Engineering protein processing of the mammary gland to produce abundant hemophilia B therapy in milk

Evaluation of the α-casein (CSN1S1) locus as a potential target for a site-specific transgene integration

Transgenic animals are an important tool in biotechnology, including the production of recombinant proteins in the milk. Traditionally, expression constructs are based on hybrid vectors bearing mammary gland specific regulatory elements from the α-casein (Csn1s1), β-casein (Csn2), whey acidic protein (WAP), or β-lactoglobulin (BLG) genes. Overexpression from the randomly integrated vectors typically provides high levels of expression, but has drawbacks due to unpredictable genome localization. CRISPR-Cas9 targeted transgene integration into the endogenous casein locus could alleviate the need for extensive animal screening to achieve high and reproducible expression levels. We decided to evaluate such a “precise” integration approach, placing the human granulocyte–macrophage colony-stimulating factor (hGMCSF) gene under control of the mouse endogenous alpha-S1-casein (Csn1s1) promoter. We designed two types of transgene integrations: a knock-in in the second exon of the Csn1s1 (INS-GM) and a full-size Csn1s1 replacement with hGMCSF (REP-GM) which was never tested before. The INS-GM approach demonstrated low transgene expression and milk protein levels (0.4% of Csn2 transcripts; 2–11 µg/ml hGMCSF). This was probably caused by the absence of the 3’-polyadenylation signal in the hGMCSF transgene. REP-GM animals displayed high transgene expression, reaching and slightly exceeding the level of the endogenous Csn1s1 (30–40% of Csn2 transcripts), but yielded less hGMCSF protein than expected (0.2–0.5 mg/ml vs 25 mg/ml of Csn1s1), indicating that translation of the protein is not optimal. Homozygous inserts leading to the Csn1s1 knock-out did not have any long standing effects on the animals’ health. Thus, in our experimental design, site-specific transgene integration into the casein locus did not provide any significant advantage over the overexpression approach.

Use of Genome Editing Techniques to Produce Transgenic Farm Animals

Recombinant proteins are essential for the treatment and diagnosis of clinical human ailments. The availability and biological activity of recombinant proteins is heavily influenced by production platforms. Conventional production platforms such as yeast, bacteria, and mammalian cells have biological and economical challenges. Transgenic livestock species have been explored as an alternative production platform for recombinant proteins, predominantly through milk secretion; the strategy has been demonstrated to produce large quantities of biologically active proteins. The major limitation of utilizing livestock species as bioreactors has been efforts required to alter the genome of livestock. Advancements in the genome editing field have drastically improved the ability to genetically engineer livestock species. Specifically, genome editing tools such as the CRISPR/Cas9 system have lowered efforts required to generate genetically engineered livestock, thus minimizing restrictions on the type of genetic modification in livestock. In this review, we discuss characteristics of transgenic animal bioreactors and how the use of genome editing systems enhances design and availability of the animal models.

Application of Genetically Engineered Pigs in Biomedical Research

Progress in genetic engineering over the past few decades has made it possible to develop methods that have led to the production of transgenic animals. The development of transgenesis has created new directions in research and possibilities for its practical application. Generating transgenic animal species is not only aimed towards accelerating traditional breeding programs and improving animal health and the quality of animal products for consumption but can also be used in biomedicine. Animal studies are conducted to develop models used in gene function and regulation research and the genetic determinants of certain human diseases. Another direction of research, described in this review, focuses on the use of transgenic animals as a source of high-quality biopharmaceuticals, such as recombinant proteins. The further aspect discussed is the use of genetically modified animals as a source of cells, tissues, and organs for transplantation into human recipients, i.e., xenotransplantation. Numerous studies have shown that the pig (Sus scrofa domestica) is the most suitable species both as a research model for human diseases and as an optimal organ donor for xenotransplantation. Short pregnancy, short generation interval, and high litter size make the production of transgenic pigs less time-consuming in comparison with other livestock species This review describes genetically modified pigs used for biomedical research and the future challenges and perspectives for the use of the swine animal models.

Production of biologically active human factor IX-Fc fusion protein in the milk of transgenic mice

OBJECTIVE

To investigate the feasibility of producing human IgG1 Fc fragment fused factor IX (FIX-Fc) in the milk of transgenic animals, for an alternative possible solution to the unmet need of FIX-Fc products for hemophilia B treatment.

