Clinicopathologic features of CDK6 translocation-associated B-cell lymphoproliferative disorders

Abstract

Cyclin-dependent protein kinase 6 (CDK6), in cooperation with cyclin Ds, drives cell cycle progression from G1 to S phase through phosphorylation and subsequent inactivation of retinoblastoma 1 protein. Alteration of this pathway results in both nonhematologic and hematologic malignancies, which include a small subset of B-cell lymphoproliferative disorders (BLPDs). We identified 5 cases of BLPD that carried CDK6 chromosomal translocations and characterized their clinical, pathologic, immunophenotypic, and genetic features. Common clinical characteristics included marked neoplastic lymphocytosis, systemic lymphadenopathy, splenomegaly, and bone marrow involvement. Three patients were diagnosed with low-grade B-cell lymphoma and had an indolent clinical course, and 2 patients (one who transformed to large B-cell lymphoma, and the other who was initially diagnosed with a high-grade B-cell lymphoma) had an aggressive clinical course. Immunophenotypically, the neoplastic B cells expressed CD5, CDK6, and cytoplasmic retinoblastoma 1 protein in all cases, expressed phospho-RB, p27kip1, and cyclin D2 in most cases, and uniformly lacked expression of all other cyclins. In 4 cases, the CDK6 translocation partner was kappa immunoglobulin light-chain gene; and in the fifth case, the CDK6 translocation partner was unknown. These distinct clinicopathologic and cytogenetic features distinguish the CDK6 translocation-associated BLPDs (CDK6-BLPDs) from other mature B-cell lymphomas.

Figure 1. Identification of CDK6 translocations by FISH analysis

A: Isolated nuclei from a normal donor bone marrow aspirate (A) were hybridized using CDK6 BAP probes. The FISH pattern of two fusion signals (2F) indicates the absence of a CDK6 break point. B–D: Isolated nuclei from paraffin-embedded sections of bone marrow biopsy from patient 1 were hybridized with CDK6 BAP probes (B), IGK BAP probes (C), and IGK-CDK6 D-FISH probes (D). Patient 1 had positive CDK6 (1R1G1F) (B) and IGK (1R1G1F) (C) break points. The two CDK6-IGK fusion signals (1R1G2F) in the lower right nucleus (D) indicate CDK6-IGK translocation. The upper left nucleus (D) is negative for the CDK6-IGK translocation (2R2G).

Figure 2. Morphologic features of the peripheral blood and bone marrow from a CDK6-BLPD

A–B: The morphologic features of circulating neoplastic lymphocytes in peripheral blood (A) and bone marrow aspirate (B) (patient 1) (GW stain, x600); C–D: Bone marrow biopsy (patient 1) at a lower magnification (H&E stain, x100) (C) and higher magnification (H&E stain, x400) (D) showed the peritrabecular and interstitial infiltration by the morphologically distinct small lymphocytes. E–F: The absence of intrasinusoidal distribution of the neoplastic B-cells was demonstrated by immunohistochemical stains on bone marrow biopsy sections (patient 1) with antibodies to CD20 (E) and p27kip1 (F). Note the intact bone marrow sinuses (arrowhead). G–I: H&E stained sections of the involved spleen (patient 3) showed morphologic features that were most consistent with SMZL. Micrographic images of a lower (x40) (G) and a higher magnification (x400) of the involved white pulp (H) and red pulp (I) demonstrated a zonal distribution of the neoplastic B-cells. “W” indicates the white pulp; “R”, the red pulp.

Figure 3. Immunophenotypic features of CDK6-BLPD

A–D and E–H: Flow cytometric studies of the circulating neoplastic lymphocytes in peripheral blood from patients 2 and 5 showed distinct populations of kappa immunoglobulin light-chain restricted B-cells that coexpressed bright or dim CD5, respectively. I–L: The micrographic images of the immunohistochemical studies of the involved spleen from patient 4 (x400) showed partial/dim CD5 coexpression (L) by the neoplastic CD20-positive B-cells (J). I and J: Paraffin H&E and CD3 immunohistochemical stains of the spleen section.

Figure 4. Expression of CDK6 and other cell cycle-related genes

Paraffin H&E and immunohistochemical stains of the spleens from patient 2 (A–E) and 3 (F–J) using antibodies directed against CDK6 (B and G), cyclin D2 (C and H), phospho-Rb (D and I), and p27kip1 (E and J) were performed. Both involved spleens showed cytoplasmic and nuclear staining for CDK6 (B and G) and nuclear staining for cyclin D2 (C and H), phospho-Rb (D and I), and p27kip1 (E and J).