The spectrum of KIAA0196 variants, and characterization of a murine knockout: implications for the mutational mechanism in hereditary spastic paraplegia type SPG8
Orphanet J Rare Dis. 2015-11-16; 10(1):
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1. Orphanet J Rare Dis. 2015 Nov 16;10:147. doi: 10.1186/s13023-015-0359-x.
The spectrum of KIAA0196 variants, and characterization of a murine knockout:
implications for the mutational mechanism in hereditary spastic paraplegia type
Jahic A(1), Khundadze M(2), Jaenisch N(3), Schüle R(4)(5)(6), Klimpe S(7), Klebe
S(8), Frahm C(3), Kassubek J(9), Stevanin G(10), Schöls L(4)(5), Brice A(10),
Hübner CA(2), Beetz C(11).
(1)Department of Clinical Chemistry and Laboratory Medicine, Jena University
Hospital, Jena, Germany.
(2)Institute of Human Genetics, Jena University Hospital, Jena, Germany.
(3)Hans Berger Department of Neurology, Jena University Hospital, Jena, Germany.
(4)Hertie-Institute for Clinical Brain Research, Department of Neurodegenerative
Diseases, University of Tübingen, Tübingen, Germany.
(5)German Research Center for Neurodegenerative Diseases (DZNE), Tübingen,
(6)Dr. John T. Macdonald Foundation Department of Human Genetics and John P.
Hussman Institute for Human Genomics, Miami, FL, USA.
(7)Department of Neurology, University Medical Center of the Johannes-Gutenberg
University Mainz, Mainz, Germany.
(8)Department of Neurology, University Hospital, Freiburg, Germany.
(9)Department of Neurology, University of Ulm, Ulm, Germany.
(10)INSERM U1127, Sorbonne Universités, UPMC Univ Paris 06 UMR_S1127, CNRS
UMR7225, EPHE, Institut du Cerveau et de la Moelle épinière, Paris, France.
(11)Department of Clinical Chemistry and Laboratory Medicine, Jena University
Hospital, Jena, Germany. .
BACKGROUND: The hereditary spastic paraplegias (HSPs) are rare neurodegenerative
gait disorders which are genetically highly heterogeneous. For each single form,
eventual consideration of therapeutic strategies requires an understanding of the
mechanism by which mutations confer pathogenicity. SPG8 is a dominantly inherited
HSP, and associated with rather early onset and rapid progression. A total of
nine mutations in KIAA0196, which encodes the WASH regulatory complex (SHRC)
member strumpellin, have been reported in SPG8 patients so far. Based on
biochemical and cell biological approaches, they have been suggested to act via
loss of function-mediated haploinsufficiency.
METHODS: We generated a deletion-based knockout allele for E430025E21Rik, i.e.
the murine homologue of KIAA0196. The consequences on mRNA and protein levels
were analyzed by qPCR and Western-blotting, respectively. Motor performance was
evaluated by the foot-base angle paradigm. Axon outgrowth and relevant organelle
compartments were investigated in primary neuron cultures and primary fibroblast
cultures, respectively. A homemade multiplex ligation-dependent probe
amplification assay enabling identification of large inactivating KIAA0196
deletion alleles was applied to DNA from 240 HSP index patients.
RESULTS: Homozygous but not heterozygous mice showed early embryonic lethality.
No transcripts from the knockout allele were detected, and the previously
suggested compensation by the wild-type allele upon heterozygosity was disproven.
mRNA expression of genes encoding other SHRC members was unaltered, while there
was evidence for reduced SHRC abundance at protein level. We did, however,
neither observe HSP-related in vivo and ex vivo phenotypes, nor alterations
affecting endosomal, lysosomal, or autophagic compartments. KIAA0196 copy number
screening excluded large inactivating deletion mutations in HSP patients. The
consequences of monoallelic KIAA0196/E430025E21Rik activation thus differ from
those observed for dominant HSP genes for which a loss-of-function mechanism is
CONCLUSIONS: Our data do not support the current view that heterozygous loss of
strumpellin/SHRC function leads to haploinsufficiency and, in turn, to HSP. The
lethality of homozygous knockout mice, i.e. the effect of complete loss of
function, also argues against a dominant negative effect of mutant on wild-type
strumpellin in patients. Toxic gain-of-function represents a potential
alternative explanation. Confirmation of this therapeutically relevant hypothesis
in vivo, however, will require availability of appropriate knockin models.
PMID: 26572744 [Indexed for MEDLINE]