Novel mutations and defective protein kinase C activation of T-lymphocytes in ataxia telangiectasia

Abstract

Three ataxia telangiectasia (AT) patients have been characterized immunologically and molecularly. Patient 1 presents two nondescribed splicing mutations which affect exons 15 and 21 of the ATM gene. The maternal defect consists of a G > A transition in the first nucleotide of the intron 21 donor splicing site which results in a complete deletion of exon 21. The paternal mutation consists of an A > C transversion in the intron 14 acceptor splicing site which produces a partial skipping of exon 15. Two abnormal alternative transcripts were found, respectively, 17 and 41 nucleotides shorter. Patient 2 presents a homozygous genomic deletion of 28 nucleotides in the last exon of the gene. This deletion changes the normal reading frame after residue 3003 of the protein and introduces a premature stop codon at residue 3008 that could originate a truncated ATM protein. Patient 3, a compound heterozygote, presents a defect which consists of a G > A transition in the first nucleotide of intron 62 donor splicing site which results in a complete deletion of exon 62. The results obtained during a three year period in the proliferation assays show an impaired PMA (phorbol myristate acetate) activation in specific T lymphocyte activation pathways (CD69, CD26, CD28, CD3, PHA, PWM and Con A mediated) but not in others (CD2, ionomycin, and Ig surface receptor). The possible link among specific ATM mutations and abnormal immune responses is unknown.

Fig. 1

Ideograms, G-banding patterns and FISH of both (A) normal chromosomes 7 and 14 and (B) translocated (7; 14) chromosomes observed in the HVS-T-cell line of patient 1.

Fig. 2

Schematic representation of maternal (GenBank accession numbers: AF035325 for genomic DNA and AF035328 for cDNA) and paternal-inherited mutations (GenBank accession numbers: AF035324 for genomic DNA and AF035326 and AF035327 for cDNAs) of patient 1. The location of relevant introns and exons of genomic DNA in the ATM gene is shown in the upper panel followed by both the normal and the mutated DNA sequences. Lowercase letters indicate introns and uppercase letters indicate exons. Donor and acceptor splicing signals are boxed. A vertical arrow depicts the genomic mutation. Double arrow horizontal lines show the normal splicing sites and the alternative abnormal splicing in the patient for the maternal-inherited mutation. Two putative novel acceptor sites inside exon 15 are boxed for the paternal-inherited mutation.

Fig. 3

Schematic representation of patient 2 homozygous mutation. See legend of Fig. 2 for the meaning of the symbols. A vertical arrow shows boundaries between intron-exon or where the putative protein is changed in the patient. Above the amino acid 3002, the patient would have a normal protein however, this changes from residue 3003 on. The location of relevant portions of ATM cDNA and their corresponding amino acid sequence are shown in the bottom panel. The 5 amino acids changed in the patient's truncated-protein are shown in italics.