Purpose: To analyze somatic hypermutations in clonally rearranged IgH chain variable (V) genes of primary intraocular lymphoma (PIOL), to identify the differentiation stage of B-PIOL cells.
Methods: Sixteen cases of PIOL were diagnosed on the basis of morphology and immunohistology. In six patients, simultaneous cerebral lymphomatous involvement was known; stereotactic biopsy specimens were investigated in three cases. A polymerase chain reaction (PCR) was performed on DNA extracted from vitrectomy specimens or from paraffin-embedded sections, to amplify the clonally rearranged heavy-chain immunoglobulin (IgH) genes. The isolated PCR products were sequenced and compared with published VH germ-line segments to determine the VH usage and number of somatic mutations in the complementarity-determining region (CDR)2 and framework region (FR)3.
Results: All tumors exhibited clonal IgH rearrangements. Of the eight sequenceable cases, four had the VH4-34 gene segment, two the VH3-23, one the VH3-7, and another the VH3-30. The pattern of somatic mutations indicated selection of PIOL cells for expression of a functional antibody. The mean frequency of somatic mutations detected for the IgH gene was very high (14.5%). In three oculocerebral lymphomas, the identical B-cell clone was demonstrated in ocular and cerebral tissues.
Conclusions: The data suggest that PIOLs (1) are derived from mature B-cells that have undergone the germinal center reaction and (2) are closely related to primary cerebral nervous system lymphoma (PCNSL), due to their high mutation frequency of VH region genes. The close relationship between PIOL and PCNSL is underlined by demonstration of the same VH segment (VH4-34) in three of six cases of oculocerebral lymphoma.