Primary mediastinal B-cell lymphoma (PMBL) is distinct from other diffuse large B-cell types in its epidemiology, biology, and clinical course. Arising in the thymus, it is thought to originate in medullary B cells. It is characterized by a relatively increased incidence among young women, early local invasion, and a generally favorable prognosis after chemoimmunotherapy, often followed by consolidation radiotherapy. Failure of the initial therapy is uncommon, but it is often rapid when it does occur and extremely difficult to salvage. Gene-expression profiling has demonstrated strong similarities to nodular sclerosing Hodgkin lymphoma (HL), although no common underlying mutational events have previously been identified in more than a small number of cases. The pathogenesis of PMBL remains unclear, and for this reason, there is continuing investigation of its molecular characteristics. Led by Dr. Randy D. Gascoyne and Dr. Christian Steidl at the British Columbia Cancer Agency in Vancouver, Canada, this study has used whole-genome sequencing as the starting point for the identification of a previously unsuspected mutation target that is altered in a significant number of cases of both PMBL and HL.
Initial whole-genome sequencing of material from two cases of PMBL, to a read depth of 60x and 52x, respectively, identified mutations in two negative regulators of the JAK-STAT signaling pathway, SOCS1 and PTPN1, in both cases. Subsequent sequencing of the whole transcriptome in another five cases and three PMBL cell lines revealed a high frequency of alterations in several regulators of the JAKSTAT pathway, with SOCS1 and PTPN1 being the most commonly affected. Next, the authors sequenced all 10 exons of PTPN1 in another 70 cases of PMBL and found a total of 18 variants in 17 out of 77 cases and an additional two variants in one of the three cell lines. Due to the possible overlapping biology with HL, they also sequenced PTPN1 in 30 cases and nine cell lines, finding another 12 variants among these. In clinical cases, the mutation rate of PTPN1 was 22 percent in PMBL and 20 percent in HL. Analysis of the PTP1B protein by immunohistochemistry suggested that the mutations generally resulted in reduced or completely absent expression, depending on the exact location and type of sequence variant.
Investigating the functional consequences of these mutations showed that they impaired the phosphatase function of PTP1B, with higher levels of phospho-STAT6 seen in a reporter assay, suggesting constitutive upregulation of the signaling pathway through loss of inhibition. This hypothesis was supported by knock-down experiments using short-hairpin RNA to target PTPN1 transcripts in a wild-type HL cell line, which led to increased phosphorylation in several members of the JAK-STAT pathway and alterations in the gene-expression profile showing up-regulation of many STAT targets. Among these were oncogenes, including BCL-6, MYC, and JUN, and members of the gene family of ATP-binding cassette (ABC) transmembrane transporters, which are associated with resistance to chemotherapy.
In Brief
The evidence for de-regulation in the JAK-STAT pathway in PMBL is mounting steadily, and this study shows another way in which this pathologic process can come about. It has previously been difficult to identify a common pathogenetic pathway in PMBL, a disease with highly variable cytogenetic findings and few common alterations at the chromosomal level. However, the convergence of many different abnormalities seems to center on the JAK-STAT pathway and suggests a novel approach to treatment. The finding that some of the downstream targets of the mutations are mediators of drug resistance is also interesting, and larger studies are now needed to determine whether PTPN1 mutations convey an increased risk of primary treatment failure.
Competing Interests
Dr. Johnson indicated no relevant conflicts of interest.
