White Matter Tauopathy with Globular Glial Inclusions: A Distinct Sporadic Frontotemporal Lobar Degeneration

Frontotemporal lobar degenerations are a group of disorders characterized by circumscribed degeneration of the frontal and temporal lobes and diverse histopathological features. We report clinical, neuropathological, ultrastructural, biochemical and genetic data on seven individuals with a four-repeat (4R) tauopathy characterized by the presence of globular glial inclusions (GGIs) in brain white matter. Clinical manifestations were compatible with the behavioral variant of frontotemporal dementia (FTD) and included motor neuron symptoms; there was prominent neuronal loss in the frontal and temporal cortex, subiculum and amygdala. The surrounding white matter showed abundant GGIs composed of abnormal filaments present mostly in oligodendrocytes. The severity of white matter tau abnormalities correlated with a reduction in myelin and axons and with microglial activation. Western blotting of sarkosyl-insoluble tau demonstrated the presence of two major tau bands of 64 and 68 kDa. No mutations in the microtubule-associated protein tau (MAPT) gene were detected in two affected individuals. We propose that 4R tau-immunoreactive GGIs are the neuropathologic hallmark of a distinct sporadic tauopathy with variable clinical presentations that include FTD and occasionally upper motor neuron disease. This type of tauopathy with GGIs expands the group of neurodegenerative disorders in which oligodendroglial pathology predominates, beyond the synucleinopathy multiple system atrophy disorders.