HYPOTHESIS
Hypothesis
Are neuronal intranuclear inclusions the common
neuropathology of triplet-repeat disorders with
polyglutamine-repeat expansions?
Stephen W Davies, Kathryn Beardsall, Mark Turmaine, Marian DiFiglia, Neil Aronin, Gillian P Bates
Neuronal intranuclear inclusions have been found in the brain of a transgenic mouse model of Huntington’s disease
and in necropsy brain tissue of patients with Huntington’s disease. We suggest that neuronal intranuclear inclusions
are the common neuropathology for all inherited diseases caused by expansion of polyglutamine repeats. We also
suggest that patients with a pathological diagnosis of neuronal intranuclear hyaline inclusion disease may also have
polyglutamine repeat expansions.
The onset of neurological signs in a transgenic animal
model of Huntington’s disease is preceded by the
appearance of huntingtin protein in neuronal nuclei.
The protein is identified by electron microscopy as a
characteristic neuronal intracellular inclusion (NII). We
have found these structures in neuronal nuclei in the
cerebral cortex and caudate or putamen of necropsy
brain tissue from patients with Huntington’s disease,
and believe that disruption of the neuronal nucleus by
NIIs may be the pathogenic mechanism in seven other
neurodegenerative diseases caused by expansion of
polyglutamine repeats.
Previous reports have identified patients with a wide
range of neurological symptoms accompanied by a
neuropathological diagnosis of neuronal intranuclear
hyaline inclusion disease, established by the presence of
NIIs, identical in ultrastructural appearance to those we
have seen in Huntington’s disease. We suggest that
many of these patients may have inherited disease due
to polyglutamine repeat expansion.
subunit of a Purkinje-cell-specific calcium channel), the
proteins are of ubiquitous expression and of unknown
function. They have been named huntingtin
(dentatorubropallido-Luysian
atrophy),
atrophin
(Huntington’s disease), and ataxins 1–7 (spinocerebellar
ataxia types 1–7). Each disease is characterised by a
unique pattern or neuron death within the central
nervous system. However, the juvenile forms of these
diseases, characterised by inheritance of large
polyglutamine expansions, are invariably accompanied
by a more extensive neuropathology than the adultonset forms.14 For example, juvenile-onset Huntington’s
disease is frequently accompanied by loss of cerebellar
Inherited neurodegenerative disorders
caused by polyglutamine-tract expansion
Molecular genetic studies have identified eight
neurodegenerative diseases caused by inheritance of the
expanded trinucleotide repeat, CAG: Huntington’s
dentatorubropallido-Luysian
atrophy;2,3
disease;1
spinobulbar muscular atrophy or Kennedy’s disease;4,5
spinocerebellar ataxia type 1,6,7 type 2,8,9,10 type 3 (or
Machado Joseph disease),11 type 6,12 and now type 7.13
In each case the expansion of a CAG-repeat sequence in
DNA results in the translation of an extended glutamine
sequence within the corresponding protein. With the
exception of spinobulbar muscular atrophy (androgen
receptor) and spinocerebellar ataxia type 6 (alpha
Lancet 1998; 351: 131–33
Department of Anatomy and Developmental Biology, University
College London, London, UK (S W Davies PhD, K Beardsall MRCP,
M Turmaine); Department of Neurology, Massachusetts General
Hospital, Harvard Medical School, Boston, MA, USA
(M DiFiglia PhD, N Aronin MD); Division of Medical and Molecular
Genetics, UMDS, Guy’s Hospital, London, UK (G P Bates PhD)
Correspondence to: Dr S W Davies, Department of Anatomy and
Developmental Biology, University College London, Rockefeller
Building, London WC1E 6JJ, UK
THE LANCET • Vol 351 • January 10, 1998
Ultrastructural appearance of an intranuclear inclusion (NII) in a
striatal neuron in a mouse transgenic for the Huntington’s
disease mutation
The NII (large arrow) is accompanied by prominent invaginations in the
nuclear envelope and an apparent increase in the density of nuclear pores
(small arrows).
