Localization of insulin-2 (Ins-2) and the obesity mutant tubby (tub) to distinct regions of mouse chromosome 7

Evolution of the CDKN1C-KCNQ1 imprinted domain

Background

Genomic imprinting occurs in both marsupial and eutherian mammals. The CDKN1C and IGF2 genes are both imprinted and syntenic in the mouse and human, but in marsupials only IGF2 is imprinted. This study examines the evolution of features that, in eutherians, regulate CDKN1C imprinting.

Results

Despite the absence of imprinting, CDKN1C protein was present in the tammar wallaby placenta. Genomic analysis of the tammar region confirmed that CDKN1C is syntenic with IGF2. However, there are fewer LTR and DNA elements in the region and in intron 9 of KCNQ1. In addition there are fewer LINEs in the tammar compared with human and mouse. While the CpG island in intron 10 of KCNQ1 and promoter elements could not be detected, the antisense transcript KCNQ1OT1 that regulates CDKN1C imprinting in human and mouse is still expressed.

Conclusion

CDKN1C has a conserved function, likely antagonistic to IGF2, in the mammalian placenta that preceded its acquisition of imprinting. CDKN1C resides in synteny with IGF2, demonstrating that imprinting of the two genes did not occur concurrently to balance maternal and paternal influences on the growth of the placenta. The expression of KCNQ1OT1 in the absence of CDKN1C imprinting suggests that antisense transcription at this locus preceded imprinting of this domain. These findings demonstrate the stepwise accumulation of control mechanisms within imprinted domains and show that CDKN1C imprinting cannot be due to its synteny with IGF2 or with its placental expression in mammals.

Insulin is imprinted in the placenta of the marsupial, Macropus eugenii

Therian mammals (marsupials and eutherians) rely on a placenta for embryo survival. All mammals have a yolk sac, but while both chorio-allantoic and chorio-vitelline (yolk sac) placentation can occur, most marsupials only develop a yolk sac placenta. Insulin (INS) is unusual in that it is the only gene that is imprinted exclusively in the yolk sac placenta. Marsupials, therefore, provide a unique opportunity to examine the conservation of INS imprinting in mammalian yolk sac placentation. Marsupial INS was cloned and its imprint status in the yolk sac placenta of the tammar wallaby, Macropus eugenii, examined. In two informative individuals of the eight that showed imprinting, INS was paternally expressed. INS protein was restricted to the yolk sac endoderm, while insulin receptor, IR, protein was additionally expressed in the trophoblast. INS protein increased during late gestation up to 2 days before birth, but was low the day before and on the day of birth. The conservation of imprinted expression of insulin in the yolk sac placenta of divergent mammalian species suggests that it is of critical importance in the yolk sac placenta. The restriction of imprinting to the yolk sac suggests that imprinting of INS evolved in the chorio-vitelline placenta independently of other tissues in the therian ancestor of marsupials and eutherians.

Mash2 acts cell autonomously in mouse spongiotrophoblast development

The Mash2 gene, which encodes a basic helix-loop-helix transcription factor, is one of the mammalian homologues of the Drosophila achaete-scute genes. It is strongly expressed in diploid trophoblast cells of the postimplantation mouse embryo. Targeted mutagenesis of Mash2 revealed that loss of function results in embryonic lethality at midgestation, due to placental failure associated with a lack of spongiotrophoblast and reduced labyrinthine trophoblast layers. For the further study of Mash2 function in development of the trophoblast cell lineage, we have performed chimeric analysis combining Mash2 mutant and wild-type embryos. We have addressed the question of whether the phenotype of the Mash2 mutant embryo, which affects all of the three trophoblast cell layers, is caused by a cell autonomous or non-autonomous defect and whether Mash2 is required in both spongiotrophoblast and labyrinthine trophoblast development. Our results showed no contribution of Mash2 mutant cells to the spongiotrophoblast layer in chimeric placentae at 10.5 and 12.5 days postcoitum, suggesting that the product of the Mash2 gene is required cell autonomously during the development of the spongiotrophoblast. However, it seems that Mash2 is not required for development of labyrinthine trophoblast or giant cells, since high contributions of Mash2 mutant cells were observed in those trophoblast cell layers in the chimeric placentae analyzed. We can therefore conclude that the primary and cell-autonomous function of Mash2 appears to be an involvement in the development of diploid trophoblast cells in the ectoplacental cone to form the spongiotrophoblast cell layer of the mature chorioallantoic placenta.

Mapping of mouse obesity genes: A generic approach to a complex trait

Identification of genes underlying any complex trait such as obesity is an important and difficult problem in genetics. Traditional candidate gene approaches cannot be relied on to identify all of the genes influencing a complex trait, and positional cloning is very laborious. With the advent of new tools and methods, however, comprehensive approaches to the identification of any genes underlying complex traits are now available. Quantitative trait locus (QTL) mapping is a general technique to map Mendelian factors influencing complex traits. The QTL approach involves the crossing of two strains that differ in the trait of interest to produce F2 or back-cross progeny, individually phenotyping and genotyping each progeny, and statistically associating the typed markers and the phenotype. QTL mapping has been used in the last 4 years to map genes for a wide variety of traits, including body weight and growth, obesity, atherosclerosis and susceptibility to cancer in the mouse, and hypertension, hyperactivity and arthritis in the rat. QTL mapping has also been used to map genes in pigs, poultry, cows, fish and plants. Once a trait has been located in a chromosomal subregion, identifying the underlying gene remains a significant problem. A monogenic model must be developed, isolating one gene influencing a trait from other genes affecting the same phenotype. Then the positional candidate strategy, which relies on a combination of mapping to a chromosomal subregion followed by a survey of the interval to see if attractive candidates reside there, becomes practical.

