Organization of the bovine casein gene locus

Polymorphism of CSN1S1 (g.12164G>A) and CSN2 (g.8913C>A) genes in pure and cross dairy goats

Casein genes directly control milk protein of animals. CSN1S1 (αS1-Casein) and CSN2 (β-Casein) genes influence on milk protein fractions. Genetic polymorphisms of CSN1S1 gene at g.12164G>A locus and CSN2 gene at g.8913C>A locus were identified by PCR-RFLP technique. Animal samples were pure dairy goats providing PE (5 hds.), Saanen (8 hds.) and their crosses providing Sapera (50% Saanen, 50% PE) (51 hds.) and SaanPE (75% Saanen, 25% PE) (3 hds.) from IRIAP dairy goat station. Allele frequency, genotype frequency, heterozygosity value, and Hardy-Weinberg (H-W) equilibrium value were analyzed by Popgen32 program. CSN1S1_g.12164G>A locus resulted in two alleles, i.e. G allele (192 bp, 145 bp, and 101 bp) and A allele (337 bp and 101 bp). The G allele from the highest frequenciest was successively Saanen (0.625), Sapera (0.578), PE (0.400), and SaanPE (0.333). Most dairy goats were heterozygote (Ho>He) and in H-W equilibrium (q2 count < q2P0.05). Whereas CSN2_g.8913C>A locus was monomorphic for possesing only C allele (233 bp and 162 bp), without A allele (416 bp). The existent g.12164G>A SNP of the CSN1S1 gene of could be a potencial molecular selection marker of milk protein content in dairy goat.

Investigating the genetic polymorphism of sheep milk proteins: a useful tool for dairy production

Sheep is the second most important dairy species after cow worldwide, and especially in the Mediterranean and Middle East regions. In some countries, the difficult environmental conditions require a peculiar adaptation and, in these contexts, sheep are able to provide higher quality protein than cattle. In the least-developed countries, the amount of dairy sheep and ovine milk production is progressively increasing. In order to improve dairy productions, in particular those with local connotations, it is necessary to obtain in-depth information regarding milk quality and rheological properties. The genetic polymorphisms of milk proteins are often associated with quantitative and qualitative parameters in milk and are potential candidate markers that should be included in breeding strategies similar to those already available for cattle. Due to the current and growing interest in this topic and considering the large amount of new information, the aim of this study was to review the literature on sheep milk protein polymorphisms with a particular emphasis on recent findings in order to give scientists useful support. Moreover, the effects of different protein variants on milk yield and composition are discussed.

The influence of extracellular matrix and prolactin on global gene expression profiles of primary bovine mammary epithelial cells in vitro

An in vitro bovine mammosphere model was characterized for use in lactational biology studies using a functional genomics approach. Primary bovine mammary epithelial cells cultured on a basement membrane, Matrigel, formed three-dimensional alveoli-like structures or mammospheres. Gene expression profiling during mammosphere formation by high-density microarray analysis indicated that mammospheres underwent similar molecular and cellular processes to developing alveoli in the mammary gland. Gene expression profiles indicated that genes involved in milk protein and fat biosynthesis were expressed, however, lactose biosynthesis may have been compromised. Investigation of factors influencing mammosphere formation revealed that extracellular matrix (ECM) was responsible for the initiation of this process and that prolactin (Prl) was necessary for high levels of milk protein expression. CSN3 (encoding kappa-casein) was the most highly expressed casein gene, followed by CSN1S1 (encoding alphaS1-casein) and CSN2 (encoding beta-casein). Eighteen Prl-responsive genes were identified, including CSN1S1, SOCS2 and CSN2, however, expression of CSN3 was not significantly increased by Prl and CSN1S2 was not expressed at detectable levels in mammospheres. A number of novel Prl responsive genes were identified, including ECM components and genes involved in differentiation and apoptosis. This mammosphere model is a useful model system for functional genomics studies of certain aspects of dairy cattle lactation.

