Background
Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder involving the motor system with a progressive, fatal course within 3–5 years after onset. Most cases of ALS are sporadic (sALS), while approximately 5–10% cases are familial ALS (fALS). ALS-causing mutations have been so far identified in more than 25 genes, including Cu/Zn superoxide dismutase 1 (
SOD1) [
1], TAR DNA-binding protein-43 (
TARDBP) [
2,
3], fused in sarcoma (
FUS) [
4,
5], optineurin (
OPTN) [
6], chromosome 9 open reading frame 72 (
C9orf72) [
7,
8], and sequestosome 1 (
SQSTM1) [
9]. Although
C9orf72 is the predominant mutant gene in the Caucasian ALS population, it is rather rare in non-Caucasian patients with ALS [
10,
11]. Mutations in
SOD1 are one of the most common and important causes of ALS, accounting for 23% of fALS and ~ 7% of apparently sALS worldwide [
12]. More than 185 mutations in
SOD1 have been reported to date [
13]. Multiple efforts have focused on targeted therapeutic approaches for
SOD1-related ALS (
ClinicalTrials.gov: NCT01041222: SOD1Rx [
14]; NCT00706147: Arimoclomol [
15]; NCT01083667: Pyrimethamine [
16]).
In the present study, we described the genetic and natural history profiles of ALS patients with SOD1 mutations obtainted from a national referral hospital site in China. The effort to clinically stratify mutations in SOD1 according to the patients’ natural histories should be beneficial for future genetic counselling and the selection of genetically and clinically homogeneous patients for SOD1-targeted clinical trials for ALS.
Discussion
We identified 66 patients harbouring 43 confirmed SOD1 mutations among 923 sALS and 159 fALS patients. Because of the number of patients available, the present study represents the largest effort to study SOD1 mutations to date in a non-Caucasian ALS population, including 47 fALS patients and 19 sALS patients with SOD1 mutations. The extensive survey and analysis of ALS-related SOD1 mutations and the patients’ natural histories provide data that could serve in the design and interpretation of future clinical trials targeted at patients with SOD1 mutations.
In Chinese
SOD1 mutant patients with ALS, p.His47Argwas most frequently identified (8/66, 12%); this mutation is found mainly in Asian ALS patients (more than 15 pedigrees have been reported, only 3 of which are of Caucasian descents) [
31]. P.His47Arg was also the leading mutation in Japanese
SOD1-mutant patients [
32]. In our study, almost all patients with p.His47Arg presented with the characteristic phenotype of exclusive spinal muscle initiations but mild disease course, i.e., an older AAO, extended diagnostic delay, slower progression, and longer survival time (Table
2), with the exception of one patient with a survival time of only 14 months. Conversely, the most globally predominant
SOD1 gene mutation, D90A [
32], was absent in our study, and only one case with A4V (p.Ala5Val, 1/66) and one case with I133T (p.Ile114Thr, 1/66) were found; these two mutations account for ~ 50% and ~ 65% of
SOD1 mutations in the United States [
29] and Canada [
33], respectively. This result is not uncommon since the frequency of specific mutations can vary among different countries or even among different regions of the same country. For example, the A4V mutation has also been rarely observed in Europe. Another case in point is the mutation cluster of L48F-
SOD1, which has been described in central Italy but is quite rare in other Italian regions [
34].
Of note is the gender difference in patients harbouring
SOD1 mutations. Male predominance was present but mild in
SOD1-mutant patients (M: F of 1.2 overall, 1.3 in fALS, and 0.9 in sALS); these ratios are lower than those in other reports from China (5/3 = 1.6) [
35], Taiwan (7/5 = 1.4) [
36], and Iran (4/3 = 1.3) [
37] despite the small sample sizes in those studies. These ratios are also lower than those in both the present cohort (689/393 = 1.75) and in the ALS population in China (1.47–1.7) [
35,
38]; however, they are comparable to that of
SOD1-mutant patients found in the United States (1.3, 175 fALS included) [
29].
