Our mutational screening study of Sardinian patients with ALS found that a single missense mutation in TARDBP accounts for approximately one-third of all ALS cases in this island population. Furthermore, these ALS patients shared a large risk haplotype across the TARDBP locus, indicating that they had a common ancestor and were part of a larger kindred. This founder effect is comparable with that recently described for the chromosome 9p21 ALS-FTD locus in the Finnish population4
and highlights the power of performing genetic studies in population isolates with a higher incidence of a disease. In this regard, it is noteworthy that nearly a quarter of the Sardinian ALS cases in our consecutive series of prevalent patients had a known family history of ALS, which is considerably higher than the approximately 5% reported by population-based epidemiological studies in other European populations.13,14
The TARDBP p.A382T missense mutation has been previously described in 2 French patients with ALS and 11 patients living on mainland Italy.19–21
Microsatellite analysis suggested that at least a portion of these French and Italian patients carried the same 2.4-Mb haplotype on chromosome 1.21 The high marker density of the Illumina HumanHap660W genotyping array used in our study allowed this haplotype to be narrowed to a 663-Kb region containing the TARDBP gene locus. DNA was not available to test if the mainland Italian or French samples carried the same haplotype, but the geographical closeness of these cases makes it likely that all of them arose from a single mutational event.
Nearly two-thirds of the patients (n=24) carrying the p.A382T mutation were classified as having sporadic ALS. Although it is possible that some of these cases arose from multiple mutational events affecting the same nucleotide, our data strongly support the notion that these cases are part of the same extended pedigree: genome-wide analysis demonstrated that both the familial and sporadic samples shared an average of 21% of their genome with each other, indicating that they had a common ancestor as recently as 3 generations ago. Cryptic relatedness, in which 2 individuals are related to each other without their knowledge, is a well-described issue in ALS and may arise from poor diagnosis in past generations, incomplete knowledge of family history, and even varying manifestations of motor and cognitive dysfunction among mutation carriers within the same family.22
The relative late age of symptom onset in patients carrying the p.A382T mutation also indicates that this pathogenic variant is only fully penetrant by the eighth decade of life and that mutation carriers in previous generations may have died of other diseases prior to developing motor neuron degeneration. An alternative possibility is that the p.A382T mutation represents a risk factor for ALS in these sporadic cases rather than being the direct cause of disease. Although this possibility cannot be fully excluded, the lack of this known pathogenic mutation in neurologically normal Sardinian control samples (or in other populations), the shared haplotype, and relatedness among the cases carrying the mutation would not support this hypothesis.
Sardinian patients carrying the p.A382T mutation had a heterogeneous spectrum of ALS phenotypes, a pattern that has been observed for mutations in other ALS genes.23
Despite these observations, the flail arm variant of ALS occurred with greater than expected frequency among mutation carriers. This clinical variant has also been previously described in patients with TARDBP mutations,24
and all 7 French patients with the p.A382T mutation manifested a lower motor neuron predominant form of the disease with onset in the upper limbs.20
We observed extrapyramidal symptoms in multiple Sardinian patients with ALS carrying the TARDBP p.A382T mutation, representing the first time that parkinsonism has been described in individuals with mutations in this gene.25
Some caution is required in interpreting these findings. First, one of the patients with extrapyramidal symptoms also manifested external ophthalmoplegia, a finding more commonly associated with mitochondrial disorders. Though supranuclear palsy has already been described in a patient with a different TARDBP mutation,26
the mitochondrial genome of our case was not sequenced, so the existence of a second mitochondrial disease-causing variant in addition to his known pathogenic p.A382T mutation remains possible. Second, it is possible that the neuroleptic medications taken by the third patient for treatment of childhoodonset tic disorder contributed to his extrapyramidal symptoms, though it is noteworthy that these medications were discontinued more than 30 years prior to the onset of parkinsonism. Third, parkinsonism is a well-recognized feature of ALS,27,28
suggesting that the finding of 3 cases in a cohort of this size may have been a chance occurrence. In addition, the diagnosis of extrapyramidal signs in patients with ALS is often complicated by the presence of upper and lower motor neuron signs inherent to the disease. Finally, autopsy material was not available from any of the patients with extrapyramidal symptoms, so histopathological confirmation of substantia nigra involvement was not possible.
Despite this, we believe that our observations, taken together with the previous report of chorea in a patient with a TARDBP mutation,26
expand the clinical spectrum associated with mutations in this gene to include basal ganglia dysfunction. Given that pathological deposition of TDP-43 in the central nervous system has been described in a variety of neurological disorders,29
it is perhaps not surprising that TARDBP mutations can be associated with a wide range of neurodegeneration processes with corresponding clinical manifestations. The basis of this pathological and phenotypic heterogeneity is not clear, though genetic or environmental factors may influence the precise clinical manifestations of TARDBP mutation carriers. These genotype-phenotype observations suggest that screening of Sardinian patients with diverse neurological diseases for the TARDBP p.A382T mutation may reveal additional unrecognized clinical consequences of this pathogenic variant.