In recent times there have been numerous investigations [2
] indicating that a tau-negative, ubiquitin-positive pathology, known as FTLD-U, is a frequent, if not the most common, histological change underlying FTLD. Nonetheless, in most studies [2
], little or no attempt has been made to differentiate cases according to the phenotypic appearance of the UBQ changes, with all cases being simply referred to as FTLD-U. Only in one previous study [13
] did the authors correlate the appearance of the ubiquitin pathology with clinical phenotype, but then mainly from the perspective of differentiating between cases of FTD with or without accompanying MND. In this latter study, this was done according to differences in the density and morphology of UBQ changes in the cerebral cortex, hippocampus and striatum [13
]. In the present study, we have classified 60 cases of FTLD-U drawn from two UK Centres (Manchester and Newcastle) into three distinct histological subtypes according to the appearance and anatomical distribution of the UBQ pathology within the cerebral cortex and hippocampus. We have been able to extend previous observations [13
] and show that clinical representations of FTLD other than FTD + MND (i.e. FTD, SD and PNFA) are strongly associated with each of the different histological subtypes. The results of our study may have important nosological implications for the classification of FTLD.
The histological subtype we have designated as type 1 is characterised by many UBQ immunoreactive neurites, and UBQ intraneuronal cytoplasmic inclusions with “cat’s eye” or “lentiform” NII, within layer II of the cerebral cortex, and variable numbers of UBQ cytoplasmic inclusions within granule cells of the dentate gyrus. Nineteen patients (32%) showed type 1 histology, 17 with NII. This type of histology was typically seen in patients with clinical FTD or PNFA. On the basis of their examinations of UBQ immunostained sections of cerebral cortex, hippocampus and striatum, Katsuse and Dickson [13
] described one form of UBQ pathology, which they termed FTLD-MNI (FTLD with motor-neuron disease (MND)-type inclusions but without MND) in 47/52 patients with FTLD-U and another form which they called FTLD-MND (FTLD with MND). Although here we did not examine the striatum, their descriptions of FTLD-MNI and FTLD-MND [13
] bear close resemblance to those which we have termed type 1 and type 3 histology, respectively. Nonetheless, in contrast to present findings, where NII were universal in type 1 cases, these were reported by Katsuse and Dickson to occur in only 26 of their FTLD-MNI cases [13
]. It is possible that selection criteria account for these differences. In the present study, we differentiated cases of type 1 histology from those with type 2 histology (see above for criteria) in whom NII were not seen, whereas in the study by Katsuse and Dickson [13
], because they did not attempt to further subtype their FTLD-MNI cases, it is possible that these could have included cases of (our) type 2 histology in whom NII would not have been anticipated.
In the present study, type 2 histology, with abundant UBQ neurites but few or no cytoplasmic inclusions or NII, was the most common histology present in patients with SD-8/9 Manchester patients with SD showed type 2 histology and 8/9 patients with type 2 histology had SD. FTLD-U had been previously reported by Rossor et al [26
] to be the underlying histology in three patients with SD, and from the histological descriptions provided it would appear that these three patients also displayed UBQ pathological changes similar to type 2 histology described here, although this was not so remarked upon at the time.
Type 3 histology, characterised by numerous neuronal UBQ cytoplasmic inclusions but few or no UBQ neurites was seen in all six Manchester patients with FTD + MND. Although others [3
] have suggested that the type 1 histology may be present in some cases of FTD + MND, in the present series such cases were not encountered. However, it is accepted that the small numbers of such patients studied here may have precluded the finding of cases with alternative UBQ histologies. In four of these (patients# 25–28) both cerebral cortical and hippocampal neurones were extensively involved, though in the other two (patients #36 and 37) only hippocampal neurones were severely affected. However, there were eight other patients with this type of histology, albeit in most instances restricted to hippocampus, none of whom showed any clinical or pathological evidence of MND. Seven of these patients had FTD alone and one had PNFA, emphasising the overlapping pathology between FTD and FTD + MND, particularly as far as the hippocampus is concerned [31
When we compared duration of illness across the histological phenotypes we found that it was shorter in patients with type 3 histology than in other patients. These findings are most likely explained by the inclusion of those patients with FTD + MND within this histological subtype, for whom a shorter duration of illness has been previously reported [6
]. In such individuals it is likely that the disease is terminated early through the presence of bulbar muscle wasting and respiratory failure consequent upon MND.
