Precursory or early lesions of follicular lymphoma: clinical features, pathology, and genetics

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Follicular lymphoma (FL) is an indolent B-cell lymphoma with a germinal center (GC) B cell phenotype that typically harbors t(14;18)(q32;q21). t(14;18) juxtaposes IGH on 14q32 and BCL2 on 18q21, resulting in overexpression of the anti-apoptotic BCL2 protein. However, t(14;18) is also found in the peripheral blood or lymphoid nodes (LNs) of otherwise healthy individuals. Moreover, overt FL has several additional gene alterations involved in epigenetic modification, JAK/STAT signaling, immune modulation, and NF-κB signaling, indicating multi-step lymphomagenesis in FL. There are two early or precursory lesions of FL: t(14;18)-positive cells in the peripheral blood of otherwise healthy individuals and in situ follicular B-cell neoplasm (ISFN). t(14;18)-positive cells are found in 10%–50% of healthy populations, and their incidence and frequency increase with age. The detection of t(14;18) in peripheral blood is a predictive factor for an increased risk of overt FL development. In contrast, ISFN is a histopathologically recognizable precursory lesion, in which t(14;18)-positive cells are confined to the GC of otherwise reactive LNs. ISFN is usually detected incidentally, with an incidence ranging from 2.0% to 3.2%. Occasional ISFN cases have concurrent or metachronous clonally related overt FL or aggressive B-cell lymphoma of a GC phenotype. t(14;18)-positive cells in peripheral blood and isolated ISFN, by themselves, are asymptomatic with limited clinical significance; however, investigations of t(14;18)-positive precursory or early lesions offer meaningful insights into the pathogenesis of FL. This review summarizes the epidemiology, clinical features, pathology, and genetics of precursory or early lesions of FL.