The role of native bovine α-lactalbumin in bovine mammary epithelial cell apoptosis and casein expression

Folding variants of α-lactalbumin (α-la) are known to induce cell death in a number of cell types, including mammary epithelial cells (MEC). The native conformation of α-la however has not been observed to exhibit this biological activity. Here we report that native bovine α-la reduced the viability of primary bovine mammary epithelial cells (BMEC) and induced caspase activity in mammospheres, which are alveolar-like structures formed by culturing primary BMEC on extracellular matrix in the presence of lactogenic hormones. These observations suggest a possible role for bovine α-la in involution and/or maintaining the luminal space in mammary alveoli during lactation. In addition, co-incubation of bovine α-la in an in-vitro mammosphere model resulted in decreased β-casein mRNA expression and increased αs1- and κ-casein mRNA expression. This differential effect on casein expression levels is unusual and raises the possibility of manipulating expression levels of individual caseins to alter dairy processing properties. Manipulation of α-la levels could be further investigated for its potential to enhance milk protein expression and/or improve lactational persistency by influencing the balance between proliferation and apoptosis of BMEC, which has a major influence on the milk-producing capacity of the mammary gland.

Short communication: characterization of a kappa-casein genetic variant in the Chinese yak, Bos grunniens

A PCR-single strand conformation polymorphism protocol has been developed for rapid genotyping of the yak kappa-casein gene. A total of 307 yaks from the Tianzhu White, Jiulong, Maiwa, and Datong breeds in China were genotyped at the kappa-casein locus using the protocol developed in the present study. A polymorphism of kappa-casein gene exon 4 has been identified in Tianzhu White breed by evaluating genomic DNA. The polymorphic site consists of a single nucleotide substitution G-->C at position 362 of the exon 4, resulting in an AA substitution from Arg to Pro at position 121 of the AA sequence and in 2 alleles named, respectively, G and C based on nucleotide 362. The occurrence of allele C in the Tianzhu White breed was high with an allele frequency of 0.15. However, allele C appears to be absent in the yaks from Jiulong, Maiwa, and Datong breeds.

Lactoferrin decreases primary bovine mammary epithelial cell viability and casein expression

The concentration of lactoferrin (LTF) in milk varies during lactation, rising sharply during involution. We proposed that LTF might have a regulatory role in involution and investigated its effects in vitro on the viability of bovine mammary epithelial cells (BMEC) and on casein expression in bovine mammospheres. Mammospheres capable of milk protein expression were formed by culturing primary BMEC on extracellular matrix in the presence of lactogenic hormones. Exogenously added LTF decreased β-casein and κ-casein mRNA expression in mammospheres while transfection of a short interfering RNA (siRNA) to suppress LTF expression resulted in increased casein mRNA expression. We believe that LTF exerts its effect on casein gene expression by up-regulating interleukin-1β (IL-1β) as IL-Iβ gene expression was elevated in mammospheres treated with LTF. LTF also decreased viability of BMEC grown as monolayers and as mammospheres. Interestingly, LTF was only effective in reducing casein mRNA expression and viability in mammospheres when added at concentrations found during early involution but was inactive when used at concentrations found in milk. We suggest that LTF has a regulatory role during early involution, decreasing casein expression and reducing BMEC viability.

Characterization of the CHORI-240 BAC clones containing the bovine CSN1S1, CSN2, STATH, CSN1S2 and CSN3 genes

Nineteen BAC clones were identified by hybridization of the bovine genomic BAC library CHORI-240 with mixed CSN1S1- and CSN3-specific probes. Two of the clones were shown to contain the genes CSN1S1, CSN1S2, CSN2, STATH and CSN3, and five were proved to include the genes CSN2, STATH, CSN1S2 and CSN3. These data showed that the BAC contig was established for the whole casein cluster, including all known five genes.

Casein Haplotype Structure in Five Italian Goat Breeds

The aim of this work was to investigate the genetic structure of the casein gene cluster in 5 Italian goat breeds and to evaluate the haplotype variability within and among populations. A total of 430 goats from Vallesana, Roccaverano, Jonica, Garganica, and Maltese breeds were genotyped at alphas1-casein (CSN1S1), alphas2-casein, (CSN1S2), beta-casein (CSN2), and kappa-casein (CSN3) loci using several genomic techniques and milk protein analysis. Casein haplotype frequencies were estimated for each breed. Principal component analysis was carried out to highlight the relationship among breeds. Allele and haplotype distributions indicated considerable differences among breeds. The haplotype CSN1S1*F- CSN1S2*F-CSN3*D occurred in all breeds with frequencies >0.100 and was the most common haplotype in the Southern breeds. A high frequency of CSN1S1*0-CSN1S2*C-CSN3*A haplotype was found in Vallesana population (0.162). Principal component analysis clearly separated the Northern and Southern breeds by the first component. The variability of the caprine casein loci and variety of resulting haplotypes should be exploited in the future using specific breeding programs aiming to preserve biodiversity and to select goat genetic lines for specific protein production.

