MPO

Summary

MPO expression is strongly associated with granulocytic maturation. In acute leukaemia MPO has high specificity for the diagnosis of AML, although expression may be observed on more primitive AML, or where maturation is non-granulocytic e.g. erythroid or megakaryocyte lineages and on some monocytic cases. Expression is not observed in typical ALL, if MPO is detected in ALL then this supports the diagnosis of mixed-phenotype acute leukaemia (MPAL).

Normal expression and function

Myeloperoxidase is a lysosomal enzyme used by neutrophils to kill pathogens. It is present in neutrophils and eosinophils at all stages of maturation, and in developing basophils. MPO may be expressed at low intensity in monocytes but erythroid precursors and lymphocytes are MPO-negative.

Diagnostic role

• MPO is expressed in around 60% of AML cases - generally those with granulocytic maturation and is weakly expressed by promonocytes
• In otherwise typical ALL a small number of MPO-expressing cells may be seen although precise criteria vary (most often <5%)
• In cases of mixed phenotype acute leukaemia (MPAL), expression of mpo can alone be used to establish myeloid differentiation

Other relevant information:

Technically enzymatic measures of MPO (cytochemical or sudan black B staining) are probably the least sensitive detection methods. Flow cytometric detection of MPO has lower sensitivy than immunohistochemical detection on trephine; this may be because immunocytochemistry detects expression of a precursor enzyme form of MPO as well as thefunctional form. For this reason, if there is doubt about MPO expression then MPO staining of trephine biopsies can be useful.

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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	BL	T ALL	ETP ALL	MPAL	Hgn
40-80%	20-40%*	<5%	<5%	<5%	<5%	5-20%	<5%

Notes: *Expression is characteristically weak in CMML

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

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

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