Monosomy 21, a Sole Abnormality in an Elderly Man with Non-CLL-Type Monoclonal B-cell Lymphocytosis

Monoclonal B-cell lymphocytosis (MBL) is a light-chain restricted proliferation of mature B cells fewer than 5000 cells/μL without additional clinical or hematologic abnormalities. Sibling studies of individuals genetically susceptible to chronic lymphocytic leukemia (CLL) first identified monoclonal B cells in otherwise healthy persons, and studies show a 3% to 14% prevalence for MBL in persons over 40 years of age. Non-CLL-type MBL accounts for less than 20% of all MBL cases, and its progression is incompletely characterized. Here we present the case of an 85-year-old man with CD5-, CD19+, CD20 bright, and lambda-restricted lymphoid cells whose immunophenotypic findings are suggestive for a precursor lesion to marginal zone lymphoma (MZL). Karyotyping showed monosomy 21 without additional cytogenetic changes in three of the 35 cells examined. Monosomy 21 as a sole abnormality in CLL has been detected in just 11 cases between 1984 and 2003. As a sole abnormality in splenic and nodal marginal zone lymphoma, only three instances of monosomy 21 have been recorded on the Mitelman Database of Chromosome Aberrations and Gene Fusions in Cancer. The significance of monosomy 21 as a marker for oncogenesis remains unclear.