What we have presently termed type 1 histology occurs in certain families with autosomal dominant transmission of disease known to be linked to chromosome 17q21 [8
]. Recent work [1
] has shown that FTLD in many of these latter families is caused by mutations in progranulin gene (PGRN
), located just 1.8 mb distant from MAPT
locus on chromosome 17q21. Such observations raise the possibility that type 1 histology, and perhaps the NII component thereof, might act as a surrogate marker of PGRN
mutation-related FTLD. This argument is supported by several observations.
Firstly, that two of the present patients with FTLD reported in Baker et al. [1
], one with Q130SfsX124 mutation and one with Q468Xmutation (patients #4 and #12, respectively in the present study), in whom the disease was not previously known to be linked to chromosome 17, did indeed have this pathology. Since then we have further found in patient #13 the C31LfsX34 mutation reported initially in a Canadian family UBC-17 with chromosome 17-linked tau negative histology and published in Baker et al. [1
], and a novel PGRN
mutation (V452WfsX38) in patient #7 (Pickering-Brown, unpublished data), both with type 1 histology. We are continuing to analyse the remaining ten patients with this histological subtype for PGRN
mutations to provide further support for this viewpoint. Furthermore, since the original studies by Baker et al. [1
] and Cruts et al. [6
], many other additional FTLD-U cases with PGRN
mutations have also been identified by other workers, and NII were present in all cases where post mortem brain analysis had been done (Mackenzie–manuscript submitted).
Secondly, none of the 16 patients with type 2 histology showed NII, even though there was positive family history of FTLD in two of these (patients #43 and #47). So far, we have not detected PGRN mutation in any of these 16 patients, nor have we found any such mutations in a further 25 clinically confirmed living patients with SD (most of whom can be presumed to have type 2 histology) (Pickering-Brown, unpublished data), suggesting PGRN mutations are unlikely to underpin this form of UBQ pathology. Nonetheless, paradoxically, patient #14 with SD did in fact show type 1 histology and had a positive family history; PGRN analysis in this patient has not as yet revealed PGRN mutation (Pickering-Brown, unpublished data).
Thirdly, NII were not present in any patient with type 3 histology, including the six patients with FTD + MND. Similarly, although Katsuse and Dickson [13
] reported NII to be present in 26/43 cases of FTLD-U, none of their cases had clinical and pathological MND. However, in another study of 34 patients with FTLD-U, 11 patients showed NII of whom 3 had FTD and clinical and pathological MND [3
]. It is not known in either of these latter studies whether any or all such cases with NII bore PGRN
mutation. We have so far been unable to detect PGRN
mutation in any of the present 6 clinically and pathologically confirmed cases of FTD + MND, nor have we found any such mutations in a further 22 clinically confirmed living patients with FTD + MND and presumed type 3 histology (Pickering-Brown, unpublished data).
