Isabella Cascino, Giuliana Papoff, Adriana Eramo, and Giovina Ruberti

Department of Immunobiology, Institute of Cell Biology, National Research Council, Rome, Italy.

Received 12/01/95; Accepted 01/11/96; On-line 1/1/96

Our group and others have reported that normal human PHA-activated lymphocytes express, in addition to the full length mRNA, less abundant, shorter Fas mRNA species (28-31). The genomic intron/exon organization of the regions surrounding the deleted sequences demonstrated that the transcripts derive by alternative splicing of the Fas gene. A more descriptive nomenclature of the Fas variants has been proposed based on the Fas/Apo-1 gene structure (32, 33): FasExo6Del (previously called FasTMDel, ref. 28,29); FasExo3,4Del (previously called FasDel2, ref. 29), FasExo3,4,6Del (previously called FasDel3, ref. 29); FasExo4Del and FasExo4,6Del (30) and FasExo4,7Del (31). With the exception of FasExo6Del, which is characterized by an in frame deletion of exon 6, in all of these variants, the deletions result in a different reading frame with premature termination codons. Thus, the variants should code for smaller mature Fas proteins with a C-terminal end of 21 or 38 amino acids that differ from those of the membrane-bound form of Fas (29). Fas (CD95) is widely expressed on both hematopoietic and non hematopoietic tumor cells (34-37). Information on the expression of these splicing variants is far from complete. Besides in PBMC, several variants are known to be expressed in tumor cells. For example, FasExo6Del expression has been reported in human hepatoma (38) and osteosarcoma cells (39).

Two variants have been detected only in pathological conditions and correspond to mutations in splicing recognition sites. However, it must be pointed out that there is no definitive proof that the expression of the last two variants is strictly confined to pathological situations.

FasExo8Del has been reported in apoptosis-resistant clones derived from a human lymphoma cell line. This variant codes for a truncated Fas molecule that lacks the intracellular death-signalling domain. The involved mutation was identified as a deletion-insertion in the intron 7/Exon 8 region of the Fas gene. Notably, this mutation affects the phenotype in a dominant negative fashion, i.e. in the presence of the normal receptor (26, 27).

FasExo3Del has been reported in a patient with ALPS, in addition to normal-sized Fas mRNA and to the previously described FasExo3,4Del. This patient showed a mutation in the 5' splice site of intron 3 (22).

The human Fas isoforms so far described in physiological or pathological conditions are schematically represented in Fig. 1.