deltaCD20 & rituximab-resistance

[Guest guest]

Andy Gach suggested that I post the following on

the list. I originally posted it yesterday on the list.

Al Janski

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Re: Rituxan less effective in CLL than in other B cell malignancies

At 12:06 PM 8/3/2010, Dwyer wrote:

>A study recently published

>[<http://dx.doi.org/10.1016/j.leukres.2010.06.031>http://dx.doi.org/10.1016/j.l\

eukres.2010.06.031],

>has had a look at the effectiveness of Rituxan treatment in CLL.

>CLL patients generally have a low level of

>CD20. Surprisingly, ............ CD20 mRNA

>levels were normal or close to normal,

>..........Researchers concluded that CD20

>protein is substantially decreased in CLL 'by

>rituximab' due to a post-transcriptional defect.

The wording in the abstract of the above paper is

a bit confusing. The authors (Sarro et al.)

conclude, in the abstract, that " CD20 protein is

substantially decreased in CLL due to a post-

transcriptional defect " . However, the " decrease "

was not a comparison of CLL cells in pre- vs.

post- rituximab treatment. The authors were

comparing CD20 protein levels in CLL cells with

CD20 protein levels in normal B-cells, and, in

which case, they should have said the CD20

protein levels were 'lower' (instead of

" decreased " ) in CLL cells than in normal B-cells.

As an interesting aside, Sarro et al. cite a

recent paper (Ref. #9, Henry et al., Mar/2010,

http://tinyurl.com/22qo3gy ) that contains

observations about their newly discovered variant

of CD20 protein, i.e. " deltaCD20 " . [Note: Henry

et al. use the Greek symbol of delta, which does

not translate on this listserve, so I used " delta " instead.]

deltaCD20 protein was detected in several B-cell

leukemias (including CLL) and lymphomas, but

deltaCD20 was not detectable in normal B-cells.

Henry et al. concluded that increasing levels of

deltaCD20 correlated with rituximab-resistance in

malignant B-lymphocytes in general, but they only

measured this correlation in Burkitt lymphoma

cells. The actual measurements observed

deltaCD20 increasing with increasing exposure to

rituximab, up to a time point (21 days after the

last rituximab exposure) when resistance to rituximab was established.

The authors did observe deltaCD20 in CLL cells,

but they did not measure rituximab-resistance in CLL cells.

Nevertheless, as some of the following details

suggest, deltaCD20 may be an interesting

endogenous molecule to monitor on a research basis in CLL experiments.

The size of the deltaCD20 is only 50% of the size

of the full-length ( " wild-type " ) CD20

protein. deltaCD20 does 'not' result from

natural degradation of CD20, but deltaCD20 is

translated into protein from a different mRNA

molecule, in which portions of the wild-type CD20

mRNA (wt-mRNA) are missing, with the remaining

portions of the wt-mRNA spliced together to form

the deltaCD20 mRNA, which is referred to in the

title of Henry et al. as the " alternative CD20 transcript variant " .

The portions of wtCD20 that are missing in

deltaCD20 include portions of the protein that

are involved in anchoring the protein to the cell

membrane, where it would be presented

for binding to molecules, like

rituximab. Interestingly, the missing portions

also include the site that binds to rituximab,

and, thus, rituximab is not expected to bind to deltaCD20.

Portions of wtCD20 that are retained in deltaCD20

include the amino acids that are

phosphorylatable, which are involved in the

activation of B-cells. Consequently, deltaCD20

may retain activities associated with

antigen-activation of malignant B-cells, despite

the expected inability of deltaCD20 to be

presented at the surface of these cells.

The levels of mRNA coding for deltaCD20 in CLL

cells were lower in cells from blood & marrow

than in cells from lymph nodes, spleen, or

pleural effusion. However, deltaCD20 mRNA levels

in CLL cells from all sources were generally

lower than deltaCD20 levels observed in malignant

B-cells of the other types of leukemias and lymphomas tested by the authors.

Only 0.81% of total CD20 mRNA codes for deltaCD20

in CLL cells, compared with up to 14% of total

CD20 mRNA coding for deltaCD20 in diffuse large

B-cell lymphoma, with other B-cell leukemias and

lymphomas with intermediate levels of deltaCD20 mRNA.

Overall, based on the small amount of CLL data in

Henry et al., one might not expect deltaCD20 to

be a significant player in CLL, but deltaCD20 may

be a molecule worth monitoring, e.g. in studying

rituximab-resistance in CLL.

Additionally, because deltaCD20 was not

detectable in normal B-cells, I would not be

surprised if deltaCD20 plays some role in the

post-transcriptional defect, suggested by Sarro

et al., in which CD20 mRNA levels in CLL cells

are comparable to those in normal B-cells, yet

CD20 protein levels in CLL cells are only 40% (or

less) of the protein levels in normal B-cells.

REFERENCE:

Identification of an alternative CD20 transcript

variant in B-cell malignancies coding for a novel

protein associated to rituximab resistance

Blood, 25 March 2010, Vol. 115, No. 12, pp.

2420-2429. Prepublished online as a Blood First

Edition Paper on January 20, 2010; DOI 10.1182/blood-2009-06-229112.

Carole Henry13,*, Marina Deschamps14,*,

Pierre-Simon Rohrlich13,5, Jean-René Pallandre13,

Jean- Rémy-13, Callanan6,7,

andra Traverse-Glehen8, Camille

GrandClément13, Francine Garnache-Ottou13,8, Remy

Gressin6,7, Deconinck13,7, Gilles Salles8,

Robinet9, Pierre Tiberghien13,5, Christophe

Borg13,5, and Christophe Ferrand14,7

http://tinyurl.com/22qo3gy

Al Janski