Tag Archives: Clones

Gene cutting.

Second gene-silencing mechanism found, could lead to viable clones and safer in vitro

A new cellular gene-silencing mechanism has been identified and could hold the key to safer in vitro fertilization, even the cloning of animals.

Gene cutting.

Image credits Arek Socha.

We each inherit two working copies of most genes from our parents, one from the maternal and one from the paternal side. But for a tiny minority of genes, or allele, only one copy can be allowed to function while the other remains inactivated from inception until the moment we die. This mechanism is called imprinting, and faulty imprinting can cause a host of genetic syndromes, such as Angelman’s (too much imprinting, so both genes are inactivated) or Beckwith-Wiedemann syndrome (too little imprinting, so both alleles are expressed).

Imprinting is why a lion and a tiger can have two types of offspring. If the female is lion, the couple will sire a tigon, which generally-speaking are smaller than both the parent species. But if the female is a tiger, they will sire a liger — which is much larger in general than any of the initial two. The differences in size and appearance come down, in part, to imprinting differences in maternal- and paternal-inherited genes.Usually, imprinting takes place naturally during inception, through a process called methylation — basically, methyl groups are added to a gene to shut it down.

But in artificial fertilization methods, such as in vitro for humans or straight-up cloning of mammals, imprinting can sometimes be faulty or bypassed altogether. However, a new discovery from the Howard Hughes Medical Institute might hold the key to reversing faulty imprinting. The team, whose correspondent author is Investigator Yi Zhang, found another mechanism cells can use to silence imprinted genes — by attaching specially-modified proteins called histones to the problematic alleles.

These genes are histone-y

The researchers succeeded in shutting down the activity of some imprinted genes in mice by modifying a histone known as H3K27 to carry methyl groups. They also identified 76 genes in mice that likely belong to the imprinted gene group, which is a pretty big number: until now, roughly 150 imprinted genes have been found in mice and roughly half that in humans.

There’s still a lot of work to be done on imprinting, Zhang says, but finding a second mechanism underpinning it just goes to show how important imprinting is from evolution’s point of view. It’s possible that the one the team describes in their paper evolved as a back-up to catch any improperly-imprinted alleles before they can cause any damage.

Imprinting disorders seem to develop more often in children conceived in vitro or through similar methods, the paper notes. It’s still unclear as to why. It could be that imprinting problems are inherently tied to infertility itself, or it may well be that these procedures somehow interfere with imprinting and we just don’t know it yet. But Zhao thinks their findings could give hope to couples who’re having difficulties conceiving and are pursuing assisted reproductive technologies that their child will be healthy.

Furthermore, improper imprinting could be why we’ve had so little success in cloning a healthy animal. Usually, the process requires that imprinting marks be scrapped in the precursor cells and then re-added in the eggs and sperm. Previous research lends weight to the idea that even minor bugs in this erase-rewrite phase can have dramatic effects on the development of clone embryos.

“The new imprinting mechanism may eventually offer a target for treating such developmental failures,” Zhang concludes.

The paper “Maternal H3K27me3 controls DNA methylation-independent imprinting” has been published in the journal Nature.

Dolly sheep clones show no long-term health issues

The four Nottingham Dollies. Credit: The University of Nottingham

The four Nottingham Dollies. Credit: The University of Nottingham

In an age where cloning animals is becoming more common and human genetic modification is right around the corner, scientists are still trying to figure out how safe the cloning process is. Now, three weeks after the 20th anniversary of the birth of Dolly the sheep, a study by researchers from The University of Nottingham suggests that four genomic clones of Dolly reached their 8th birthdays – around 65 in human years – with no health problems.

The four Finn Dorset sheep – also referred to as the Nottingham Dollies – are named Debbie, Denise, Dianna and Daisy. They were born in 2007 during the research of the late Keith Campbell, who was attempting to improve the efficiency of the somatic-cell nuclear transfer (SCNT) technique. Using the mammary gland cell line that led to the birth of Dolly, Campbell successfully cloned the Finn Dorsets.

The new research is the first detailed examination of age-related non-communicable disease in cloned offspring and was led by Kevin Sinclair, a close colleague of Campbell’s from The University of Nottingham.

“Despite technological advances in recent years’ efficiency of SCNT remains low but there are several groups across the world working on this problem at present and there is reason to be optimistic that there will be significant improvements in future,” Sinclair said.