Polymorphism of the bovine CSN1S1 promoter: linkage mapping, intragenic haplotypes, and effects on milk production traits

The bovine CSN1S1 5' flanking region (CSN1S1-5') was screened for polymorphisms in different cattle breeds. Single-strand conformation polymorphisms (SSCP) and sequence analyses revealed four alleles (1-4), two of them being new allelic forms (3 and 4). Sequences were deposited in GenBank with accession numbers AF549499-502. In alleles 1 and 4, potential transcription factor binding sites are altered by the mutations. Using SSCP analysis, all four alleles were identified in German Holsteins. Six intragenic haplo-types comprising CSN1S1-5' (alleles 1, 2, 3, 4) and exon 17 (CSN1S1*B and C) genotypes were found. Linkage mapping using half-sib families from the German QTL project positioned CSN1S1 between the markers FBN14 and CSN3, with 5.6 cM distance between CSN1S1 and CSN3. Variance analysis, using family and CSN1S1 promoter genotypes as fixed effects, of breeding values and deregressed proofs for milk production traits (milk, fat, and protein yield and also fat and protein percentage) revealed significant effects on protein percentage when all families and genotypes were considered. Contrast calculations assigned a highly significant effect to genotype 24, which was associated with highest LS-means for protein percentage breeding values. As CSN1S1 is one of the main caseins in milk, this could be an effect of mutations in regulatory elements in the promoter region. An effect on milk yield breeding values was indicated for genotype 12, but is probably caused by a linked locus.

Mineralized tissue and vertebrate evolution: the secretory calcium-binding phosphoprotein gene cluster

Gene duplication creates evolutionary novelties by using older tools in new ways. We have identified evidence that the genes for enamel matrix proteins (EMPs), milk caseins, and salivary proteins comprise a family descended from a common ancestor by tandem gene duplication. These genes remain linked, except for one EMP gene, amelogenin. These genes show common structural features and are expressed in ontogenetically similar tissues. Many of these genes encode secretory Ca-binding phosphoproteins, which regulate the Ca-phosphate concentration of the extracellular environment. By exploiting this fundamental property, these genes have subsequently diversified to serve specialized adaptive functions. Casein makes milk supersaturated with Ca-phosphate, which was critical to the successive mammalian divergence. The innovation of enamel led to mineralized feeding apparatus, which enabled active predation of early vertebrates. The EMP genes comprise a subfamily not identified previously. A set of genes for dentine and bone extracellular matrix proteins constitutes an additional cluster distal to the EMP gene cluster, with similar structural features to EMP genes. The duplication and diversification of the primordial genes for enameldentinebone extracellular matrix may have been important in core vertebrate feeding adaptations, the mineralized skeleton, the evolution of saliva, and, eventually, lactation. The order of duplication events may help delineate early events in mineralized skeletal formation, which is a major characteristic of vertebrates.

Structure and regulation of the murine gamma-casein gene

The murine casein locus consists of five genes, which are coordinately regulated during mammary development. The levels of casein-specific mRNAs in mammary epithelial cells increase during the second half of pregnancy and remain high during lactation. The murine gamma-casein gene, which corresponds to the alphaS2-casein gene in ruminants, was isolated from a mouse bacterial artificial chromosome (BAC) library (strain 129SV). The gene contains 14 exons, which are distributed over 14 kb of DNA sequence. The expression pattern of the murine gamma-casein gene mimics that of the neighbouring beta-casein gene in terms of developmental induction in vivo. In cell culture, both the beta- and gamma-casein promoter are synergistically induced by prolactin and glucocorticoids. Glucocorticoid induction is critically dependent on prolactin-mediated activation of STAT5 in both promoters. Several consensus STAT5 binding sites were identified in the gamma-casein promoter, some of which may have an additive effect on prolactin induction. mRNA levels of gamma- and beta-casein are similar in lactating mammary tissue. However, promoter segments derived from the gamma-casein gene are significantly less active in cell culture than comparable fragments of the beta-casein promoter. Promoter hybrids between the gamma- and beta-casein promoters revealed that the critical sequences which are responsible for the different in vitro activity are located in a short promoter proximal region.

