It was 20 years ago today that Dolly the sheep was born, the ultimate product of a series of cutting-edge cloning experiments that would soon rock the research world. Animals had been created from embryonic cells before, but Dolly was the first mammal successfully cloned from an adult cell. Her birth, reported the following year in Nature, made Dolly and the scientists who created her instant celebrities. And the wider public realised that the dystopian science fiction vision of factories churning out hundreds of clones as a way to preserve loved ones, or to provide medical spare parts, or simply as military cannon fodder was now technically feasible. Governments immediately set about passing legislations to try and prevent this.
Dolly was the product of research by PPL Therapeutics and the Roslin Institute in Edinburgh into the development of transgenic animals to manufacture therapeutic proteins, specifically by secreting them in milk. Professor Keith Campbell, to whom British embryologist and team leader Professor Sir Ian Wilmut assigns 66% of the credit for Dolly, had concentrated on developing the technology using sheep “because sheep in Scotland are very, very, very cheap.” PPL went on to develop a recombinant form of the plasma protein alpha-1-antitrypsin (AAT) to treat cystic fibrosis and Fibrin I wound sealant before shutting down in 2004.
Dolly’s celebrity was the result of the technique used to create her somatic cell nuclear transfer. In Dolly’s case, a cell from the mammary gland of a 6-year-old Finn Dorset sheep was fused with an unfertilised egg cell taken from a Scottish Blackface sheep which had had its nucleus removed. The fertilised egg was then allowed to develop to blastocyst, in vitro, before being implanted into the uterus of a third and then fourth sheep. The choice of udder cells as the source of DNA led to her being named after Dolly Parton, who was reported to have been flattered and supposedly retorted this widely-publicized comment: “There’s no such thing as baaaaaad publicity”
The real significance of this work was not reproduction without sex, but that differentiated somatic cells from an adult were shown to be usable as progenitors for all cells in the body through epigenetic reprogramming, and thus capable of creating an entire organism, a genetically identical copy of the parent/donor of the nucleus. It is worth highlighting, however, that the offspring, while [chromosomally] genetically identical, still had mitochondrial DNA derived from the egg donor. In fact, somatic cell nuclear transfer has been identified as a potential route by which sufferers and carriers of mitochondria-based diseases could avoid transmitting them to their children by fusion with a donor egg from an unaffected female.
At the time that Dolly was born, somatic cell nuclear transfer was labour-intensive and inefficient. To give you an idea how hard this was, Dolly (initially identified as 6LL3) was the only lamb born alive from 277 attempts! It was reported that 29 embryos were successfully created, and subsequently implanted into 13 surrogate mothers, but Dolly was the only pregnancy that went to full term. In the 20 years since her birth, improvements to this technique have made it more reliable and affordable and accessible—so now, as well as precious livestock, well- heeled pet owners with a spare US$50,000 to $100,000 can have their favourite pet cloned! Ah, puppy love.
In the aftermath of the media hype surrounding Dolly’s arrival, US President Clinton, the Vatican and the European Commission all initiated expert reviews of the implications, and rumours of human clones have periodically circulated the Internet since, alongside references to Frankenstein, scientists playing God, government conspiracies, and Nazi doctors.
Dolly went on to have six lambs—Bonnie in 1998, twins Sally and Rosie in 1999, and Lucy, Darcy and Cotton in 2000. In late 2001, at the age of 5, Dolly was diagnosed with arthritis in her left knee, which led to much speculation that cloned animals would be susceptible to premature ageing, although it has been noted that Dolly’s pampered existence—highly unusual for a sheep—is also very likely to have contributed! The underlying cause of Dolly’s arthritis was never determined, but anti-inflammatory treatment resolved clinical signs within months.
In 2003, investigation of a cough led to diagnosis of ovine pulmonary adenomatosis (OPA), which had already affected a quarter of the 16 animals housed in the same barn. OPA is a progressive disease in sheep caused by the Jaagsiekte sheep retrovirus (JSRV). Jaagsiekte means driving or chasing sickness in Afrikaans and refers to respiratory distress in affected animals, as if they were breathless from being chased. Viral infection of bronchiolar-alveolar epithelial cells transforms those cells, leading to cell proliferation, tumour development, and fluid production. The disease is more common in animals housed indoors where aerosol and close contact transmission of the virus are facilitated. A CT scan in 2003 revealed the full extent of the disease’s progression and tumour formation in Dolly’s lungs, and, as she was already anaesthetised to perform the scan, the decision was made, out of kindness, to euthanase her the same day.
Dolly’s death at the age of only 6½ again fueled speculation that she had been old before her time, although her confinement indoors to protect her from “lunatics who would want to harm her, criminals who wanted to kidnap her, and local students who might pull pranks,” as Wilmut noted in his book ‘After Dolly: The Uses and Misuses of Human Cloning,’ was certainly a contributing factor for the disease that led to her death.
In 1999, the team published analysis of Dolly’s telomeres (repetitive DNA sequences at the end of chromosomes whose unique folding pattern affords stability and protection). Telomeres are iteratively shortened with each cell division and are consequently considered a useful indicator of cellular ageing. The paper reported that Dolly possessed shortened telomeres, and that this decrease was consistent with the age of the donor tissue as well as telomere erosion during in vitro culture. In other words, Dolly may have had a genetic age of greater than 6 years at birth. The paper also reported that the nuclear transfer process does not restore telomere lengths, although other literature suggests that telomerase activity may effect repairs. Dolly’s lambs were reported to have normal telomeres and it has subsequently been shown that the offspring of cloned animals have normal telomeres.
From the day Dolly was announced to the world, her existence has been controversial and complicated, and informing debate over the medical and ethical implications of humanity’s technical ability to produce a clone from adult cells is perhaps Dolly’s most lasting legacy. Recent publication of the production of viable bacteria from synthetic DNA, and of the synthesis of entire yeast chromosomes, can only reinforce the belief that the technology will soon exist (if it does not already) to manufacture humans in test tubes.
And now that transplant surgery has become as routine as such a complicated procedure can be, the limited supply of suitable donor organs is the often heart-breaking limiting factor for those waiting for a replacement organ, and so research into alternatives including xenotransplantation and growing humanised organs in vitro or in animals (usually pigs) continues.
Not baaad Dolly.
How to cite:
Welch, Jonathan. The Dolly Experiment. Eureka blog. July 5, 2016. Available: http://eureka.criver.com/the-dolly-experiment/