis for GSI resistance in T cell acute lymphoblastic leukemia (T-ALL), we studied T-ALL cell lines with constitutive expression of the NOTCH intracellular domain (NICD), but that lacked C-terminal truncating mutations in NOTCH1. Each of the seven cell lines examined and 7 of 81 (8.6%) primary T-ALL samples harbored either a mutation or homozygous deletion of the gene FBW7, a ubiquitin ligase implicated in NICD turnover. Indeed, we show that FBW7 mutants cannot bind to the NICD and define the phosphodegron region of the NICD required for FBW7 binding. Although the mutant forms of FBW7 were still able to bind to MYC, they do not target it for degradation, suggesting that stabilization of both NICD and its principle downstream target, MYC, may contribute to transformation in leukemias with FBW7 mutations. In addition, we show that all seven leukemic cell lines with FBW7 mutations were resistant to the MRK-003 GSI. Most of these resistant lines also failed to down-regulate the mRNA levels of the NOTCH targets MYC and DELTEX1 after treatment with MRK-003, implying that residual NOTCH signaling in T-ALLs with FBW7 mutations contributes to GSI resistance.

Notch signaling has a crucial role in T cell acute lymphoblastic leukemia (T-ALL), but γ-secretase inhibitors (GSIs), which block the Notch pathway, cause intestinal toxicity that limits their use. Adolfo Ferrando and his colleagues now report that glucocorticoids can reverse the gut toxicity of GSIs, and GSIs can restore sensitivity of T-ALL cells to glucocorticoids, suggesting that this combination may have clinical utility in T-ALL and other diseases ( pages 20–21 ). Gamma-secretase inhibitors (GSIs) block the activation of the oncogenic protein Notch homolog-1 (NOTCH1) in T cell acute lymphoblastic leukemia (T-ALL). However, limited antileukemic cytotoxicity and severe gastrointestinal toxicity have restricted the clinical application of these targeted drugs. Here we show that combination therapy with GSIs plus glucocorticoids can improve the antileukemic effects of GSIs and reduce their gut toxicity in vivo. Inhibition of NOTCH1 signaling in glucocorticoid-resistant T-ALL restored glucocorticoid receptor autoupregulation and induced apoptotic cell death through induction of the gene encoding BCL-2–like apoptosis initiator-11 (BCL2L11). GSI treatment resulted in cell cycle arrest and accumulation of goblet cells in the gut mediated by upregulation of the gene encoding the transcription factor Krüppel-like factor-4 (Klf4), a negative regulator of the cell cycle required for goblet cell differentiation. In contrast, glucocorticoid treatment induced transcriptional upregulation of cyclin D2 (Ccnd2) and protected mice from developing the intestinal goblet cell metaplasia typically induced by inhibition of NOTCH signaling with GSIs. These results support a role for glucocorticoids plus GSIs in the treatment of glucocorticoid-resistant T-ALL.

Adolfo Ferrando and colleagues identify frequent inactivating mutations and deletions in the X chromosome gene PHF6 in T-cell acute lymphoblastic leukemia. PHF6 mutations are found almost exclusively in males and are associated with leukemias driven by aberrant expression of TLX1 and TLX3. Tumor suppressor genes on the X chromosome may skew the gender distribution of specific types of cancer1,2. T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematological malignancy with an increased incidence in males3. In this study, we report the identification of inactivating mutations and deletions in the X-linked plant homeodomain finger 6 (PHF6) gene in 16% of pediatric and 38% of adult primary T-ALL samples. Notably, PHF6 mutations are almost exclusively found in T-ALL samples from male subjects. Mutational loss of PHF6 is importantly associated with leukemias driven by aberrant expression of the homeobox transcription factor oncogenes TLX1 and TLX3. Overall, these results identify PHF6 as a new X-linked tumor suppressor in T-ALL and point to a strong genetic interaction between PHF6 loss and aberrant expression of TLX transcription factors in the pathogenesis of this disease.