IDENTIFICATION OF A NOVEL FOLATE RECEPTOR, A TRUNCATED RECEPTOR, AND RECEPTOR-TYPE-BETA IN HEMATOPOIETIC-CELLS - CDNA CLONING, EXPRESSION, IMMUNOREACTIVITY, AND TISSUE-SPECIFICITY

The expression of a membrane-associated folate receptor (FR) was elevated in spleen samples from patients with chronic (CML) and acute (AML) myelogenous leukemias compared with normal spleen. Contrary to earlier reports, antibodies to a purified FR from placenta cross-reacted quantitatively with this protein in solution radioimmunoassays. Similar to FR-alpha (KB cells) and FR-beta (placenta), the protein was released from the membrane by phosphatidylinositol-specific phospholipase C, indicating a glycosylphosphatidylinositol (GPI) membrane anchor. Screening of a cDNA library from CML spleen with a heterologous murine FR cDNA and also amplification of FR cDNAs from spleen and bone marrow in CML, AML, chronic lymphocytic leukemia (CLL), and acute lymphocytic leukemia (ALL) by polymerase chain reaction (PCR) using degenerate oligonucleotides yielded cDNA clones representing FR-beta, a novel FR (type gamma), and an aberrant transcript of FR-gamma with a 2 base pair deletion resulting in a truncated 104-residue polypeptide; FR-alpha was not detected in these tissues. The cDNA for FR-gamma predicts a 243-residue polypeptide with an amino acid sequence homology of 71% and 79% with FR-alpha and FR-beta, respectively, a 23-residue aminoterminal signal peptide, and 3 potential sites for N-linked glycosylation. Transfection of COS-1 cells with the cDNA for FR-gamma resulted in low expression of a [H-3]folic acid binding protein on the cell surface that was GPI-anchored. PCR analysis of total RNA from a number of normal and malignant tissues and cell lines indicated a limited tissue specificity of FR-gamma. FR-gamma and FR-beta could thus be potential differentiation markers in hematopoiesis and potential therapeutic targets in certain malignancies.