“These improvements will stem from a better understanding of the underlying biology related to the earliest stages of mammalian development,” he added. “In turn this could lead to the realistic prospect of using SCNT to generate stem cells for therapeutic purposes in humans as well as generating transgenic animals that are healthy, fertile and productive. However, if these biotechnologies are going to be used in future we need to continue to test their safety.”

During the course of the study, the four clones underwent a series of comprehensive assessments for non-communicable diseases such as obesity, hypertension, and osteoarthritis. These assessments included X-rays, magnetic resonance imaging (MRI) scans, tests for glucose tolerance and insulin sensitivity, radio-telemetric assessments to determine heart rate and blood pressure and musculoskeletal examinations.

The results showed that the four clones showed no major health issues and no signs of premature aging – one of the early concerns of the cloned offspring. Although more research will need to be conducted until SCNT’s safety is certain, the current results are promising.

“It is well established that prior to conception and in the early stages of pregnancy during natural or assisted reproduction subtle chemical changes can affect the human genome leading to development and late-onset chronic diseases,” Sinclair said. “Given that SCNT requires the use of assisted reproductive procedures it is important to establish if similar diseases or disorders exist in apparently healthy aged cloned offspring.”

Journal Reference: Healthy ageing of cloned sheep. 26 July 2016. 10.1038/ncomms12359

Genetically-altered pigs to become humanity’s source for “spare” organs

Among all the species with which we share the animal kingdom, pigs are the ones whose organs are best suited for transplant in human bodies — they are approximately the same size as our organs and have similar structures, making reconnecting blood vessels much easier. Pigs tend to have large litters and reproduce quickly, making them a very large, very accessible source of “spare parts.”

Never too early to start training that liver.
Image via imgur

So far so good, but why aren’t we all running around with an extra pig spleen or a couple of bonus pig kidneys cleaning our blood? Well, there is a itty bitty hurdle when using pig organs — our bodies freak out when we transplant them. Pig organs are coated with specific sugar molecules that trigger an acute rejection response in human bodies — our antibodies attach themselves to these sugar molecules and destroy the newly transplanted pig organ. Hoping to overcome this problem, researchers are working to create pigs that lack the gene that serves as a template for these sugars.

There are two research efforts being poured into this project currently — Randall Prather at the University of Missouri in Columbia created four cloned piglets from which one had one copy of the sugar-producing gene inactivated (each organisms has two copies of each gene, one from the maternal and one from the paternal side.) The piglets were born in September and October, and a description of Prather’s work was published in the journal Science. The other, a team working for PPL Therapeutics PLC of Scotland, the company that played a part in cloning Dolly the sheep, also announced the birth of a litter of five cloned piglets on December 25th, who’ve also had a copy of the gene inactivated.

The next step involves selectively breeding the pigs, to produce animals lacking both copies of the gene. Theoretically, the organs of these modified pigs could be transplanted into humans without the body rejecting the foreign tissue.

The new results are a significant advance over many other attempts at genetic modification in animals because in both of the studies, the scientists were able to modify—in this case, “knock out”—a gene at a specific location. Although genes from other organisms have been inserted into the genomes of sheep, cattle, and pigs, scientists have had little control over where on a chromosome the new gene is incorporated.

“This is the first time a specific genetic modification has been made in the pig,” said Prather.

Prather’s team, made up of fellows of the University of Missouri and colleagues at the Immerge BioTherapeutics Inc. in Charlestown, Massachusetts, worked directly on fetal pig cells, altering their genetic make-up. These cells were used to grow 3,000 embryo clones that were implanted into 28 surrogate sows, with only seven piglets born, three of which died later.

What started five years ago with the cloning of Dolly, the expectation of creating identical, genetically-controlled organs for transplant into humans, only got one step closer to reality with the cloning of these piglets.

But it’s not all roses — one concern that has dampened the prospects of xenotransplantation is the possibility of spreading viruses from one species to another. Porcine endogenous retrovirus (PERV), for example, is part of a pig’s natural genetic makeup and does not cause any disease in the animal. There is no guarantee, however, that PERV would be harmless in humans.

Still, xenotransplantation might soon become a common practice, as there is an enormous demand for organ transplants that human donors alone will never be able to fill.