16 October 2009

NUS Scientists Pioneer the World's First Semi-Cloned Fish

Landmark findings offer novel and important insights into reproductive medicine; first-ever haploid cell line for easy analysis of disease development and progression

The world has heard of Dolly the cloned sheep. NUS introduces Holly, the world's first semi-cloned fish. Using the medaka fish model, a team of scientists from the National University of Singapore (NUS) successfully demonstrated, for the first time, the feasibility of semi-cloning as a technique for reproduction. Led by Associate Professor Hong Yunhan from the NUS Department of Biological Sciences, the three-member research team created Holly, a fertile female medaka fish, by haploid embryonic stem cell nuclear transfer into a normal egg. This important work was published in the 16 October issue of the highly acclaimed Science magazine.

Application in Reproductive Medicine and Technology

The findings have important implications for reproductive medicine and technology. Human-assisted reproduction by somatic cell nuclear transfer (i.e. current technique for cloning) has been widely debated because of its low efficiency and the technique also raised ethical concerns about producing an offspring that would be identical to the donor.

Using an approach called 'semi-cloning', researchers from NUS successfully created a fertile offspring with a new and unpredictable combination of genetic traits from both parents similar to normal fertilisation. Before this landmark experiment, semi-cloning remained hypothetical because viable offspring has not yet been obtained. At NUS, the research team demonstrated the feasibility of semi-cloning in reproductive medicine or technology for treating infertility.

In conventional cloning technique using somatic nuclear transfer, the nucleus is removed and a diploid nucleus is inserted into the egg, leading to the replacement of a haploid egg nucleus by a diploid somatic nucleus. The reconstituted egg is placed in the uterus to be developed into the cloned organism.

"While traditional cloning uses a 'replacement' strategy, semi-cloning adopts a 'combination' strategy," Associate Professor Hong explained. In semi-cloning, scientists combine two haploid nuclei - one haploid nucleus from the embryonic stem cell mimicking the sperm nucleus, and the nucleus of an unfertilised egg (from the mother) - using nuclear transfer.

"In the practice of nuclear transfer for the production of Holly, we introduced a haploid embryonic stem cell nucleus into a mature egg. In this way, scientists mimic the reproduction process artificially, creating offspring that carry genetic traits from both parents similar to bisexual reproduction," Prof Hong added.

In the case of Holly, the semi-cloned medaka fish went on to produce normal, healthy offspring that carry the same genetic marker as her and her parents, indicating that the technique retains genetic stability and integrity, hence providing a powerful tool for transmission of genetic information to the offspring, similar to normal reproduction.

The successful demonstration of the feasibility of semi-cloning spells hope for treatment of infertility, for instance to treat men without mature sperm.

Application for Genetic Analysis of Diseases

Scientists have since long attempted to establish haploid stem cells (i.e. cells with only one single set of chromosomes). Using the medaka fish model system, the NUS research team also generated haploid embryonic stem cells. These cells could be used to study the effect of recessive mutations of essential genes that may not be apparent in normal diploid cells (i.e. cells with two sets of chromosomes). In particular, recessive mutations of genes essential for tumour suppression, cell proliferation or pluripotency (i.e. capability of producing any cells such as neurons and muscles) can display clear phenotypes or diseases for direct analyses in Petri dishes.

"Our success provides important insights into the possibility of developing mammalian, such as mouse and even human haploid embryonic stem cell cultures for disease analysis," said Prof Hong.

This study is supported by NUS, the Singapore Agency for Science, Technology & Research (A*STAR) and the Singapore Ministry of Education. Going forward, the NUS team is studying these haploid ES cells for high-throughput precise genome engineering such as gene mutations or corrections, whereby disease phenotypes of essential genes are directly analysed in a yeast-like system. The team is also interested in engaging in local and international collaborations to develop similar haploid ES cells from mammals such as mice and humans.