Furthermore, significant beneficial effects of female gender on diagnostic delay, disease progression and survival time were observed, and we postulate two explanations for this result. 1) The mutations with poorer prognosis presented more often in male patients. For example, 17 patients had a diagnostic delay of less than 6 months (the first quartile), 13 of which were males; similarly, 13 patients had an ΔFS of more than 0.90 (the third quartile), 8 of which were males. Finally, 23 patients had a survival time of less than 31 months (the first quartile), 16 of which were males. 2) For the identical mutation, the male patients presented a poorer prognosis than females. In total, 8 patients carried the p.H47R mutation. Among these patients, the 3 male patients presented a mean diagnostic delay of 54.7 months, an ΔFS of 0.12, and a survival time of 70 months, while these values in the other 5 female patients were 67.4 months, 0.08, and 120.5 months, respectively. The difference in the diagnostic delay between genders existed in both fALS (
p = 0.042) and sALS (
p = 0.027), while gender differences in disease progression (
p = 0.031) and survival time (
p = 0.009) were present in only sALS. The finding of a longer survival time in female patients is consistent with what we previously found in the overall sALS population (female: male, 87 months vs 63 months,
p = 0.008) [
38]. To our knowledge, such a gender difference in
SOD1-related ALS patients’ natural histories has not been previously described. The rarity of this finding may be due to having an insufficient number of ALS cases with
SOD1 mutations or a lack of detailed follow-up data. It remains unclear why female patients present a more remarkable survival advantage in the sporadic population. A possible explanation for this discrepancy is that fALS is inherited as a mendelian disease (monogenic), while sALS is likely an oligogenic disease [
39]. We speculate that in fALS cases, the impact of a causative gene is predominant; while in sALS cases, the effect of each variant in certain gene is minor, and the modifying effect of gender could therefore be manifested. Since extended diagnostic delay has been associated with longer survival in ALS patients in several studies [
38,
40], it is reasonable that females with a longer diagnostic delay had better survival in the present study. The effect of gender on survival time has been mentioned multiple times with contrasting findings in ALS populations, with some studies suggesting worse survival for women [
41,
42] and others suggesting better survival [
43,
44]. One possible explanation for gender differences could be the role of gonadal hormones since these hormones, particularly oestrogen, have been proven to be neuroprotective. A very recent report [
45] demonstrated a negative association between ALS and hormonal contraception use in women, reporting a dose-response effect. Another potential reason for the gender difference is that smoking is a risk factor for ALS; patients who smoke may have a shorter survival time [
46,
47], and there is a higher percentage of male smokers in the Chinese ALS population than female smokers (male: 44.7% vs female: 1.7%) [
38]. Another supporting and interesting interpretation is from a meta review [
48] of a
SOD1 G93A mouse model of ALS. That report was suggestive of gender- and genetic background-related effects on disease course, i.e., female-related neuroprotective effects on lifespan and disease duration were observed for B6SJL mice but not for mice on a C57BL/6 background, implying that the inherent genetic differences observed between backgrounds had some interactive effect on the presentation of the female hormone-related protective effect.
The mean AAO of the
SOD1-mutant patients in this study was 43.92 (95% Cl 41.61–46.23) years, which was younger than that of 45.5 years reported in a southwest China [
35] study that identified only 8 patients carrying mutant
SOD1 genes among 499 ALS patients. This mean AAO is also younger than that of the overall Chinese ALS population (49.7, 95% Cl 49.2–50.3) [
38] and younger than that of
SOD1-mutant patients reported in Canada (48.9) [
49] and the United States (46.9–49.7) [
29,
50]. As described in a previous study,
SOD1-mutant patients in a specific population were also younger than the overall ALS patients in the same population (total in Canada: 59.5) [
49]. Patients carrying
SOD1 mutations with bulbar onset, who usually present a later AAO, were less common than overall ALS patients in China according to our previous report [
38] (8.1% vs 14%), which may partially explain why the mean AAO of
SOD1-carrying patients was younger than that of the overall ALS patients in the same population.
Another striking finding of this study is the distinctive profile of disease progression and survival in the overall
SOD1-mutant patients. The proportion of subpopulations was approximately 5:3:2 (slow: intermediate: fast). The overall median survival time of 89 months, much longer than that of 17.5 months reported in a recent
SOD-related clinical trial [
15], could be due to the rarity of the A4V mutation in the Chinese ALS population, which was characterized by a survival time of 1–2 years. The median survival of non-A4V
SOD1 patients in the United States was 6.8 years, comparable to that of our patients. Nevertheless, the discrepancy in the
SOD1-related natural histories between Chinese and Caucasians should be recognized in the design and strategy of
SOD1-targeting clinical trials.
The present study distinguished seven novel variants in the included ALS patients. These variants were evaluated as VUS or likely pathogenic according to the ACMG Standards and Guidelines [
28] (Table
1). It is noteworthy that the nonsense variant of p.Glu133Ter was not classified as evidence of very strong pathogenicity because loss of function (LOF) is not believed to be the primary pathomechanism of
SOD1-related ALS.
Since this cohort was based on a national referral site for ALS, caution should be applied when using these data to deduce the frequency of SOD1 mutations in Chinese ALS patients. Additionally, the relatively younger mean AAO of SOD1-mutant patients and the longer median survival time compared with those of overall ALS patients in China (mean AAO: 43.92 years vs 49.8 years, median survival time: 89 months vs 71 months) made it more likely that these patients reported to the referral hospital. The calculated frequencies of 30% for fALS and 2% for sALS were likely an overestimation. Nevertheless, these numbers are the result of the greatest effort to date to explore the prevalence of SOD1 mutations in the Chinese ALS population.