Novel SAA gene products in Mus caroli mice defined by rapid amplification of complementary DNA ends (3′ and 5′ RACE)

Abstract

The structure of the SAA gene family has been defined primarily in BALB/c mice, a strain which is the prototype for the large group of inbred strains designated haplotype A. Four different SAA genes have been identified: SAA₁, SAA₂, SAA₃, and SAA₄ (originally described as SAA₅). The gene products differ in isoelectric point, with experimentally determined pI values of 6.45, 6.3, 9.2, and 8.1, respectively. Members of a smaller subset of inbred strains designated haplotype B, including SJL/J, express a gene whose product is identical in pI to the SAA₁ gene product in BALB/c mice and a variant of the BALB/c SAA₂ gene whose product is more acidic (pI 5.9) because of the substitution of aspartic acid for alanine at position 101.

Like inbred mice, wild derived Mus caroli mice respond to inflammatory stimulation with an increase in SAA gene expression; isoelectric focusing analysis ofM. caroli acute phase plasma revealed two major apoSAA isoforms, one corresponding to the haplotype A SAA₂ gene product and the other to the haplotype B SAA₂ gene product. The derived amino acid sequences ofM. caroli SAA gene products were investigated using reverse transcriptase polymerase chain reaction (RT-PCR) and rapid amplification of complementary DNA ends (3′ and 5′ RACE) techniques. The theoretical pIs of proteins encoded by two distinct M. caroli SAA specific cDNA clones were identical to the theoretical pI of the acidic variant of apoSAA₂ expressed in haplotype B mice (reported as pI 5.9). However the M. caroli SAA cDNAs differ in derived amino acid sequence from that variant and from each other. Our results indicate additional diversity within the murine SAA gene family and demonstrate that apoSAA isoforms among various strains of mice cannot be considered identical solely on the basis of isoelectric point. The approach described here may have general applicability in phylogenetic studies of the SAA gene family.

Key Words:

• SERUM AMYLOID A
• APOSAA
• ACUTE PHASE RESPONSE
• RACE