Extra

CD34

• Often regarded as a marker of primitive phenotype, CD34 is expressed by many cases of ALL (around 50%) and slightly fewer cases of AML around (40%)
• CD34 is also expressed by regenerating cells in marrow, and particularly by regenerating B lymphocytes (haematogones)

what is its normal function and expression CD34 is a membrane protein that is likely to play a role in the retention of cells within the stem cell niche where it may also contribute to their subsequent maturation. CD34 is acquired at the stage of pluripotent stem cells and is retained by committed stem cells before being lost as the cells mature. CD34 is also expressed by mast cells and by some non-haematopoietic cells particularly endothelial cells.

what is its diagnostic role

• CD34 is commonly used to identify the primitive nature of early lymphoid or myeloid cells i.e. blast cells of ALL and AML
• Rare very primitive cases of acute leukaemia may be identified solely through their expression of CD34 and CD45
• CD45 is also expressed by regenerating cells in marrow and also by regenerating haematogones so caution is necessary in interpretation in regenerating marrows

other relevant information Histopathologists often use CD34 to demonstrate the number of primitive cells in bone marrow trephine biopsies e.g. in the assessment of primitive cell number in myelodysplaia; however a range of other cell types in bone marrow also express CD34 in marrow, and these include vascular endothelial cells and mast cells.

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summary tables

1. expression by acute leukaemias and by haematogones freq pro-b all freq early pre-b all freq pre-b all poss b all freq pro-t all freq mature-t all freq aml hi haemato-gones

• in aml expression is rare in apl and is less frequent in npm1-mutated patients, Burkitt

2. expression by b-lymphoproliferative disorders rare cll rare pll rare mcl rare fl link rare hcl rare hclv rare mzl rare lpl rare pcs