Anne-Sophie Lebre

PML nuclear bodies and neuronal intranuclear inclusion in polyglutamine diseases

In polyglutamine diseases, accumulation in the nucleus of mutant proteins induces the formation of neuronal intranuclear inclusions (NIIs). The nucleus is compartmentalized into structural and functional domains, which are involved in NII formation. Promyelocytic leukemia protein (PML), a major component of nuclear bodies, and mSin3A, a component of the transcription co-repressor complex, were used to investigate how the intranuclear domains/sites relate to NII formation in SCA2, SCA3, SCA7, SCA17 and DRPLA brains. We demonstrate that the size of PML-positive intranuclear structures was larger in pathological brains than in control ones and that these structures contained mutant proteins. PML colocalized only with small NIIs, which maintained the ring-like structure of normal nuclear bodies. Enlarged ring-like PML-positive structures, devoid of mutant proteins, were also found and might represent structures where mutant polyglutamine proteins have been successfully processed. These data suggest that NIIs originate from nuclear bodies, where mutant proteins accumulate for degradation.

Amyotrophic lateral sclerosis with neuronal intranuclear protein inclusions.

A 46-year-old patient developed amyotrophic lateral sclerosis (ALS) characterized by rapid progression. She needed respiratory assistance after a course of 9xa0months. She died 4.5xa0years after onset. Autopsy showed dramatic atrophy of the spinal cord, sparing only the posterior tracts, associated with neuronal loss and astrogliosis in various areas including the anterior horns, motor cortex, striatum, thalamus, and substantia nigra. Ubiquitin immunohistochemistry showed rare skein-like inclusions in the surviving spinal and medullary motor neurons. Eosinophilic inclusions were found in the nuclei of pyramidal neurons in the hippocampus. These inclusions were immunoreactive to antibodies against ubiquitin, promyelocytic leukemia gene product, proteasome, and ataxin-3. They were not immunoreactive to antibodies against tau, cystatin C, neurofilament, alpha-synuclein, SOD-1, and polyglutamine (1C2), and were not stained by ethidium bromide. Similar inclusions were found in the motor cortex. The immunoreactivity of the inclusions was similar to that encountered in diseases associated with CAG repeats, except for the negativity of the immunolabelling with 1C2. At the ultrastructural level, the nuclear inclusions were made of straight filaments (10–12xa0nm in diameter) arranged at random, reminiscent of the polyglutamine intranuclear hyaline inclusions.

Maladies par expansion de polyglutamine: Données moléculaires et physiopathologiques

Les progres recents realises dans les domaines de la genetique et de la biologie moleculaire ont profondement modifie la connaissance des maladies neurologiques hereditaires. Notamment, une nouvelle classe de mutations a ete impliquee dans une quinzaine daffections : il sagit des mutations dynamiques ou mutations dites instables due a lexpansion de trinucletodes repetes [7]. Ces trinucleotides repetes sont polymorphes dans la population generale, et il existe un seuil de repetitions, variable en fonction du locus considere, audela duquel le phenotype se manifeste. Les mutations sont qualifiees dinstables car le nombre de repetitions porte par lallele pathologique varie souvent au cours de sa transmission a la descendance. Deux classes de mutations dynamiques sont decrites ( tableau 1 ). La premiere classe regroupe les expansions generalement de grande taille situees dans des regions non codantes de genes qui comportent des motifs (CGG)n, (CTG)n, (GAA)n ou (CAG)n. Ce type dexpansions est observe dans le syndrome de lX fragile (FRAXA) et dans le retard mental lie a lX (FRAXE), la dystrophie myotonique de Steinert, les ataxies cerebelleuses SCA8 et 12 (SCA, spinocerebellar ataxia ) et lataxie de Friedreich. La deuxieme classe regroupe les expansions de taille moderee de trinucleotides CAG ou GCG dans des regions codantes de genes. Ce groupe daffections comporte la maladie de Huntington, latrophie dentatorubro-pallido-luysienne, la maladie de Kennedy, cinq formes dataxies cerebelleuses autosomiques dominantes et la dystrophie musculaire oculo-pharyngee. Nous nous interesserons ici aux maladies qui impliquent des expansions instables de trinucletodies CAG dont la consequence est une expansion de polyglutamine dans la proteine correspondante ( tableau 2 ). Il sagit de maladies neurodegeneratives pour lesquelles aucun traitement autre que symptomatique nest disponible et dont levolution est, pour la plupart, irreversible et fatale en 10 a 20 ans. Il sagit de maladies de transmission autosomique dominante (sauf la maladie de Kennedy dont la transmission est recessive liee a lX), qui partagent des caracteristiques cliniques communes parmi lesquelles le phenomene danticipation, cest-a-dire un âge de debut plus precoce et/ou une severite accrue de la maladie au cours des generations successives. Lanticipation resulte du caractere instable de ces mutations. Il a ete postule que lexpansion de polyglutamine dans ces proteines entraine un gain de fonction implique dans la mort neuronale, cela par un mechanisme physiopathologique probablement commun a toutes ces maladies.

Chapter 9 – Spinocerebellar Ataxia 7 (SCA7)

Publisher Summary nSpinocerebellar ataxia 7 (SCA7) is a progressive autosomal dominant neurodegenerative disorder characterized clinically by cerebellar ataxia associated with progressive macular dystrophy. The disease affects primarily the cerebellum and the retina, and many other central nervous system (CNS) structures as the disease progresses. SCA7 is caused by expansion of an unstable trinucleotide CAG repeat encoding a polyglutamine tract in the corresponding protein, ataxin-7. Normal SCA7 alleles contain 4–35 CAG repeats, whereas pathological alleles contain from 36–306 CAG repeats. Ataxin-7 is a protein of unknown function expressed in many tissues including the CNS, but its mutation leads to selective neuronal death only in the brain. Cellular models and SCA7 transgenic mice are generated which constitute valuable resources for studying the disease mechanism. They are also useful for screening and evaluating possible therapeutic strategies.