Gene transfer strategies in animal transgenesis

Position effects in animal transgenesis have prevented the reproducible success and limited the initial expectations of this technique in many biotechnological projects. Historically, several strategies have been devised to overcome such position effects, including the progressive addition of regulatory elements belonging to the same or to a heterologous expression domain. An expression domain is thought to contain all regulatory elements that are needed to specifically control the expression of a given gene in time and space. The lack of profound knowledge on the chromatin structure of expression domains of biotechnological interest, such as mammary gland-specific genes, explains why most standard expression vectors have failed to drive high-level, position-independent, and copy-number-dependent expression of transgenes in a reproducible manner. In contrast, the application of artificial chromosome-type constructs to animal transgenesis usually ensures optimal expression levels. YACs, BACs, and PACs have become crucial tools in animal transgenesis, allowing the inclusion of distant key regulatory sequences, previously unknown, that are characteristic for each expression domain. These elements contribute to insulating the artificial chromosome-type constructs from chromosomal position effects and are fundamental in order to guarantee the correct expression of transgenes.

Multispecies comparison of the casein gene loci and evolution of casein gene family

Caseins, the major milk proteins, are present in a genomic cluster spanning 250-350 kb. The divergence at the coding level between human, rodent, and cattle sequences is rather extensive for most of the genes in this region. Nevertheless, comparative analysis of genomic sequences harboring the casein gene cluster region of these species (with equal evolutionary distances 79-88 Myr) shows that the organization and orientation of the genes is highly conserved. The conserved gene structure indicates that the molecular diversity of the casein genes is achieved through variable use of exons in different species and high evolutionary divergence. Comparative analysis also revealed the presence within two species of uncharacterized casein family members and ruled out the previously held notion that another gene family, located in this region, is primate-specific. Several other new genes as well as conserved noncoding sequences with potential regulatory functions were identified. All genes identified in this region are, or are predicted to be, secreted proteins involved in mineral homeostasis, nutrition, and/or host defense, and are mostly expressed in the mammary and/or salivary glands. These observations suggest a possible common ancestry for the genes in this region.

Structure of the rabbit alphas1- and beta-casein gene cluster, assignment to chromosome 15 and expression of the alphas1-casein gene in HC11 cells

Several casein (CSN) genes (CSN1, 2, 10 and alphas2-CSN) have been described and shown to be clustered in mouse, man and cattle. These genes are expressed simultaneously in the mammary gland during lactation, but they are silent in most mammary cell lines, even in the presence of lactogenic hormones. However, it has been shown that the CSN2 gene, and this gene only, can be induced in certain mammary cell lines, such as HC11. In the present paper, we describe three overlapping bacterial artificial chromosome (BAC) clones which harbor both the rabbit CSN1 and CSN2 genes. These two genes are in a convergent orientation, separated by an intergenic region of 15 kb. DNA from one of the CSN/BAC clones was used as a probe for in situ hybridization to show that the CSN1 and CSN2 gene cluster is located on chromosome 15 band q23 and not on chromosome 12 as had been previously reported. Each of the three CSN/BAC DNAs was transfected into HC11 cells. In the presence of lactogenic hormones, the rabbit CSN1 gene was clearly expressed from all three CSN/BAC DNAs, whereas the rabbit CSN2 gene, which at the most possesses a 1 kb upstream region in one of the CSN/BAC DNAs, was not expressed at detectable levels on Northern blots. The transfected HC11 cells now express both rabbit CSN1 and mouse CSN2 genes. These transfected cells will be used as a model to study the role of CSN1 in milk protein secretion.