RESULTS

Six founder lines of transgenic mice harboring FIX-Fc cassette designed to be expressed specifically in the mammary gland were generated. FIX-Fc protein was secreted into the milk of transgenic mice with preserved biological activity (with the highest value of 6.2 IU/mL), similar to that of the non-fused FIX transgenic milk. RT-PCR and immunofluorescence analysis confirmed that FIX-Fc was specifically expressed in the mammary gland. The blood FIX clotting activities were unchanged, and no apparent health defects were observed in the transgenic mice. Moreover, the stability of FIX protein in milk was increased by the Fc fusion.

CONCLUSIONS

It is feasible to produce biologically functional FIX-Fc in the mammary gland of transgenic mice. Our preliminary results provide a foundation for the potential scale-up production of FIX-Fc in the milk of dairy animals.

An exonic splicing enhancer mutation in DUOX2 causes aberrant alternative splicing and severe congenital hypothyroidism in Bama pigs

Pigs share many similarities with humans in terms of anatomy, physiology and genetics, and have long been recognized as important experimental animals in biomedical research. Using an N-ethyl-N-nitrosourea (ENU) mutagenesis screen, we previously identified a large number of pig mutants, which could be further established as human disease models. However, the identification of causative mutations in large animals with great heterogeneity remains a challenging endeavor. Here, we select one pig mutant, showing congenital nude skin and thyroid deficiency in a recessive inheritance pattern. We were able to efficiently map the causative mutation using family-based genome-wide association studies combined with whole-exome sequencing and a small sample size. A loss-of-function variant (c.1226 A>G) that resulted in a highly conserved amino acid substitution (D409G) was identified in the DUOX2 gene. This mutation, located within an exonic splicing enhancer motif, caused aberrant splicing of DUOX2 transcripts and resulted in lower H₂O₂ production, which might cause a severe defect in thyroid hormone production. Our findings suggest that exome sequencing is an efficient way to map causative mutations and that DUOX2^D409G/D409G mutant pigs could be a potential large animal model for human congenital hypothyroidism.

Summary: Here, we show that an exonic splicing enhancer variant in DUOX2 (c.1226 A>G) causes aberrant splicing of DUOX2 transcripts, resulting in lower H₂O₂ production, to cause severe congenital hypothyroidism in Bama pigs.

A galactopoiesis accordant yield of functional recombinant human factor IX from homozygous transgenic pigs requires a large amount of vitamin K supplementation

Transgenic pigs failed to accord milk yield curve to lactate rhFIX-a vitamin K (VK) dependent protein even fed with VK enriched to 8 times higher than nutritional requirement. A further higher VK supplementation may be required. Homozygous transgenic sows (n = 4, 200 kg) at their 3rd nursing were divided into control and treatment groups and respectively received VK enriched and further menadione (soluble VK) supplemented diet (220 mg/kg VK enriched diet) for 33 days. At next lactation, control sows than received treatment and previous treated were fed on control diet. Results revealed that menadione treatment increased milk bioactivity of rhFIX from the 7th day of 73 to the 21st day of 153 IU/mL; it gradually decreased to 96 IU/mL on 35th day of lactation. Under control feeding, bioactivity remained relatively unchanged. However, milk rhFIX concentration and ratio of activated rhFIX responded little to the treatment. The menadione-induced bioactivity curve agrees with the known lactation pattern of sow means rhFIX secretion is still galactopoietic but requires high VK intake to show. The ineffectual VK spend on lactational carboxylation might be common in other mammary VK dependent expression system but can be effectively overcome by a high supplementation of menadione with a 5-folds improvement in quality.

One-step Multiplex Transgenesis via Sleeping Beauty Transposition in Cattle

Genetically modified cattle are important for developing new biomedical models and for an improved understanding of the pathophysiology of zoonotic diseases. However, genome editing and genetic engineering based on somatic cell nuclear transfer suffer from a low overall efficiency. Here, we established a highly efficient one-step multiplex gene transfer system into the bovine genome.

Genetically engineering milk

It has been thirty years since the first genetically engineered animal with altered milk composition was reported. During the intervening years, the world population has increased from 5bn to 7bn people. An increasing demand for protein in the human diet has followed this population expansion, putting huge stress on the food supply chain. Many solutions to the grand challenge of food security for all have been proposed and are currently under investigation and study. Amongst these, genetics still has an important role to play, aiming to continually enable the selection of livestock with enhanced traits. Part of the geneticist's tool box is the technology of genetic engineering. In this Invited Review, we indicate that this technology has come a long way, we focus on the genetic engineering of dairy animals and we argue that the new strategies for precision breeding demand proper evaluation as to how they could contribute to the essential increases in agricultural productivity our society must achieve.