CD117

Summary

CD117 is a widely expressed molecule that in haematology is used most often as a marker of primitive myeloid cells where it contributes to a diagnosis of AML. However expression is absent in around 20-30% of typical AML and there is recognised aberrant expression by up to 30% of ALL cases (more frequently in T lineage). CD117 is also expressed by mast cells, and weakly on some plasma cells

Normal expression and function

CD117 was originally described in 1987 as c-kit (the cellular homolog of the feline sarcoma viral oncogene v-kit). Stem cell factor (SCF) binds to CD117 initiating signals that variously control apoptosis, differentiation, proliferation, chemotaxis or cellular adhesion. In myeloid cells, CD117 is expressed by early haematopoietic progenitor cells then expression is lost as the cells mature.

Diagnostic role

• In acute myeloid leukaemia expression of CD117 is seen in 70-80% of cases including less differentiated forms, it may be expression by any subtype, but is less likely in cases with monocytic, erythroid, or megakaryocytic maturation
• Aberrant expression in acute lymphoid leukaemias is reported - more frequently in T-lineage ALL (particularly more primitive types). Expression in B-lineage ALL is less frequent.
• Expression by mature B cell or mature T cell types is rare, although CD117 expression is described on some CD8 positive mature T cell neoplasms. *Around 30 of myeloma cases express CD117, but often expression is weak which limits its value in this disorder
• CD117 can be used to identify mast cells, but does not distinguish normal from neoplastic cells.

Other useful information

Outside of the haematopoietic system CD117 has a wide expression that includes germ cells, cajal cells of the gastrointestinal tract and epithelial cells in skin and breast. Gain-of-function mutations are seen in tumours affecting mast cells, myeloid cells, systemic mastocytosis, acute myeloid leukemia or cajal cells gastrointestinal stromal tumors - GOST tumours.

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SUMMARY TABLES

Expression in primitive cell types
AML	CMML	B ALL	BL	T ALL	ETP ALL	MPAL	Hgn
40-80%	20-40%	5-20%	<5%	5-20%	20-40%	20-40%	20-40%

Notes: The main application is in AML. Aberrant expression is seen in ALL particularly in T-lineage, although it may be less likely in Burkitt lymphoma. CD117 expression can indicate MPAL or ETP-ALL if other markers are consistent.

Expression in mature B cell neoplasms
CLL	MCL	FL	MZL	HCL	DLBCL	LPL	PCL
<5%	<5%	<5%	<5%	<5%	<5%	<5%	20-40%

Notes: expression is uncommon in mature B-lymphoproliferative disorders, but may occur in plasma cells

Expression: mature T cell neoplasms
ATLL	CTCL Sezary	T-PLL	T-LGL	NK-LGL
limited	limited	limited	limited	limited

Notes: limited reports are consistent with expression being uncommon in T-lymphoproliferative disorders, but may occur in some cases of T-PLL

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