Human genomic sequences corresponding to murine CD3 eta-related transcripts: lack of conservation or expression of homologous human products

Abnormal expression of various molecular forms and distribution of T cell receptor zeta chain in patients with systemic lupus erythematosus

OBJECTIVE

T cells from the majority of patients with systemic lupus erythematosus (SLE) display antigen receptor-mediated signaling aberrations associated with defective T cell receptor (TCR) zeta chain expression. The TCR zeta chain, a critical signaling molecule, exists in multiple molecular forms and membrane fractions with distinct functions in antigen-mediated signaling processes. This study was undertaken to investigate the complete spectrum of expression of the different forms and distribution of the TCR zeta chain in SLE T cells.

METHODS

T cells were isolated from 48 SLE patients and 21 healthy subjects. The expression of various forms of the TCR zeta chain was investigated by immunoblotting with specific antibodies. The lipid raft-associated form of the zeta chain was determined by quantitating the solubilized zeta chain after disruption of the lipid rafts by cholesterol depletion using methyl-betacyclodextrin. The distribution of the zeta chain was investigated by fluorescence microscopy.

RESULTS

The phosphorylated 21- and 23-kd forms and the detergent-insoluble membrane-associated form of the TCR zeta chain and alternatively spliced zeta chain were significantly decreased in SLE T cells. In contrast, major ubiquitinated forms of the zeta chain were increased in these cells. We also identified up-regulation of a novel 14-kd form of the zeta chain in SLE T cells. Resting SLE T cell membranes had an increased percentage of the residual membrane-bound zeta chain in the lipid rafts. Fluorescence microscopy findings indicated that the residual zeta chain is more clustered on the cell membranes of SLE T cells.

CONCLUSION

These results suggest that, in addition to the 16-kd form, expression of other molecular forms and fractions of the TCR zeta chain as well as its membrane distribution are abnormal in SLE T cells. Increased lipid raft association and surface clustering of the zeta chain may explain the molecular mechanisms underlying the signaling abnormalities in these cells.

T cell signaling abnormalities in systemic lupus erythematosus are associated with increased mutations/polymorphisms and splice variants of T cell receptor zeta chain messenger RNA

OBJECTIVE

T cells from patients with systemic lupus erythematosus (SLE) display antigen receptor-mediated signaling aberrations associated with defective T cell receptor (TCR) zeta chain protein and messenger RNA (mRNA) expression. This study was undertaken to explore the possibility that coding-region mutations/polymorphisms of the TCR zeta chain could account for its decreased expression and altered signaling in SLE T cells.

METHODS

TCR zeta chain mRNA from 48 SLE patients, 18 disease controls, and 21 healthy volunteers was reverse transcribed, amplified by polymerase chain reaction, and cloned, and complementary DNA (cDNA) was sequenced. DNA sequences from multiple clones were analyzed for silent single-nucleotide polymorphisms, mutations, and splice variations, to promote the identification of heterozygosity.

RESULTS

DNA sequence analysis revealed several widely distributed missense mutations and silent polymorphisms in the coding region of the TCR zeta chain, which were more frequent in SLE patients than in patients with other rheumatic diseases or healthy controls (P < 0.0001). Several of the missense mutations were located in the 3 immunoreceptor tyrosine activation motifs or the GTP binding domain, and this could lead to functional alterations in the TCR zeta chain. A splice variant of the TCR zeta chain with a codon CAG (glutamine) insertion between exons IV and V was found in half of the SLE and control samples. Two larger spliced isoforms of the TCR zeta chain, with an insertion of 145 bases and 93 bases between exons I and II, were found only in SLE T cells. We also identified various alternatively spliced forms of the TCR zeta chain resulting from the deletion of individual exons II, VI, or VII, or a combined deletion of exons V and VI; VI and VII; II, III, and IV; or V, VI, and VII in SLE T cells. The frequency of the deletion splice variants was significantly higher in SLE than in control samples (P = 0.004). These variations were observed in cDNA and may not reflect the status of the genomic DNA.

CONCLUSION

These findings demonstrate that heterogeneous mutations/polymorphisms and alternative splicing of TCR zeta chain cDNA are more frequent in SLE T cells than in T cells from non-SLE subjects and may underlie the molecular basis of known T cell signaling abnormalities in this disease.

Polymorphisms/mutations of TCR-zeta-chain promoter and 3' untranslated region and selective expression of TCR zeta-chain with an alternatively spliced 3' untranslated region in patients with systemic lupus erythematosus

A vast majority of systemic lupus erythematosus (SLE) patients display decreased expression of TCR zeta-chain mRNA, a critical signaling molecule implicated in the selection of the TCR repertoire and in the prevention of autoimmunity. To identify the molecular mechanisms involved in the downregulation of TCR zeta-chain transcripts in SLE T cells, we investigated the possibility of polymorphisms/mutations in the promoter and the 3' untranslated region. PCR, cloning and sequence analysis of the promoter region from the genomic DNA showed significantly higher number of polymorphisms in SLE T cells compared to non-SLE control subjects (P = 0.044). Promoter sequence was also analysed from granulocytes to delineate the possibility of somatic mutations in activated SLE T cells. Promoter polymorphisms were significantly higher in granulocytes of SLE patients compared to non-SLE controls (P = 0.048), suggesting that these polymorphisms were of genomic origin. Nucleotide analysis of the promoter sequence revealed a -76T insertion compared to the published sequence, in all of the SLE samples and controls. RT-PCR analysis of the TCR zeta-chain 3' untranslated region showed a 344 bp product in addition to the expected 906 bp product. Cloning and sequence analysis of the 344 bp product indicated that it is an alternatively spliced form with both splicing donor and acceptor sites, resulting in deletion of nucleotides 672-1233 of TCR zeta-chain mRNA. Unlike the nomal TCR zeta-chain, the expression of TCR zeta-chain with the alternatively spliced 344 bp 3' untranslated region was higher in SLE T cells compared to non-SLE controls. The number of mutations/polymorphisms in the 906 bp TCR zeta-chain 3' untranslated region were significantly higher in SLE T cells compared to non-SLE subjects (P = 0.032). Frequent mutations/polymorphisms and aberrant splicing of the downstream 3' untranslated region may affect the stability and/or transport of TCR zeta-chain mRNA, leading to its downregulation in SLE T cells.