Identification and characterization of the mouse obesity gene tubby: a member of a novel gene family

The mutated gene responsible for the tubby obesity phenotype has been identified by positional cloning. A single base change within a splice donor site results in the incorrect retention of a single intron in the mature tub mRNA transcript. The consequence of this mutation is the substitution of the carboxy-terminal 44 amino acids with 24 intron-encoded amino acids. The normal transcript appears to be abundantly expressed in the hypothalamus, a region of the brain involved in body weight regulation. Variation in the relative abundance of alternative splice products is observed between inbred mouse strains and appears to correlate with an intron length polymorphism. This allele of tub is a candidate for a previously reported diet-induced obesity quantitative trait locus on mouse chromosome 7.

A candidate gene for the mouse mutation tubby

A mutation in the tub gene causes maturity-onset obesity, insulin resistance, and sensory deficits. In contrast to the rapid juvenile-onset weight gain seen in diabetes (db) and obese (ob) mice, obesity in tubby mice develops gradually, and strongly resembles the late-onset obesity seen in the human population. Excessive deposition of adipose tissue eventually leads to a twofold increase of body weight. Tubby mice also suffer retinal degeneration and neurosensory hearing loss. The tripartite character of the tubby phenotype shows striking similarity to human obesity syndromes, such as Alström and Bardet-Biedl. Here we report the identification of a G --> T transversion in a candidate gene that abolishes a donor splice site in the 3' coding region and results in a larger transcript containing the unspliced intron. This alteration is predicted to replace the 44-carboxyterminal amino acids with a 20-amino-acid sequence not found in the wide-type protein. Additionally, a second, prematurely truncated transcript with the unspliced intron is observed in testis messenger RNA and a 2-3-fold increase in brain mRNA is observed in tubby mice compared to B6. The phenotype features of tubby mice may be the result of cellular apoptosis triggered by expression of the mutuated tub gene.

Identification of four chromosomal loci determining obesity in a multifactorial mouse model

We previously described a new mouse model for multigenic obesity, designated BSB. We now report the use of a complete linkage map approach to identify loci contributing to body fat and other traits associated with obesity in this model. Four loci exhibiting linkage with body fat, or with the weights of four different fat depots, residing on mouse chromosomes 6, 7, 12, and 15, were identified and confirmed by analysis of additional BSB mice. Each of the four loci differed with respect to their effects on the percent of body fat, specific fat depots and plasma lipoproteins. The loci exhibited allele-specific, non-additive interactions. A locus for hepatic lipase activity was co-incident with the body fat and total cholesterol loci on chromosome 7, providing a possible mechanism linking plasma lipoproteins and obesity. The chromosome 7 locus affecting body fat, total cholesterol and hepatic lipase activity was isolated in congenic strains whose donor strain regions overlap with the chromosome 7 BSB locus. These results provide candidate genes and candidate loci for the analysis of human obesity.

Localization of the murine cholecystokinin A and B receptor genes

We have determined the chromosomal locations of the two cholecystokinin (CCK) receptor genes in the mouse. Genetic localization utilized an interspecific backcross panel formed from the cross (C57BL/6J x Mus spretus) F1 x Mus spretus. Genomic DNAs from 94 individuals in the backcross were analyzed by Southern hybridization with rat CCKA and CCKB receptor cDNA probes. Unique map positions were determined by haplotype analysis with 650 previously mapped loci in the mouse backcross. The CCKA receptor gene (Cckar) mapped to mouse Chromosome (Chr) 5, in tight linkage with the DNA marker D5Bir8. The CCKB receptor gene (Cckbr) mapped to mouse Chr 7, tightly linked to the beta-hemoglobin locus (Hbb). This localization places Cckbr in the same region as the mouse obesity mutation tubby (tub), which also maps near Hbb (2.4 +/- 1.4 cM). Since CCK can function as a satiety factor when administered to rodents, localization of Cckbr near the tub mutation identifies this receptor as a possible candidate gene for this obesity mutation.

Glycogen synthase: a putative locus for diet-induced hyperglycemia

Inbred mouse strains fed a diabetogenic diet have different propensities to develop features analogous to type 2 diabetes mellitus. To define chromosomal locations that control these characteristics, recombinant inbred strains from diabetes-prone C57BL/6J (B/6J) and diabetes-resistant A/J strains were studied. Insulin levels and hyperglycemia correlated with two different regions of mouse chromosome 7 (two point LOD scores > 3.0). For insulin levels, 15 of 16 recombinant inbred strains were concordant with a region that contains the tubby mutation that results in hyperinsulinemia. For hyperglycemia, 19 of 23 strains were concordant with the D7Mit25 marker and 20 of 23 strains with the Gpi-1 locus on proximal mouse chromosome 7. Using more stringent criteria for hyperglycemia, 10 of 11 strains characterized as A/J or B/6J like were concordant with D7Mit25. This putative susceptibility locus is consistent with that of the glycogen synthase gene (Gys) recently suggested as a candidate locus by analyses of type 2 diabetes patients. Fractional glycogen synthase activity in isolated muscle was significantly lower in normal B/6J diabetic-prone mice compared with normal diabetic-resistant A/J mice, a finding similar to that reported in relatives of human patients with type 2 diabetes. These data, taken together, raise the possibility that defects in the Gys gene may in part be responsible for the propensity to develop type 2 diabetes.