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EURO-HSP GA, Copenhagen, september 12-13 2014
A concise survey about researches on Hereditary Spastic Paraplegia (HSP),
with recent breakthroughs, august 2014
HSP is a neurological disease characterized by progressive weakness and spasticity of the
lower limbs due to degeneration of corticospinal axons.
Facts about HSP-related genes
Seminal discoveries, first of SPG7 gene (Casari G et al. Cell, 1998), then of SPG4 gene (Hazan
et al, Nature Genet. 1999 ) triggered genetic researches on HSP. A huge number of different
SPG genes were subsequently identified; in keeping in mind that a single mutation of one of
these genes enables HSP to develop.
SPG4 encoding Spastin, and SPG11 encoding Spatacsin (Stevanin et al., Nature Genet. 2007,)
stand up as the leader genes in term of mutation frequency for dominant and recessive
autosomal transmission of the disease, respectively (see Diagram below).
To day, from more than 70 chromosomal loci (short pieces of genomic DNA harbouring a
culprit mutated gene) about 50 genes have been identified, for both dominant and
recessive transmission. New Generation Sequencing (NGS) applied on the whole exome (all
sequences transcribed from genomic DNA), as recently proposed, does currently speed up
the rate of discovery of new genes: 24/50 found within 2012-2014!
Hence HSP appears as a paradigm of highly heterogeneous group of monogenic hereditarytransmitted diseases.
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Frequency of mutated SPG genes
Autosomal dominant
Autosomal recessive
Estimation of mutated SPG genes frequency (over 1300 cases of persons eliciting HSP, gathered at the Institut
du Cerveau et de la moelle, ICM). (left, Autosomal Dominant; right, Autosomal Recessive forms of
transmission; “inconnues” in French means unknown))
Courtesy of Pr Giovanni Stevanin, ICM, Paris, France
What’s going wrong at cellular and molecular levels?
The key question is understanding the instrumental role of each SPG encoded protein, that
operates in the corticospinal tract that enables the completion of this major neurological
“motorway” conducting locomotion order from the brain (cortex) to the end of the spine.
Gradually, biochemical and cellular studies are going to define a specific job for each protein.
If we understand what’s going wrong at cellular and molecular levels, new paths defining
therapeutic targets would be reachable
To that aim, loss or gain functions strategies applied on the tested proteins, insects
(drosophila), fishes (zebra fish) or vertebrates (mouse), enable researchers to set up
biological models able to study the HSP diseases. From obtained data, a general scheme had
been drawn out where encoded SPG proteins are gathered together according to 5
biochemical functions, as shown below:
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Scheme resuming HSP proteins and their related instrumental functions
for corticospinal neuron to proceed (from nucleus to axon and dendritic ramifications ).
Note that SPG4-encoded Spastin plays, at least, 3 functions
Courtesy of Pr Giovanni Stevanin, 2011, ICM, Paris, France
Moreover, and importantly, also obtained during the last decade, there is evidence of
physical and functional interactions between the HSP proteins . This leads to the concept a
network that functionally links the HSP proteins together to form what is called an
interactome, more specifically the “HSP-ome”, as shown below. Today researchers assess
the level of interactivity (or “force”) existing between all these proteins, aiming at finding
out the internal cellular pathways involved. Physical interaction between the mutated HSP
protein and modifier proteins -that directly interact with it- within this HSPome might
explain the variability of expression among HSP diseases; indeed, it is well-known that in the
same family eliciting HSP condition, affected persons may exhibit variable expression of the
disease.
Strikingly, resulting from a worldwide collaborative effort, recent study (Novarino G. et al,
Science jan. 2014, attached 1), the HSPome approach has allowed: a) pointing out new
putative HSP candidate genes, b) linking corticospinal motor neuron disease (HSP) to
common neurodegenerative diseases such as Amyotrophic Lateral sclerosis, Parkinson, or
Alzheimer diseases.
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Extracted from the study of Novarino G et al Science 2014 (attached)
This study stresses the evidence that HSP genes are connected in their function and that
rare genetic mutations may converge on a few key biological processes the underlying
pathways of which may lead to potential therapeutic targets.
The Spastin, the encoded SPG4 protein as a model of HSP physio-pathological research
Given its highest frequency of mutation in hereditary paraplegic patients, the Spastin protein
highlights the whole field of HSP research.
Inside the corticospinal motor neuron, microtubules are necessary for many functions, the
axonal transport of the corticospinal neurons, in particular. Microtubules, polymers of
tubulin, are submitted to continuous cycles of breaking and rebuilding according to a
dynamic process. Spastin plays a key role in this process since it severs (breaks) microtubule
structures that then are recycled. In a mouse model harboring a deletion of Spg4, French
researchers showed that animals develop progressive axonal degeneration characterized by
focal axonal swellings associated with impaired axonal transport (Tarrade A, Fassier C et al.
Human Molecular Genetics, 2006, attached). Recently using primary cultures of cortical
neurons from this spastin-mutant mouse model, the same team assessed microtubule
dynamics and axonal transport (Fassier C. , Tarrade A. et al. Disease Model & Mechanisms,
2012, attached 2). Researchers showed an early and marked impairment of microtubule
dynamics all along the axons of spastin-deficient cortical neurons, which is likely to be
responsible for the occurrence of axonal swellings phenotypes and stalling of “cargos”, big
structures of transport that carry organelles on the tubulin “railways”; this creates a kind of
traffic jam that prevent organelles to allow regular axonal transport in the corticospinal
neuron. French researchers revealed that a modulation of microtubule dynamics by
microtubule-targeting drugs rescues the axonal swelling phenotype of cultured cortical
neurons. Their results contribute to a better understanding of the pathogenesis of SPG4linked HSP and ascertain the influence of microtubule-targeted drugs on the early axonal
phenotype in a mouse model of the disease.
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Based on this evidence, an Australian team investigated the potential of tubulin-targeting
drugs onto neurons derived from olfactory mucosa of patients with HSP. Results indicate
that, at low doses, drugs (taxol, vinblastin and epothylone) that are already used in the
cancer treatment rescue the tubulin-related membrane traffic defect related to spastin
mutation. Authors conclude that such treatment might be a treatment option for SPG4mutated patients (Fan Y et al. , Biology Open, 2014; attached 3). However, due to the
potential toxicity of such molecules in cancers treatment, it is necessary to be cautious about
this proposal; other experimental proofs seem to be required before foreseeing clinical trials
in this way.
Jean Bénard
Member & Scientific advisor of the EURO-HSP Board patients
Vice-President of the A.SL-HSP France Association
September 11, 2014