Bidirectional imprinting of a single gene: GNAS1 encodes maternally, paternally, and biallelically derived proteins

The GNAS1 gene encodes the α subunit of the guanine nucleotide-binding protein Gs, which couples signaling through peptide hormone receptors to cAMP generation. GNAS1 mutations underlie the hormone resistance syndrome pseudohypoparathyroidism type Ia (PHP-Ia), so the maternal inheritance displayed by PHP-Ia has raised suspicions that GNAS1 is imprinted. Despite this suggestion, in most tissues Gsα is biallelically encoded. In contrast, the large G protein XLαs, also encoded by GNAS1, is paternally derived. Because the inheritance of PHP-Ia predicts the existence of maternally, rather than paternally, expressed transcripts, we have investigated the allelic origin of other mRNAs derived from GNAS1. We find this gene to be remarkable in the complexity of its allele-specific regulation. Two upstream promoters, each associated with a large coding exon, lie only 11 kb apart, yet show opposite patterns of allele-specific methylation and monoallelic transcription. The more 5′ of these exons encodes the neuroendocrine secretory protein NESP55, which is expressed exclusively from the maternal allele. The NESP55 exon is 11 kb 5′ to the paternally expressed XLαs exon. The transcripts from these two promoters both splice onto GNAS1 exon 2, yet share no coding sequences. Despite their structural unrelatedness, the encoded proteins, of opposite allelic origin, both have been implicated in regulated secretion in neuroendocrine tissues. Remarkably, maternally (NESP55), paternally (XLαs), and biallelically (Gsα) derived proteins all are produced by different patterns of promoter use and alternative splicing of GNAS1, a gene showing simultaneous imprinting in both the paternal and maternal directions.

Interactions of the chaperone Hsp104 with yeast Sup35 and mammalian PrP

[PSI+] is a genetic element in yeast for which a heritable change in phenotype appears to be caused by a heritable change in the conformational state of the Sup35 protein. The inheritance of [PSI+] and the physical state of Sup35 in vivo depend on the protein chaperone Hsp104 (heat shock protein 104). Although these observations provide a strong genetic argument in support of the “protein-only” or “prion” hypothesis for [PSI+], there is, as yet, no direct evidence of an interaction between the two proteins. We report that when purified Sup35 and Hsp104 are mixed, the circular dichroism (CD) spectrum differs from that predicted by the addition of the proteins’ individual spectra, and the ATPase activity of Hsp104 is inhibited. Similar results are obtained with two other amyloidogenic substrates, mammalian PrP and β-amyloid 1-42 peptide, but not with several control proteins. With a group of peptides that span the PrP protein sequence, those that produced the largest changes in CD spectra also caused the strongest inhibition of ATPase activity in Hsp104. Our observations suggest that (i) previously described genetic interactions between Hsp104 and [PSI+] are caused by direct interaction between Hsp104 and Sup35; (ii) Sup35 and PrP, the determinants of the yeast and mammalian prions, respectively, share structural features that lead to a specific interaction with Hsp104; and (iii) these interactions couple a change in structure to the ATPase activity of Hsp104.

Fetal and maternal contributions to risk of pre-eclampsia: population based study

Objective: To use familial patterns of recurrence of pre-eclampsia to investigate whether paternal genes expressed in the fetus contribute to the mother’s risk of pre-eclampsia and whether mother’s susceptibility to pre-eclampsia is related to maternal inheritance by mitochondrial DNA.

