This is a detailed pathologic description of c9FTLD/ALS due to mutations in C9ORF72. The cases were both clinically and pathologically heterogeneous and fell into three pathologic categories: ALS, FTLD-MND, and FTLD-TDP. ALS cases had motor neuron degeneration, but minimal extramotor TDP-43 pathology. They all had clinical presentation consistent with ALS. FTLD-TDP had no motor neuron degeneration and widespread TDP-43 pathology. They were clinically heterogeneous. FTLD-MND had both motor neuron degeneration and widespread TDP-43 pathology. Only some of the FTLD-MND had clinically recognized evidence of motor neuron disease. There were age of onset and age at death differences between the groups. ALS and FTLD-MND were younger on average than FTLD-TDP. There was no significant difference in disease duration amongst the three pathological groups.
Two caveats should be emphasized with respect to the clinical heterogeneity observed in this series of cases. First, the clinical information in this report is not from a prospective, longitudinal study. Second, the cases were obtained from diverse clinical sources, including tertiary academic medical centers as well as non-specialty community hospitals. As previously reported, clinical presentations of c9FTD/ALS include ALS and FTD, as well as patients with mixed clinical features of FTD-MND [4
]. All of these clinical syndromes tend to be symmetrical and associated with predominance of frontal lobe involvement [41
]. Asymmetrical or focal cortical syndromes were uncommon in the present autopsy series; only one patient had prominent aphasia. A previously unrecognized clinical presentation of c9FTD/ALS in this autopsy series is amnestic dementia resembling Alzheimer's disease, sometimes clearly preceded by amnestic mild cognitive impairment. Amnestic dementia cases were relatively frequent in our cohort. They were often evaluated in Florida memory disorder clinics, and they tended to be older at death than those presenting with FTD or ALS. Almost all cases with amnestic dementia suggestive of Alzheimer's disease had hippocampal sclerosis, which supports previous findings in the elderly with hippocampal sclerosis who may clinically mimic Alzheimer's disease rather than FTD [55
]. FTLD cases with final clinical diagnoses of bvFTD tended to be younger. There was a strong correlation between microscopic evidence of hippocampal sclerosis and macroscopic evidence of hippocampal atrophy, which would make antemortem differential diagnosis difficult in these elderly patients in the absence of other more specific biomarkers. Cases with the most severe macroscopic and microscopic hippocampal involvement had FTLD-TDP compared to mild pathology in FTLD-MND. This fits with the lower frequency of hippocampal sclerosis noted in a previous series of FTLD-MND compared with FTLD-TDP [27
Although all ALS cases had clinical features compatible with this diagnosis, there was variability in the distribution of motor neuron degeneration, with half showing predominantly terminal bulbar signs and half with predominantly spinal signs. There was a greater range of clinical presentations amongst the FTLD subgroups. As noted in much larger clinical and pathological series [10
], not all c9FTD/ALS in this pathology series had a family history of neurological disease. Whether this represents true sporadic disease or incomplete ascertainment remains to be determined. Interestingly, NII were more common in cases with a positive family history, as previously reported [28
]. On the other hand, most cases of c9FTD/ALS with NII had Mackenzie Type 1 pathology, not Type 3 as originally suggested in the recent “harmonization” of the two major FTLD-TDP classification schemes [47
Macroscopic examination of the brain revealed frontal cortical atrophy in both FTLD-TDP and FTLD-MND that was greater than in ALS; atrophy of motor cortex was more frequent in ALS and FTLD-MND than in FTLD-TDP. Ventricular enlargement was greater in FTLD-TDP than both ALS and FTLD-MND. Other pathologies (i.e. Alzheimer type pathology, Lewy bodies, and vascular pathology) were infrequent and did not differ between the groups.
