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The PRKAG3 gene encodes a muscle-specific isoform of the regulatory gamma subunit of AMP-activated protein kinase (AMPK). A major part of the coding PRKAG3 sequence was isolated from horse muscle cDNA using reverse-transcriptase (RT)-PCR analysis. Horse-specific primers were used to amplify genomic fragments containing 12 exons. Comparative sequence analysis of horse, pig, mouse, human, Fugu, and zebrafish was performed to establish the exon/intron organization of horse PRKAG3 and to study the homology among different isoforms of AMPK gamma genes in vertebrates. The results showed conclusively that the three different isoforms (gamma1, gamma2, and gamma3) were established already in bony fishes. Seven single nucleotide polymorphisms (SNPs), five causing amino acid substitutions, were identified in a screening across horse breeds with widely different phenotypes as regards muscle development and intended performance. The screening of a major part of the PRKAG3 coding sequence in a small case/control material of horses affected with polysaccharide storage myopathy did not reveal any mutation that was exclusively associated with this muscle storage disease. The breed comparison revealed several potentially interesting SNPs. One of these (Pro258Leu) occurs at a residue that is highly conserved among AMPK gamma genes. In an SNP screening, the variant allele was only found in horse breeds that can be classified as heavy (Belgian) or moderately heavy (North Swedish Trotter, Fjord, and Swedish Warmblood) but not in light horse breeds selected for speed or racing performance (Standardbred, Thoroughbred, and Quarter horse) or in ponies (Icelandic horses and Shetland pony). The results will facilitate future studies of the possible functional significance of PRKAG3 polymorphisms in horses., (Copyright 2003 S. Karger AG, Basel)

The PRKAG3 gene encodes the gamma3 chain of AMP-activated protein kinase (AMPK). A non-conservative missense mutation in the PRKAG3 gene causes a dominant phenotype involving abnormally high glycogen content in pig skeletal muscle. We have determined >126 kb (in 13 contigs) of porcine genomic sequence surrounding the PRKAG3 gene and the corresponding mouse region covering the gene. A comparison of these PRKAG3 sequences and the human sequence was conducted and used to predict evolutionarily conserved regions, including regulatory regions. A comparison of the human genomic sequence and a porcine BAC sequence containing the PRKAG3 gene, revealed a conserved organization and the presence of three additional genes, CYP27A1 (cytochrome P450, family 27, subfamily A, polypeptide 1), STK36 (Serine Threonine Kinase 36), and the homolog of the unidentified human mRNA KIAA0173. Interspersed repetitive elements constituted 51.4 and 38.6% of this genomic region in human and pig, respectively. We were able to reliably align 12.6 kb of orthologous repeats shared between pig and human and these showed an average sequence identity of 72.4%. Our analysis revealed that the human KIAA0173 gene harbors alternative 5' untranslated exons originating from repetitive elements. This provides an obvious example how transposable elements may affect gene evolution., (Copyright 2003 S. Karger AG, Basel)

A high proportion of purebred Hampshire pigs carries the dominant RN- mutation, which causes high glycogen content in skeletal muscle. The mutation has beneficial effects on meat content but detrimental effects on processing yield. Here, it is shown that the mutation is a nonconservative substitution (R200Q) in the PRKAG3 gene, which encodes a muscle-specific isoform of the regulatory gamma subunit of adenosine monophosphate-activated protein kinase (AMPK). Loss-of-function mutations in the homologous gene in yeast (SNF4) cause defects in glucose metabolism, including glycogen storage. Further analysis of the PRKAG3 signaling pathway may provide insights into muscle physiology as well as the pathogenesis of noninsulin-dependent diabetes mellitus in humans, a metabolic disorder associated with impaired glycogen synthesis.