Roles for genomic imprinting and the zygotic genome in placental development

The placenta contains several types of feto-maternal interfaces where zygote-derived cells interact with maternal cells or maternal blood for the promotion of fetal growth and viability. The genetic factors regulating the interactions between different cell types within feto-maternal interfaces and the relative contributions of the maternal and zygotic genomes are poorly understood. Genomic imprinting, the epigenetic process responsible for parental origin-dependent functional differences between homologous chromosomes, has been proposed to contribute to these events. Previous studies showed that mouse conceptuses with an absence of imprinted differences between the two copies of chromosome 12 (upon paternal inheritance of both copies) die late in gestation and have a variety of defects, including placentomegaly. Here we examined the role of chromosome 12 imprinting in these placentae in more detail. We show that the spatial interactions between different cell types within feto-maternal interfaces are defective and identify abnormal behaviors in both zygote-derived and maternal cells that are attributed to the genome of the zygote but not the mother. These include compromised invasion of the maternal decidualized endometrium and the central maternal artery situated within it by zygote-derived trophoblast, abnormalities in the wall of the central maternal artery, and defects within the zygote-derived cellular layer of the labyrinth, which is in direct contact with maternal blood. These findings demonstrate multiple roles for chromosome 12 imprinting in the placenta that have not previously been associated with imprinting effects. They provide insights into the function of imprinting in placental development and have evolutionary and clinical implications.

Genetics and the origin of bird species

External (environmental) factors affecting the speciation of birds are better known than the internal (genetic) factors. The opposite is true for several groups of invertebrates, Drosophila being the outstanding example. Ideas about the genetics of speciation in general trace back to Dobzhansky who worked with Drosophila. These ideas are an insufficient guide for reconstructing speciation in birds for two main reasons. First, speciation in birds proceeds with the evolution of behavioral barriers to interbreeding; postmating isolation usually evolves much later, perhaps after gene exchange has all but ceased. As a consequence of the slow evolution of postmating isolating factors the scope for reinforcement of premating isolation is small, whereas the opportunity for introgressive hybridization to influence the evolution of diverging species is large. Second, premating isolation may arise from nongenetic, cultural causes; isolation may be affected partly by song, a trait that is culturally inherited through an imprinting-like process in many, but not all, groups of birds. Thus the genetic basis to the origin of bird species is to be sought in the inheritance of adult traits that are subject to natural and sexual selection. Some of the factors involved in premating isolation (plumage, morphology, and behavior) are under single-gene control, most are under polygenic control. The genetic basis of the origin of postmating isolating factors affecting the early development of embryos (viability) and reproductive physiology (sterility) is almost completely unknown. Bird speciation is facilitated by small population size, involves few genetic changes, and occurs relatively rapidly.

Genetic Manipulation of Alcohol Dehydrogenase Levels in Ripening Tomato Fruit Affects the Balance of Some Flavor Aldehydes and Alcohols1

Tomato (Lycopersicon esculentum) plants were transformed with gene constructs containing a tomato alcohol dehydrogenase (ADH) cDNA (ADH 2) coupled in a sense orientation with either the constitutive cauliflower mosaic virus 35S promoter or the fruit-specific tomato polygalacturonase promoter. Ripening fruit from plants transformed with the constitutively expressed transgene(s) had a range of ADH activities; some plants had no detectable activity, whereas others had significantly higher ADH activity, up to twice that of controls. Transformed plants with fruit-specific expression of the transgene(s) also displayed a range of enhanced ADH activities in the ripening fruit, but no suppression was observed. Modified ADH levels in the ripening fruit influenced the balance between some of the aldehydes and the corresponding alcohols associated with flavor production. Hexanol and Z-3-hexenol levels were increased in fruit with increased ADH activity and reduced in fruit with low ADH activity. Concentrations of the respective aldehydes were generally unaltered. The phenotypes of modified fruit ADH activity and volatile abundance were transmitted to second-generation plants in accordance with the patterns of inheritance of the transgenes. In a preliminary taste trial, fruit with elevated ADH activity and higher levels of alcohols were identified as having a more intense “ripe fruit” flavor.