All cases had ubiquitin-positive, TDP-43-negative NCI in cerebellar granular layer. Cerebellar NCI were not detected in 13 FTLD-TDP and 18 ALS cases that were negative for C9ORF72
mutations, although age-related ubiquitin pathology could be observed in the white matter in all cases [14
]. Although cerebellar NCI were not detected in cases negative for C9ORF72
, caution is advised in over-interpretation of age-related ubiquitin changes that have been previously reported in the granule cell layer of middle-age and older adults [14
FTLD cases in the present series had heterogeneity regarding the degree and pattern of cortical atrophy, hippocampal sclerosis, striatal atrophy, SN degeneration, motor neuron pathology, and TDP-43 type. Previous studies of chromosome 9p linked FTLD had suggested that TDP-43 pathology in c9FTD/ALS was most often consistent with Mackenzie Type 3 [4
]. Our study demonstrates that this is not the case, and that even amongst cases with FTLD-MND, not all had Type 3. Given the larger number of FTLD-TDP cases in this series, the most frequent type was actually Mackenzie Type 1. Some of the Type 1 cases were atypical in the frequency of pre-inclusion type neuronal lesions, a common feature of Type 3. The ALS cases had very little extramotor TDP-43 pathology, and even then, GCI were largely confined to the corticospinal tract. These observations are somewhat contrary to the observations of Geser et al
], who implied that extramotor pathology was almost inevitable in ALS.
Most FTLD cases fell into two TDP-43 types: either Mackenzie Type 1 (half with FTD and Alzheimer type dementia) or Mackenzie type 3 (half with FTD and half with clinical evidence of motor neuron disease). There were only two Mackenzie Type 2 cases (one with bvFTD and the other with Alzheimer type dementia). One of the Type 2 cases had evidence of motor neuron degeneration predominantly affecting upper motor neurons and the corticospinal tract. Neither had semantic dementia, which is a common clinical correlate of non-c9FTD/ALS Type 2 cases [28
FTLD cases differed in degree and pattern of cortical atrophy, with FTLD-TDP having more of a frontotemporal pattern of atrophy compared to FTLD-MND with mild frontal and motor involvement. Frontal cortical atrophy was often consistent with frontal-behavioral clinical presentations; whereas there was a general lack of peri-Sylvian and superior parasagittal atrophy supporting similar lack of focal presentations, such as nonfluent aphasia or corticobasal syndrome. Symmetry could not be evaluated since only one hemibrain was available for histologic studies; the other hemibrain having been frozen for biochemical and genetic studies. On the other hand, there was almost no clinical evidence of asymmetry, with only one patient having a prominent aphasia. This contrasts with asymmetrical clinical presentations (e.g., progressive nonfluent aphasia) that are not uncommon in FTLD-TDP do to GRN
Striatal degeneration was present in some FTLD cases, but it was relatively mild compared to the severe striatal atrophy seen in FTLD-FUS [29
]. Severe striatal atrophy may be used to distinguish c9FTD/ALS from FTLD-FUS. Striatal degeneration is variable in FTLD-TDP due to GRN
mutations, but can be a predominant feature in some cases [69
Substantia nigra degeneration was relatively common in c9FTD/ALS and sometimes associated with parkinsonian clinical features. Of the five cases with severe nigral degeneration, four had extrapyramidal signs, including two patients thought to have dementia with Lewy bodies. One of these patients (Case 20) was enrolled in a prospective longitudinal study of dementia with Lewy bodies. Substantia nigra neuronal loss was detected in FTLD-MND and FTLD-TDP, but not ALS. Substantia nigra degeneration is also frequent in FTLD-TDP due to GRN
mutations, detected in about 80% of cases [26
], but it does not always correlate with overt clinical parkinsonism. Parkinsonism is relatively common in FTLD [15
], and is present in as many as 67% in FTLD-TDP due to GRN
]. Parkinsonism in c9FTD/ALS was less frequent (30% for all cases and 40% for only cases with FTLD). A direct comparison of c9FTD/ALS and GRN
-related FTLD-TDP is an objective of future studies. Available evidence, however, would suggest that FTLD-TDP due to GRN
mutations is less heterogeneous that c9FTD/ALS, since virtually all reported cases have had Mackenzie Type 1 pathology [26