However, the finding of ubiquitinated NII in patients with Inclusion Body Myopathy with Pagets Disease of bone and FTD (IBMPFD) associated with mutations in the valosin-containing protein gene (VCP
] apparently similar to those seen in FTLD would argue against specificity of UBQ NII in FTLD as markers of PGRN
mutations. However, although NII in VCP
mutations contain VCP protein [7
] and may superficially resemble those in FTLD cases with PGRN
mutation, the latter do not contain VCP protein [28
]. Moreover, IBMPFD is an extremely rare disorder, and can be readily distinguished from FTLD both clinically and pathologically, in terms of the nature and distribution of UBQ pathology [7
]. Hence, patients with “cat’s eye” NII, within the context of clinical FTD not associated with muscle or bone abnormality, are unlikely to possess VCP
Furthermore, NII in FTLD are not always observed in the context of a previous positive family history. Although in the present study linkage analysis had not been performed, 15/17 patients with NII had shown positive family history consistent with autosomal dominance. Nonetheless, two patients showed no previous family history, and the presence of NII in apparently sporadic cases has been noted by others [3
]. On face value, such observations would again argue against NII being pathognomic for PGRN
mutations, though in at least some cases included in the latter studies [3
mutations have in fact now been found (Mackenzie, personal communication). Hence, whether there are indeed sporadic FTLD-U cases with NII but without PGRN
mutations must remain an open question until all such potential cases have undergone genetic analysis and the absence of such a mutation definitely ruled out.
Hence, current data would strongly suggest that the presence of NII in FTLD is indicative of PGRN
mutation, though this conclusion must remain tentative until all cases with these pathological structures, both within this present study and in those of other research groups, have been subjected to genetic analysis and the presence or absence of mutation defined. Nonetheless, should this relationship between NII and PGRN
mutations turn out to be correct, then present data showing type 1 histology to be present in 17/60 patients with FTLD-U in the Manchester and Newcastle combined series (present in 14/37 Manchester patients with FTLD-U, and in 14/73 Manchester patients with FTLD per se) would suggest that PGRN
mutations are a more common cause of (inherited) FTLD than those with MAPT
mutations, outnumbering the latter by a factor of 2:1 (only 8/73 Manchester patients with FTLD had MAPT
Interestingly, a number of FTLD clinical phenotypes were embraced by type 1 histology, but principally FTD and PNFA. Indeed, patients #4 and #7 with Q468X and V452WfsX38 mutation, respectively, displayed FTD phenotype, whereas patients #12 and #13 with Q130Sfx124 and C31LfsX34 mutations showed PNFA. Such observations argue that although type 1 histology might be associated per se with PGRN
mutations (vide supra) the brain topographic distribution of that form of UBQ pathology may depend upon factors other than the actual mutation, since it has been proposed that all PGRN
mutations induce the same pathophysiological haploinsufficiency through the creation of a null allele [1
Finally, although none of the cases studied here were associated with MAPT mutations, there appeared to be variations in frequency of MAPT haplotypes and genotypes between the different histological subtypes. For example, type 2 histology was associated with a significantly higher H1H1 frequency (but not H1 haplotype frequency), and type 3 histology with H1H2 genotype. By contrast, the extent of UBQ pathology (irrespective of histological subtype) increased with possession of H2 allele. Such observations suggest MAPT haplotype might play a part in modulation of the overall degree of UBQ pathology and/or the histological form manifested. However, these observations are based on small sample sizes, especially those relating to H2 allele, and will require validation in further, and larger, cohorts before they can be considered to be substantive.
What the ubiquitinated protein present within the neuronal cytoplasmic inclusions, NII and neurites might be remains unknown. Indeed, it is not clear whether it is actually the same protein that is accumulated at all three anatomical sites, or different site specific proteins. Certainly, the protein does not appear to be progranulin, since antibodies to this do not immunolabel any of the ubiquitinated structures present in patients with either chromosome-17 linked FTLD with PGRN
], or in patients within the present series with type 1 histology and PGRN
mutations (where known), or in others with types 2 and 3 histology (Mann, unpublished data).
In conclusion, in this present study, we have described three distinct histological profiles that encompass the pathological entity of FTLD-U. We have been able to show that the various clinical representations of FTLD are strongly associated with each of the different histological subtypes. Each of these subtypes may have separate underlying pathophysiological causes which might reflect (1) different ubiquitinated proteins accumulating, (2) different forms of the same protein, or (3) different anatomical distributions of an identical protein, within each histological subtype. By refining our histological criteria of FTLD-U it might be possible to better determine genetic or other factors which cause or increase the risk of developing the different clinical or pathological forms of FTLD.