Jane O'Dwyer jane.odwyer at anu.edu.au
Tue Oct 31 00:30:32 CET 2006

News from The Australian National University
Australian scientists have unravelled a mystery of the origins of two
debilitating human genetic diseases by studying the kangaroo and
platypus genome.

The ANU researchers studied the genes that are abnormal in Prader-Willi
Syndrome (PWS), which causes abnormal feeding behaviour leading to an
initial failure to suck, followed by voracious eating; and Angelman
Syndrome (AS), which is marked by severe mental retardation and
inappropriate laughter.

Both of these diseases are caused by an error in 'imprinted' genes.
Imprinted genes are peculiar, because some work only if they come from
the father, and others only if they're from the mother. For nearly all
of these genes, both mother's and father's copies work to make protein,
providing good backup in case one gene is mutated or deleted. 

But for about 70 imprinted genes, only one copy works and if this copy
is mutated or deleted, it causes genetic disease. Prader-Willi Syndrome
results if the gene copy from the father is mutated or deleted, because
the copy from the mother cannot substitute. The opposite is true for
Angelman Syndrome. 

The ANU student team of Rob Rapkins and Tim Hore, from Professor Jenny
Graves' laboratory at the Research School of Biological Sciences, built
on the work La Trobe University graduate Megan Smithwick to investigate
the origin of these two human diseases in the genome of the kangaroos
and the platypus. These two Aussie animals are good models for studying
reproduction genetics because they both have very different reproduction
systems - kangaroos bear very tiny live young without foetal development
and platypus lay eggs. 

"In both the kangaroo and platypus we found the AS gene but not the PWS
gene," Professor Graves said. "The big surprise was that the AS gene
turned out to be next to completely different genes from those that are
near it in the human genome. The PWS gene and other imprinted genes from
the cluster also have no copy in either the kangaroo or platypus." 

Using 'hard molecular slog' the team tracked the origin of the
Prader-Willi Syndrome gene down to a duplicate of a non-imprinted gene
in another part of the genome. With colleagues in Melbourne, Rob and Tim
showed that, unlike in humans and mouse, neither gene was imprinted in
kangaroos or platypus, showing that they evolved in humans more recently
than had been thought. 

"Imprinting was thought to have evolved when mammals abandoned
egg-laying 210 million years ago, but the absence of the imprinted genes
also in kangaroos indicates that imprinting of these genes developed in
placental mammals much more recently. 

"The imprinted domain seems to have been thrown together from bits and
pieces taken from all around the genome and this happened relatively
recently in evolutionary terms," Professor Graves said. "This research
sheds new light on the accidental way that imprinted regions can come
into being, and suggests evolution of imprinting is an ongoing process
in all mammals that bear live young, including humans," Professor Graves

The team's research, funded by a 1999 ARC Discovery grant and the ARC
Centre for Kangaroo Genomics, is published in the latest edition of the
scientific journal PloS Genetics and is available at:

More information:
Professor Jenny Graves
02 6125 2492
Jenny.Graves at anu.edu.au
Amanda Morgan
Media Adviser
The Australian National University
T:   02 6125 5575
F:   02 6125 8255
M: 0416 249 245
W: www.anu.edu.au/media

Marketing & Communications Division
The Australian National University
Canberra ACT 0200

CRICOS Provider # 00120C
ANU Experts Directory: www.anu.edu.au/experts 

Jane O'Dwyer
Media Manager
The Australian National University
T: 02 6125 5001
F: 02 6125 8255
M: 0416 249 231
W: www.anu.edu.au/media

Looking for expert comment? Try www.anu.edu.au/experts 

Marketing and Communications Division
The Australian National University
Canberra ACT 0200

CRICOS Provider #00120C

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