131
HYPOTHESIS
Purkinje neurons,15 a pattern of cell loss seldom seen in
the adult-onset disease. The striking differences
between the neuropathological changes seen between
polyglutamine-repeat diseases may become less distinct
in the juvenile forms.16
Neuronal intranuclear inclusions in
transgenic mice
Transgenic mice expressing exon 1 of the human
Huntington’s-disease gene containing an expanded
CAG repeat derived from a patient with juvenile-onset
Huntington’s disease17 develop many of the clinical
features characteristic of this disease.18,19 Within the
brains of these transgenic mice, before the onset of
neurological signs, the transgene protein containing an
expanded polyglutamine repeat appears in the nucleus20
and forms a characteristic intranuclear inclusion
(figure). The presence of this inclusion leads to
morphological changes within the nucleus. The nuclear
membrane
develops
prominent
invaginations
accompanied by an apparent increase in the density of
nuclear pores.20 The NII is additionally strongly
immunoreactive with antibodies to ubiquitin.20
These observations in transgenic mice prompted us to
analyse the distribution of huntingtin protein within the
brains of patients with Huntington’s disease. Antibodies
that recognise the N-terminal of huntingtin
immunolabel NIIs, whereas antibodies to C-terminal
portions of the protein do not.21 Antibodies to ubiquitin
also recognise NIIs. Similar structures have occasionally
been seen in necropsy or biopsy samples of cerebral
cortex or caudate nucleus from patients with
Huntington’s disease, although their relevance to the
mechanism of the disease was not appreciated.22
Intranuclear inclusions containing a protein with an
expanded polyglutamine repeat have been seen in the
brains of patients with spinocerebellar ataxia type 3,
dentatorubropallido-Luysian atrophy (M Becher and
C A Ross personal communication), as well as in
Purkinje cells in the cerebellum of patients with
spinocerebellar ataxia type 1 and mice transgenic for
this mutation,25,26 which suggests that neuronal
dysfunction is due to the mutant protein within the
nucleus. Schrezinger and colleagues27 showed that the
expansion of a polyglutamine sequence within a
truncated huntingtin protein leads to in-vitro and invivo formation of fibrils with a b-pleated sheet (amyloidlike) structures that are insoluble. The insolubility of the
mutated protein seems to be enhanced by cleavage of an
n-terminal fragment containing the polyglutamine
sequence. Antibodies that recognise this n-terminal
fragment immunolabel NIIs in the brains of patients
with Huntington’s disease, whereas antibodies directed
to more central sequences in the protein do not.21 It
remains to be established if the mutant proteins
involved in the other triplet-repeat expansion diseases
similarly form amyloid-like insoluble nuclear fibrils.
notably ataxia,28–33 choreiform movements,34–37 tremor,
rigidity, and dyskinesia—similar to those seen in
juvenile-onset or adult-onset spinocerebellar ataxias,
dentatorubropallido-Luysian atrophy, and Huntington’s
disease. The NIIs in the brains of these cases are
identical in ultrastructural appearance to those seen in
transgenic mice with Huntington’s disease and in cases
of Huntington’s disease and spinocerebellar ataxia type
3. Many of these reports describe cases of neuronal
intranuclear hyaline inclusion disease with juvenile
onset, although several are of mid-life onset and some
have a clear familiar incidence. The molecular basis for
the neuropathological changes reported is not
understood. We suggest that these patients may have
inherited a disease due to expansion of a polyglutamine
repeat. This mutation may be within one of the already
identified genes with CAG-repeat expansion or may be
a new mutation. Molecular methods can be readily used
to investigate these possibilities.8,9,38,39
Hypothesis
Several proteins (huntingtin, atrophin, androgen
receptor, and ataxins), as a result of an increase in size
of their intrinsic polyglutamine sequences, are
transported into the nucleus of neurons where they
accumulate as insoluble amyloid-like fibrils that form
NIIs. These NIIs may be the neuropathological basis for
all inherited polyglutamine-repeat expansion disorders.
The
neuropathological
diagnosis
of
neuronal
intranuclear hyaline inclusion disease in many patients
with progressive neurodegenerative disease may also be
related to the inheritance of an expanded polyglutamine
repeat.
Testing the hypothesis
The hypothesis can be tested by investigation of
the
molecular,
genetic,
and
neuropathological
characteristics of transgenic mouse models, patients
with polyglutamine-repeat expansion disease, and with
neuronal intranuclear hyaline inclusion disease.
We thank A B Young, A R Lieberman, and M Goedert for their helpful
comments.
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