TDT

Summary

Terminal Deoxynucleotidyl Transferase (TdT) a nuclear cell protein expressed by most cases of acute lymphoblastic leukaemia but not by more mature cells. Expression is not fully specific however as TdT is also strongly expressed by early regenerating B-lymphocytes (haematogones) and may be expressed in acute myeloid leukaemia.

Normal expression and function

TdT acts is a nuclear enzyme requires for the rearrangement of the VDJ genes of heavy chain of the T-cell receptor or B-cell receptor genes during the formation of mature B and T cells. expression of tdt is therefore restricted to immature lymphoid cells in bone marrow or thymus. in regenerative marrows TdT is consistently expressed on regenerative B lymphocytes (haematogones) which may represent a large proportion of marrow cells, particularly in children.

Diagnostic role

• The most useful attribute of TdT is its loss of expression by mature cells: it is good marker of primitive cells although not always expressed
• TdT is consistently expressed by acute lymphoblastic leukaemia (95%)
• TdT may be weakly expressed by Burkitt lymphoma but is most often absent, expression by mature B-cell lymphomas is rarely described
• TdT is emay be expressed by AML cells - mainly on less differentiated forms and those related to MDS (around 20%)
• TdT is expressed by haematogones becoming lost as they mature

Other relevant information:

As a nuclear marker TdT requires cells to be permeabilised for detection by flow cytometry

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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
5-20%	<5%	80-100%	5-20%	80-100%	40-80%	40-80%	80-100%

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% 5-20% 20-40% 40-80% 80-100%	? <5% 5-20% 20-40% 40-80% 80-100%	? <5% 5-20% 20-40% 40-80% 80-100%	? <5% 5-20% 20-40% 40-80% 80-100%	? <5% 5-20% 20-40% 40-80% 80-100%

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