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2 Mechanisms of T‐cell Lymphomagenesis
François Lemonnier1,2,3, Philippe Gaulard2,3,4 and Laurence de Leval5
1 Unité hémopathies Lymphoïdes, Hôpitaux Universitaires Henri Mondor, Créteil, France
2 Assistance Publique des Hôpitaux de Paris, Paris, France
3 Institut Mondor de Recherche Biomédicale, INSERMU955, Université Paris Est Créteil, Créteil, France
4 Département de Pathologie, Hôpitaux Universitaires Henri Mondor, Créteil, France
5 Institut de Pathologie, Centre Hospitalier Universitaire Vaudois et Université de Lausanne, Lausanne, Switzerland
TAKE HOME MESSAGES
The peripheral T‐cell lymphomas (PTCL) are characterized by the accumulation of mutations in genes that govern multiple epigenetic pathways, with some entities like PTCL‐follicular helper T cell and angioimmunoblastic T‐cell lymphomas (AITL), representing the subtypes most enriched for these genetic events.
Mutation‐induced activation of signaling pathways that play a key role in normal T and natural‐killer (NK) cell physiology, like the Janus kinase/signal transducers and activators of transcription pathway or T‐cell receptor signaling, are highly recurrent and common to many entities.
Extranodal NK/T‐cell lymphoma and adult T‐cell leukemia/lymphoma represent two remarkable models lymphomas induced by viruses with superimposed genetic lesions.
The tumor microenvironment, and the nature of its cellular milieu, plays an important role in PTLC lymphomagenesis, especially in AITL.
Introduction
Peripheral T‐cell lymphomas (PTCLs) collectively include neoplasms of mature (i.e. post‐thymic) T or natural killer (NK) cells. As in other cancers, the neoplastic transformation encompasses a multistep process altering pivotal cellular pathways to allow for the survival and expansion of the neoplastic clone, and the recruitment of a favorable microenvironment. Interestingly, neoplastic T or NK cells retain some features related to their cellular differentiation, which affects the clinical, pathological, and biological presentation of the diseases, as well as their outcomes. In this chapter, we review the main types of genetic alterations found in PTCL, discuss the role and importance of the tumor microenvironment and the underlying conditions favoring T‐cell transformation, and the relevance of cell‐of‐origin to T‐cell lymphoma genesis and biology.
Oncogenic Events in the Transformation of T or Natural Killer Cells
Genetic Lesions
Tumor transformation is driven by genetic events that modify a biological function. Among structural variants first described is the chromosomal translocation involving the ALK oncogene at 5q35 locus. In this translocation, ALK is fused to various partners, most often NPM (nucleophosmin), resulting in abnormal expression of ALK hybrid proteins in anaplastic large‐cell lymphomas (ALCL) [1]. Thus, anaplastic lymphoma kinase positive (ALK+) ALCL was the first genetically defined T‐cell lymphoma entity. With the development of high‐throughput sequencing methods, especially RNA sequencing, many other chromosomal translocations or gene fusions have subsequently been described in PTCL, deregulating various oncogenes such as DUSP22 [2], TP63 [2], VAV1 [3], CD28‐ICOS, or CD28‐CTLA4 [4, 5]. Besides these translocations and fusions, copy number variants (CNV) are frequent in PTCL. Some CNV are unique to some subtypes of the disease, including for example, gains in chromosomes 5 and 21 in IDH2R172‐mutated angioimmunoblastic T‐cell lymphomas (AITL) [6]. In addition, complex genetic changes appear frequent in GATA3‐positive PTCL not otherwise specified (PTCL‐NOS), which have a high frequency of alterations in PTEN‐PI3K and CDKN2A‐TP53 pathways, highlighting the implication of P53 loss in the development of genomic instability [6]. More rarely, chromothripsis, an episode of catastrophic chromosomal rearrangement [7], has been described [8], but no dedicated study has been conducted to evaluate its incidence.
Next‐generation targeted, whole‐exome or whole‐genome sequencing studies have reported single‐nucleotide variants or indel mutations in coding sequences, leading to the loss of function of a tumor suppressor gene, impacting epigenetic regulation or cell‐cycle control, or a gain of function of a proto‐oncogene, resulting for example in increased signaling, in most PTCL entities. Splice site mutations in tumor suppressor genes like TET2 or DNMT3A frequently occur and result in loss of function.
Coding sequences represent