The threshold of CAG repeat expansion in the HTT gene that causes HD is 36 CAG repeats, although 'superlong' expansions are found in individual neurons in postmortem brains. Previously, we showed that, compared to mice with <250 CAG repeats, onset of disease in R6/2 mice carrying superlong (>440) CAG repeat expansions was delayed, and disease progression was slower. Inclusion pathology also differed from 250 CAG repeat mice, being dominated by a novel kind of extranuclear neuronal inclusion (nENNI) that resembles a class of aggregate seen in patients with the adult onset form of HD. Here, we characterised neuropathology in R6/2 mice with >400 CAG repeats using light and electron microscopy. nENNIs were found with increased frequency and wider distribution with age. Some nENNIs appear to 'mature' as the disease develops, developing a multi-layered cored structure. Mice with superlong CAG repeats do not develop clinical signs until they are around 30-40 weeks of age, and they attain a normal life span (>2 years). Nevertheless, they show brain atrophy and unequivocal neuron loss from the striatum and cortex by 22 weeks of age, an age at which similar pathology is seen in 250 CAG repeat mice. Since this time-point is 'end stage' for a 250 CAG mouse, but very far (at least 18 months) from end stage for a > 440 CAG repeat mouse, our data confirm that the appearance of clinical signs, the formation of inclusions, and neurodegeneration are processes that progress independently. A better understanding of the relationship between CAG repeat length, neurodegenerative pathways, and clinical behavioural signs is essential, if we are to find strategies to delay or reverse the course of this disease.
Keywords: NII; aggregation; electron microscopy; extranuclear inclusions; huntingtin; nENNI; neurodegeneration; ubiquitin.