Leukemic B cells in chronic lymphocytic leukemia (B-CLL) typically exh
ibit low or undetectable surface Ig. Because the B29 (CD79b and Ig bet
a) and mb-1 (CD79a and Ig alpha) gene products are required for surfac
e Ig display in the B-cell receptor complex (BCR), we analyzed the exp
ression of these genes in B-CLL cells. The majority (83%) of the rando
mly selected B-CLL patient samples analyzed exhibited low or undetecta
ble surface BCR measured by mu heavy chain and B29 expression, Levels
of mb-1 mRNA in these B-CLL samples with low surface BCR were similar
to those in normal B cells. Among those with decreased surface express
ion, B29 mRNA was not detected in half of these B-CLL samples. The rem
aining B-CLL samples with diminished surface BCR contained normal leve
ls of B29 mRNA. Further analysis of cDNA clones from the majority of t
hese latter samples contained point mutations, insertions, or deletion
s that were largely located in the B29 transmembrane and cytoplasmic d
omains. These results indicate the occurrence of somatic mutations pre
dicted to affect B29 expression and/or function in the majority of B-C
LL and suggest that these aberrations underlie the diminished surface
BCR display and loss of BCR signaling characteristic of this leukemia,
(C) 1997 by The American Society of Hematology.