Extra

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

Note The colour and % refer to the proportion of cases expressing the marker. Additional comments on strength, or site of expression (if relevant) are given below the table. For full details of key click here. For further information about a particular disease state click <I> adjacent to the condition name

Expression in primitive cell types
AML	CMML	B ALL	Burkitt	T ALL	ETP ALL <I>	MPAL <I>	Hgns <I>
<5%	<5%	<5%	<5%	<5%	<5%	<5%	<5%

Notes: **While a good marker of B lineage it may not be expressed in more primitive phenotype ALL, there is also a recognised abberrant expression in AML (particularly cases with t)8;21)

Expression in mature B cell neoplasms
CLL	MCL	FL	MZL	HCL	DLBCL	LPL	PCL
<5%	<5%	<5%	<5%	80-100%**	<5%	<5%	<5%

Notes: *typically expressed in HCL where it will be detected in most cases; generally absent in other LPD although rarely expression is detected so the overall pattern is important

Expression in mature T cell neoplasms*
ATLL	CTCL/Sezary	T-PLL	T-LGL	NK-LGL
40%	<5%	<5%	<5%	<5%

Notes: Sporadic expression is recognised in many T-cell disorders at around 5% of cases, expression is more frequent in ATLL where up to 40% of cases may express CD103

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summary tables

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summary tables

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

link flow cytometry key considered a strong indicator to distinguish hcl and hclv from mzl