Localization of atherosclerosis susceptibility loci to chromosomes 4 and 6 using the Ldlr knockout mouse model

Atherosclerosis is a complex disease resulting from the interaction of multiple genes. We have used the Ldlr knockout mouse model in an interspecific genetic cross to map atherosclerosis susceptibility loci. A total of 174 (MOLF/Ei × B6.129S7-Ldlrtm1Her) × C57BL/6J-Ldlrtm1Her backcross mice, homozygous for the Ldlr null allele, were fed a Western-type diet for 3 months and then killed for quantification of aortic lesions. A genome scan was carried out by using DNA pools and microsatellite markers spaced at ≈18-centimorgan intervals. Quantitative trait locus analysis of individual backcross mice confirmed linkages to chromosomes 4 (Athsq1, logarithm of odds = 6.2) and 6 (Athsq2, logarithm of odds = 6.7). Athsq1 affected lesions in females only whereas Athsq2 affected both sexes. Among females, the loci accounted for ≈50% of the total variance of lesion area. The susceptible allele at Athsq1 was derived from the MOLF/Ei genome whereas the susceptible allele at Athsq2 was derived from C57BL/6J. Inheritance of susceptible alleles at both loci conferred a 2-fold difference in lesion area, suggesting an additive effect of Athsq1 and Athsq2. No associations were observed between the quantitative trait loci and levels of plasma total cholesterol, high density lipoprotein cholesterol, non-high density lipoprotein cholesterol, insulin, or body weight. We provide strong evidence for complex inheritance of atherosclerosis in mice with elevated plasma low density lipoprotein cholesterol and show a major influence of nonlipoprotein-related factors on disease susceptibility. Athsq1 and Athsq2 represent candidate susceptibility loci for human atherosclerosis, most likely residing on chromosomes 1p36–32 and 12p13–12, respectively.

Trials of the beta model for complex inheritance.

Theoretical advantages of nonparametric logarithm of odds to map polygenic diseases are supported by tests of the beta model that depends on a single logistic parameter and is the only model under which paternal and maternal transmissions to sibs of specified phenotypes are independent. Although it does not precisely describe recurrence risks in monozygous twins, the beta model has greater power to detect family resemblance or linkage than the more general delta model which describes the probability of 0, 1, or 2 alleles identical by descent (ibd) with two parameters. Available data on ibd in sibs are consistent with the beta model, but not with the equally parsimonious but less powerful gamma model that assumes a fixed probability of 1/2 for 1 allele ibd. Additivity of loci on the liability scale is not disproven. A simple equivalence extends the beta model to multipoint analysis.

Genetic separation of tumor growth and hemorrhagic phenotypes in an estrogen-induced tumor.

Chronic administration of estrogen to the Fischer 344 (F344) rat induces growth of large, hemorrhagic pituitary tumors. Ten weeks of diethylstilbestrol (DES) treatment caused female F344 rat pituitaries to grow to an average of 109.2 +/- 6.3 mg (mean +/- SE) versus 11.3 +/- 1.4 mg for untreated rats, and to become highly hemorrhagic. The same DES treatment produced no significant growth (8.9 +/- 0.5 mg for treated females versus 8.7 +/- 1.1 for untreated females) or morphological changes in Brown Norway (BN) rat pituitaries. An F1 hybrid of F344 and BN exhibited significant pituitary growth after 10 weeks of DES treatment with an average mass of 26.3 +/- 0.7 mg compared with 8.6 +/- 0.9 mg for untreated rats. Surprisingly, the F1 hybrid tumors were not hemorrhagic and had hemoglobin content and outward appearance identical to that of BN. Expression of both growth and morphological changes is due to multiple genes. However, while DES-induced pituitary growth exhibited quantitative, additive inheritance, the hemorrhagic phenotype exhibited recessive, epistatic inheritance. Only 5 of the 160 F2 pituitaries exhibited the hemorrhagic phenotype; 36 of the 160 F2 pituitaries were in the F344 range of mass, but 31 of these were not hemorrhagic, indicating that the hemorrhagic phenotype is not merely a consequence of extensive growth. The hemorrhagic F2 pituitaries were all among the most massive, indicating that some of the genes regulate both phenotypes.