An allele associated with a non-detectable amount of alpha s2 casein in goat milk

The goat CSN1S2 locus is characterized by the presence of three alleles, A, B and C, all associated with about 2.5 g/l of protein per allele. The SDS-PAGE analysis of 441 individual milk samples obtained from goats belonging to a population reared in Southern Italy showed that the milk produced by three goats did not apparently contain alpha s2-casein, whereas milk produced by 37 goats showed a less intense electrophoretic band of this casein fraction (about 50%). These results can be explained by hypothesizing the presence of another allele at this locus, CSN1S2o, associated with a 'null' content of alpha s2-casein. Southern blot, PCR and PCR-RFLP analyses of the DNA region containing the CSN1S2 gene of individuals producing milk with and without alpha s2-casein did not show differences between the two groups. As a consequence, goats producing milk without alpha s2-casein carry an apparently intact gene. The first results obtained by sequencing part of the CSN1S2o allele revealed a G-->A transition at nucleotide 80 of the 11th exon which creates a stop codon and could be responsible for the absence of the alpha s2-casein in goat milk. This mutation eliminates a NcoI restriction site. A test based on this polymorphism has been established in order to identify carriers of the CSN1S2o allele.

Detection of casein fragments in an invertebrate and in a vertebrate using in situ hybridization

Using a non-radioactive in situ hybridization procedure it has been demonstrated that both invertebrates such as the mollusc Mytilus galloprovincialis and lower vertebrates such as the fish Cyprinus carpio express bovine alphaS1- and kappa-casein homologous mRNAs. In particular, positive results were found in molluscan immunocytes, and in cells located in different fish tissues: intestine, endocrine pancreas and kidney. These findings suggest that the casein genes are highly conserved throughout evolution.

Comparative aspects of milk caseins

The caseins comprise the major protein component of milk of most mammals and are secreted as micelles that also carry high concentrations of calcium. They are phosphoproteins that represent the products of four genes, equivalent to those that encode the bovine alpha s1, alpha s2, beta, and kappa-caseins. There is considerable variation in the relative proportions of the particular caseins across species. The primary sequences of the alpha s1, alpha s2, and beta-caseins also show considerable species variation consistent with rapidly evolving genes that are proposed to have a common precursor. In contrast, the kappa-caseins exhibit features that demonstrate a separate origin and function where they are proposed to stabilise the micelle structure. This review focuses on comparative aspects of the caseins across a number of species for which information is now available.

Biopharmaceutical production in transgenic livestock

The production of recombinant human proteins in the milk of transgenic dairy animals offers a safe, renewable source of commercially important proteins that cannot be produced as efficiently in adequate quantities by other methods. A decade of success in expressing a variety of proteins in livestock has brought three human recombinant proteins to human clinical trials. Recent progress has drawn on molecular biology and reproductive physiology to improve the efficiency of producing and reproducing useful transgenic founder animals, and to improve the expression of heterologous proteins in their milk.

Regulation of milk protein gene expression

Studies using both transgenic mice and transfected mammary epithelial cells have established that composite response elements containing multiple binding sites for several transcription factors mediate the hormonal and developmental regulation of milk protein gene expression. Activation of signal transduction pathways by lactogenic hormones and cell-substratum interactions activate transcription factors and change chromatin structure and milk protein gene expression. The casein promoters have binding sites for signal transducers and activators of transcription 5, Yin Yang 1, CCAAT/enhancer binding protein, and the glucocorticoid receptor. The whey protein gene promoters have binding sites for nuclear factor I, as well as the glucocorticoid receptor and the signal transducers and activators of transcription 5. The functional importance of some of these factors in mammary gland development and milk protein gene expression has been elucidated by studying mice in which some of these factors have been deleted.

A distal enhancer region in the human beta-casein gene mediates the response to prolactin and glucocorticoid hormones

The 5' flanking region of the human beta-casein gene was investigated for the presence of regulatory sequences mediating the action of the lactogenic hormones prolactin and dexamethasone. DNA encompassing 9389 base pairs of the flanking region was isolated and a sequence comparison performed with regulatory regions previously identified in the beta-casein gene of rodents and ruminants. The analysis revealed the presence of a distal region between -4700 and -4550 with a high percentage of identity to the bovine beta-casein enhancer region, and a proximal region between -1 and -200 similar to the proximal promoter regions found in rodents and ruminants. Reporter gene constructs under the control of the distal or the proximal region of the human beta-casein gene were tested for their responsiveness to prolactin and dexamethasone. In transfection experiments, the distal region functioned as a lactogenic hormone inducible enhancer, whereas the proximal region exhibited low activity. In electromobility shift assays, multiple binding sites for Stat5, CCAAT/enhancer-binding proteins, and Ets domain proteins were identified in the distal human enhancer. These transcription factors have already been demonstrated as important regulators of the transcription of milk protein genes in rodents. Thus, a common set of transcription factors appears to be required for the expression of the human beta-casein gene and of milk protein genes in other species.

