Grahame Bulfield

Mapping of obesity QTLs in a cross between mouse lines divergently selected on fat content.

Abstract. A genome-wide quantitative trait locus (QTL) analysis was performed in a polygenic obesity mouse model resulting from a long-term selection experiment. The parental lines were outbred lines divergently selected for 53 generations for high-fat (fat, F line) or low-fat (lean, L line) percentage (fat%) that differed fivefold in fat% at 14 weeks of age. An F2 population of 436 mice was used for the QTL analysis with 71 markers distributed across the genome. The analysis revealed significant QTLs Fob1 (for F-line obesity QTL 1), Fob2, Fob3, and Fob4, on Chromosomes (Chrs) 2, 12, 15, and X, respectively. None of these QTLs map to regions of known single gene obesity mutations (Lepob, Leprdb, Cpefat, Ay, tub), though they map to regions of previously described obesity QTLs and candidate genes. The effects of Fob1, Fob3, Fob4 were additive, and that of Fob2 was dominant. Fob2 also showed a significant female-specific effect. Fob1, Fob2, Fob3, and Fob4 explained 4.9%, 19.5%, 14.4%, and 7.3% of the F2 phenotypic variance for fat%, respectively. This study identified four loci that contributed to the response to divergent selection and control a significant proportion of the difference in obesity between the F and L lines.

Inbred lines of mice derived from long-term growth selected lines: unique resources for mapping growth genes.

Abstract. Lines of mice selected for many generations for high or low growth in several laboratories around the world have been collected, and from these, inbred lines are being developed by recurrent full-sib mating in Edinburgh. There are seven high selected lines and four low lines (each low line is from the same base population as one of the high lines), and the histories of each are summarized. Mean body weight of males at 70 days of age in the Edinburgh laboratory in the heaviest inbred line (77 g) is 4.8-fold higher than in the lightest line (16 g), and 1.9-fold higher than in the least extreme high line (41 g). Litter size, food intake, and fat content also differ substantially. These inbred extreme selected lines are a uniquely valuable resource for QTL or gene mapping, candidate gene identification, and elucidation of epistatic effects.

Detection of quantitative trait loci from frequency changes of marker alleles under selection

A method was developed to estimate effects of quantitative trait loci (QTL) by maximum likelihood using information from changes of gene frequency at marker loci under selection, assuming an additive model of complete linkage between markers and QTL. The method was applied to data from 16 molecular and coat colour marker loci in mouse lines derived from the F2 of two inbred strains which were divergently selected on 6-week weight for 21 generations. In 4 regions of the genome, marker allele frequencies were more extreme than could be explained by sampling, implying selection at nearby QTL. An effect of about 0.5 standard deviations was located on chromosome 11, and accounted for nearly 10% of the genetic variance in the base population. QTL with effects as small as 0.2 phenotypic standard deviations could be detected. For typing of a given number of individuals, the power of detection of QTL is very high compared to, for example, analysis of an F2 population. The joint effects of linkage and selection were investigated by Monte Carlo simulation. Marker gene frequencies change little as a consequence of selection at a QTL unless the marker and QTL are less than about 20 cM apart.

Biochemical genetics of a new glucosephosphate isomerase allele (Gpi-1c) from wild mice

We have found a new allele at the structural locus for glucosephosphate isomerase (called Gpi-1c) in a population of wild mice. The Gpi-1c allele codes for an enzyme of greater cathodal electrophoretic mobility than either the Gpi-1a or Gpi-1b alleles found in the wild and in the SM/J and C57BL/6J inbred strains. Mice homozygous for Gpi-1c have erythrocyte enzyme activity reduced to 33% of normal levels, altered pH profile, lowered heat stability, and normal Kms when compared with SM/J and C57BL/6J mice. The activity of the enzyme in brain, liver, and kidney is not so markedly lowered, although the electrophoretic mobility, pH profile, and heat stability are altered in these tissues. Deficiencies of erythrocyte glucosephosphate isomerase in man, to this level, can cause severe hemolytic anemia. Homozygotes for Gpi-1c show only mild hematological symptoms. The frequency of Gpi-1c in wild populations of mice is discussed and the occurrence of a further rare allele Gpi-1d is reported.

Genetic complexity of an obesity QTL (Fob3) revealed by detailed genetic mapping.

