Human CHN1 Mutations Hyperactivate 
2-Chimaerin and Cause Duane's Retraction Syndrome
Noriko Miyake,1,2
John Chilton,3*
Maria Psatha,4*
Long Cheng,1,2
Caroline Andrews,1,2,5
Wai-Man Chan,1
Krystal Law,1
Moira Crosier,6
Susan Lindsay,6
Michelle Cheung,4
James Allen,3
Nick J. Gutowski,7,8
Sian Ellard,8,9
Elizabeth Young,8
Alessandro Iannaccone,10
Binoy Appukuttan,11
J. Timothy Stout,11
Stephen Christiansen,12
Maria Laura Ciccarelli,13
Alfonso Baldi,14
Mara Campioni,14
Juan C. Zenteno,15
Dominic Davenport,4
Laura E. Mariani,5
Mustafa Sahin,2,5
Sarah Guthrie,4
Elizabeth C. Engle1,2,5,16,17
Duane's retraction syndrome (DRS) is a complex congenital eye movement disorder caused by aberrant innervation of the extraocular muscles by axons of brainstem motor neurons. Studying families with a variant form of the disorder (DURS2-DRS), we have identified causative heterozygous missense mutations in CHN1, a gene on chromosome 2q31 that encodes 
2-chimaerin, a Rac guanosine triphosphatase–activating protein (RacGAP) signaling protein previously implicated in the pathfinding of corticospinal axons in mice. We found that these are gain-of-function mutations that increase 2-chimaerin RacGAP activity in vitro. Several of the mutations appeared to enhance 2-chimaerin translocation to the cell membrane or enhance its ability to self-associate. Expression of mutant 2-chimaerin constructs in chick embryos resulted in failure of oculomotor axons to innervate their target extraocular muscles. We conclude that 2-chimaerin has a critical developmental function in ocular motor axon pathfinding.
1 Department of Medicine (Genetics), Children's Hospital Boston, Boston, MA 02115, USA.
2 Harvard Medical School, Boston, MA02115, USA.
3 Institute of Biomedical and Clinical Science, Peninsula Medical School, Research Way, Plymouth PL6 8BU, UK.
4 MRC Centre for Developmental Neurobiology, King's College, Guy's Campus, London SE1 1UL, UK.
5 Department of Neurology, Children's Hospital Boston, Boston, MA 02115, USA.
6 MRC-Wellcome Trust Human Developmental Biology Resource (Newcastle), Institute of Human Genetics, Newcastle University, International Centre for Life, Newcastle upon Tyne NE1 3BZ, UK.
7 Department of Neurology, Royal Devon and Exeter Hospital, Barrack Road, Exeter, Devon EX2 5DW, UK.
8 Peninsula Medical School, Barrack Road, Exeter EX2 5DW, UK.
9 Department of Molecular Genetics, Royal Devon and Exeter Hospital, Barrack Road, Exeter, Devon EX2 5DW, UK.
10 University of Tennessee Health Science Center, Hamilton Eye Institute, 930 Madison Avenue, Suite 731, Memphis, TN 38163, USA.
11 Casey Eye Institute, Oregon Health and Science University, 3375 SW Terwilliger Boulevard, Portland, OR 97239, USA.
12 Department of Ophthalmology, University of Minnesota, MMC 493, 420 Delaware Street SE, Minneapolis, MN 55455, USA.
13 Fatebenefratelli Hospital, Division of Ophthalmology, Isola Tiberina, Rome 00100, Italy.
14 Department of Biochemistry "F. Cedrangolo," Section of Pathologic Anatomy, Second University of Naples, Naples 80138, Italy.
15 Department of Genetics and Research Unit, Institute of Ophthalmology "Conde de Valenciana," Mexico City 06700, Mexico.
16 Department of Ophthalmology, Children's Hospital Boston, Boston, MA 02115, USA.
17 Howard Hughes Medical Institute, Chevy Chase, MD 20815, USA.
* These authors contributed equally to this work.
Deceased.
To whom correspondence should be addressed. E-mail: elizabeth.engle{at}childrens.harvard.edu