Polymorphic insertions/deletions of both 1550nt and 100nt in two microsatellite-containing, LINE-related intronic regions of the rabbit kappa-casein gene

The most frequent allele of the rabbit kappa-casein (kappa-Cas)-encoding gene (A allele) has previously been shown to possess two sequences similar to those found in the 5' end of long interspersed repeated elements (LINE). Part of an inverted rabbit LINE is present in the first intron and part of a direct rabbit LINE in the fourth intron. We describe herewith a less frequent allele (B allele) that lacks both 100bp in the first intron and 1550bp in the fourth intron. It was not possible to identify any allele exhibiting only one of the deletions in a population of 55 rabbits. The 100bp present in the first intron of the A allele, but absent from the B allele, are located at the 5' end of the inverse complementary LINE and include the poly (T) track of the LINE. The 1550bp present in the fourth intron of the A allele, but absent from the B allele, include the entire direct LINE sequence. Therefore, the B allele only possesses one partial LINE sequence that is located in the first intron and is truncated when compared to the copy found in the first intron of the A allele. The B allele might thus be more recent than the A allele. Differences between the sequences of transcripts corresponding to each allele are limited to two silent mutations and three modifications in the 3' UTR. In the mammary glands of lactating rabbits, which are homozygous for both alleles, kappa-Cas mRNA accumulate to similar levels and are translated into identical kappa-Cas that are secreted at similar concentrations into milk.

High-level expression of bovine alpha s1-casein in milk of transgenic mice

The bovine alpha s1-casein gene, isolated from a cosmid library, was introduced into the murine germline. Transgene expression occurred in all transgenic mice, and was confined to the lactating mammary gland. Half of the mouse lines (five out of ten) expressed at relatively high expression levels (> 1 mg ml-1). The highest levels of expression were obtained with a transgene containing 14.2 kb of 5' flanking sequence, in two cases expression levels comparable to (10 mg ml-1) or well above (20 mg ml-1) alpha s1-casein levels in bovine milk were obtained. Transcription initiation occurred at the same site in the bovine alpha s1-casein gene in transgenic mouse as in the cow. A marked induction of expression occurred at parturition rather than at mid-pregnancy, and thus resembled the bovine rather than the murine developmental expression pattern. Bovine alpha s1-casein specific immunoblotting and RIA were developed for characterization and quantification of the recombinant protein. Using these assays, the properties of the recombinant protein could not be distinguished from those of the natural bovine protein. In spite of the high-level tissue-specific and correctly regulated developmental expression of the transgene, expression levels were integration-site dependent. This may indicate that not all cis-acting regulatory elements involved in bovine alpha s1-casein expression were included in the transgene.

Structure and expression of the mouse casein gene locus

The analysis of yeast artificial chromosomes (YACs) containing the complete mouse casein gene locus revealed the presence of five casein genes, alpha-, beta-, gamma-, delta-, and kappa-casein, in this order, in the locus. The alpha- and beta-casein genes are only 10 kb apart and have convergent transcriptional orientations. The distance between the beta-casein gene and the alpha s2-like gamma-casein gene is about 70 kb, and these genes have divergent transcriptional orientations. The gamma- and delta-casein genes, both encoding a alpha s2-like casein, are linked within 60 kb and convergently transcribed. The kappa-casein gene is located about 100 kb from the delta-gene. Except for the presence of the delta-casein gene, the organization of the mouse casein locus resembles that of the bovine locus, including the transcriptional orientation of the genes. In contrast to the other casein genes, which are strongly induced at mid-lactation, expression of the delta-casein gene is abruptly induced upon parturition. Comparative analysis of alpha s2-like sequences from various species suggests that the ancestral alpha s2-like gene duplicated around the time of radiation of the rodent and artiodactylid ancestors.