WITHIN RECENT years, the emergence of molecular genetic techniques has provided new tools allowing the accurate analysis of the clonality status in both T-cell and B-cell infiltrates. The methods used for clonality assessment are based on the genomic organization of the genes coding for T-cell receptor (TCR) and immunoglobulin chains, which display a similar pattern characterized by clusters of exons separated by large intronic sequences, being rearranged at the DNA level during the maturation process of lymphocytes.1 Thus, the Southern blot analysis of genes coding for TCRB or for TCRG chains was the method used in initial studies that showed evidence for the monoclonal nature of cutaneous T-cell neoplasms, such as Sézary syndrome and mycosis fungoides (MF).2 However, Southern blot is a time-consuming method that requires the usage of radioactive oligonucleotidic probes and detects a monoclonal TCR rearrangement only if its ratio exceeds 5% of all TCR rearrangements among a lymphocytic infiltrate. More recently, the use of Southern hybridization has been challenged by highly sensitive, nonradioactive methods for the routine analysis of clonality. These assays are based on the amplification by the polymerase chain reaction (PCR) of TCR V/(D)/J gene rearrangements, followed by an agarose or polyacrylamide gel electrophoresis.3 Thus, the PCR amplification of theTCRG V/J junctional DNA sequences is now widely used for the routine analysis of the T-cell clonality.3 The choice of the TCRG locus for this aim relies on the simplicity of its genomic organization and on its consistent rearrangement in mature T cells, whether they express at their surface the TCRAB or theTCRGD heterodimer. The huge diversity of the rearrangedTCRGV/GJ junctional DNA in terms of length and of sequence allows discrimination between 2 distinct rearrangements after an electrophoresis step.3 PCR has also been applied for studies of the clonality status in B-cell expansions through the analysis of the V/J rearrangements of the immunoglobulin heavy chain locus.4 Indeed, PCR-based assays have been shown approximatively 10-fold more sensitive than Southern blot in the detection of a monoclonal subset amongst a polyclonal population, allowing the detection of a clonal TCR-immunoglobulin junctional DNA representing as little as 0.5% to 1% of all rearrangements.3,5 Furthermore, the use of denaturing conditions for the electrophoretic step in recently developed techniques such as denaturing gradient gel electrophoresis may be useful to discriminate between 2 rearrangements sharing an identical size but showing different oligonucleotide sequences.6
Bachelez H. The Clinical Use of Molecular Analysis of Clonality in Cutaneous Lymphocytic Infiltrates. Arch Dermatol. 1999;135(2):200–202. doi:10.1001/archderm.135.2.200
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