Obesity is proving to be a serious health concern in the developed world as well as an unwanted component of growth in livestock production. While recent advances in genetics have identified a number of monogenic causes of obesity, these are responsible for only a small proportion of human cases of obesity. By divergent selection for high and low fat content over 60 generations, we have created Fat (F) and Lean (L) lines of mice that represent a model of polygenic obesity similar to the situation in human populations. From previous crosses of these lines, four body fat quantitative trait loci (QTL) were identified. We have created congenic lines (Fchr15L), by recurrent marker-assisted backcrossing, to introgress the QTL region with the highest LOD score, Fob3 on Chr 15, from the L-Iine into the F-line background. We have further mapped this QTL by progeny testing of recombinants, produced from crosses between the F-line and congenic Fchrl5L mice, showing that the Fob3 QTL region is a composite of at least two smaller effect QTL—the proximal QTL Fob3a is a late-onset obesity QTL, whereas the distal Fob3b is an early-onset obesity QTL.

Histidinaemia in mouse and man

A recently discovered mutant in the mouse was found to have very low levels of histidase. It is an autosomal recessive. In its enzymic and metabolic properties it appears to be a homologue of human histidinaemia. While the homozygous mouse mutants show no overt abnormalities, offspring of histidinaemic mothers display a balance defect resulting in circling behaviour. This is associated with vestibular damage during in utero development. Mental retardation caused by human maternal phenylketonuria may have a similar aetiology.

Histidine decarboxylase phenotypes of inbred mouse strains: a regulatory locus (Hdc) determines kidney enzyme concentration.

Mouse kidney histidine decarboxylase (HDC) provides a model system to study genetic control of a hormone-regulated enzyme (inducible by estrogen and thyroxine; repressible by testosterone). Five major HDC phenotypes scored on the basis of (i) enzyme activity and (ii) the difference in activity between the sexes (females usually higher than males) have been discovered by screening 38 strains of mice. One genetic difference between high-activity strains (DBA/2 and C3H/He) and low-activity strains (C57BL/6 and C57BL/10) has been examined in detail. The phenotypic difference segregates as a single gene in both conventional crosses and between recombinant inbred (RI) strains. Immunoprecipitation has shown that the activity difference is due to an alteration in the number of enzyme molecules. The phenotypic difference between high and low strains can therefore be attributed to different alleles of a single regulatory locus, Hdc; the alleleHdcddetermines low HDC concentration, and the allele Hdcdhigh concentration. Hdc has been mapped to chromosome 2 using data from both comparisons of strain distribution patterns of previously mapped loci within RI strains and a conventional three-point cross. The probable gene order is B2m-pa-Hdc, with map distances of 3.1±1.7 and 2.0±1.4 cM, respectively.

Semi-automated assays for enzymopathies of carbohydrate metabolism in liver and erythrocytes, using a reaction rate analyser

Abstract It has been possible to adjust 20 enzymes of carbohydrate metabolism of the liver and 17 of erythrocytes to the optimum reagent concentrations for use on a semi-automatic reaction rate analyser. The assays and the conditions for their reliable use are described in full. This system is in use in screening for inborn errors of carbohydrate metabolism in the mouse. In man specific erythrocyte enzymopathies are known for 11 and liver enzymopathies for 2 of the enzymes chosen. As more will be found in the future this system will be useful for quick and routine diagnosis of these inborn errors in man and mouse.

Rates of change of genetic parameters of body weight in selected mouse lines

A method based on the animal model is described which allows the estimation of continuous changes in variance components over time using restricted maximum likelihood (REML). The method was applied to the analysis of a selection experiment in which a foundation population formed from a cross between two inbred strains of mice (C57BL/6J and DBA/2J) was divergently selected for 6 week body weight over 20 generations. The analysis suggested that there was an increase in phenotypic variance of about 50% in the low selected lines over the course of the experiment which was attributed to increases in the environmental and additive variance components. Variance changes in the High selected lines were generally smaller than in the Low lines, although there was an estimated 20% increase in the environmental variance. Simple models to explain these effects involving dominance, linkage and epistasis were explored. Testing which of these was responsible for the variance changes noted in this experiment (if any) is difficult, although the epistasis and dominance models require less stringent conditions than the linkage model, and the dominance model is supported by evidence of heterosis in the F1.

Farm animal biotechnology

As we enter a new millennium, the research with the greatest likely impact on both the biological sciences and the biotechnology industry will be the sequencing of the human and other genomes. Widespread interest in farm animal genomics as a method for identifying genes controlling commercially important traits started only a decade ago. Although the genomics of farm animals was relatively late to arrive on the scene compared with the genomics of crop plants, it has the advantage of being able to access the enormous amount of human genome information.