Spinobulbar signs were present in three of eight FTLD-MND cases, and of the five patients without clinical evidence of motor neuron degeneration (cases 9-13), two had a family history of ALS. Pyramidal tract degeneration and motor neuron neuronal loss and gliosis was variable in these five cases and severe in only one (case 11), a 62-year-old woman with mild dysphagia and a sister with ALS. Subclinical motor neuron degeneration is increasingly recognized in FTD [40
], and electrodiagnostic tests may be needed to reveal subtle evidence of motor neuron degeneration. In this retrospective series, only one of the FTLD cases (case 7, a 59-year-old man with a clinical diagnosis of FTLD-MND) had electromyography and nerve conduction velocity studies. Despite lack of antemortem evidence of motor neuron disease in some cases, the present study shows significant pathologic overlap between FTLD-MND (n = 8) and ALS (n = 5) cases. The major areas of neuropathologic overlap between FTLD-MND and ALS are presence of NCI in motor neurons, prominent GCI in motor cortex and corticospinal tract, and presence of Bunina bodies in lower motor neurons. Bunina bodies were detected in all cases of ALS (5 of 5) and all but one case of FTLD-MND (7 of 8). The case without Bunina bodies had severe motor neuron loss precluding their detection, which is similar to paucity of Lewy bodies in the substantia nigra in end stage Lewy body disease. Two FTLD-TDP cases with very sparse Bunina bodies were likely an incidental finding since they were not associated with motor neuron loss or NCI with TDP-43 immunohistochemistry. The presence of GCI was a characteristic feature of TDP-43 pathology in FTLD-MND and ALS, which was much less common in FTLD-TDP. Oligodendroglial inclusions have been noted previously in ALS and FTLD-TDP [2
], and the present study suggest that this histologic feature marks overlap between a subset of c9FTD/ALS with motor neuron degeneration, but that it is not specific to c9FTD/ALS. In fact, other than cerebellar inclusions, there was no specific pathologic feature that distinguishes c9FTD/ALS.
There are strengths and weaknesses in this study. A strength of the study is that all cases were referred to a single brain bank where neuropathologic procedures are standardized. All cases underwent an in-depth neuropathologic examination by one neuropathologist, including semiquantitative assessment of a range measures using immunohistochemistry with a sensitive and specific phospho-TDP-43 monoclonal antibody [24
], processed in batch with an automated tissue stainer to assure uniformity. All cases were also processed with sensitive methods for detecting motor neuron degeneration, and all cases had assessment of characteristic ubiquitin-immunoreactive cerebellar NCI. The major weakness is that cases were obtained from various sources, and there was a wide range in the quality of the medical documentation. Most cases had far from complete family histories. Only the Mayo Clinic patients were enrolled in prospective longitudinal studies, the others were a retrospective cohort.
This pathologic study of c9FTD/ALS demonstrates that although all cases had TDP-43 pathology, there is greater clinical and pathological heterogeneity than previously thought, including amnestic type dementia in an elderly cohort with hippocampal sclerosis and bvFTD in a younger cohort without motor neuron degeneration. The length of the hexanucleotide expansion in C9ORF72
is unknown in our cases. It remains to be determined if the expansion length can influence the clinical presentation or the nature of neuropathologic findings. This is an objective for future studies, as well as comparison of c9FTD/ALS with sporadic FTLD cases matched for TDP-43 Types and with the other relatively common genetically-determined FTLD-TDP, that due to mutations in GRN
]. This study further suggests that the harmonization of TDP-43 types proposed for FTLD-TDP [47
] may have been premature, at least with respect to genetic associations, since not all c9FTD/ALS are Mackenzie Type 3 (harmonization Type B), and in the present series, Mackenzie Type 1 (harmonization Type A) was actually more common, especially in those patients who presented with dementia (amnestic or frontal-behavioral variants) and no evidence of motor neuron degeneration.