Preliminary Observations on APOE ϵ4 Allele and Progression of Disability in Multiple Sclerosis

Background  Apolipoprotein E expression is increased in regenerating neural tissue and the APOE ϵ4 allele is associated with impaired neuronal repair. Since repair is essential for the restoration of central nervous system function following multiple sclerosis (MS) relapses, APOE genotype may influence clinical progression of the disease.

Objective  To examine the association of the APOE genotype with disease susceptibility and progression in MS.

Patients and Methods  APOE genotyping was determined by polymerase chain reaction and restriction enzyme digestion in 47 patients with MS who had been followed up every 3 months for 2 years as part of an open-label clinical trial with glatiramer acetate. The Expanded Disability Status Scale (EDSS) was used to assess clinical progression.

Results  Nine patients were heterozygous and 1 patient was homozygous for the APOE ϵ4 allele, for a frequency of 12% (11/94), which is similar to that of the general Israeli population. The APOE ϵ4 carriers had a mean ± SE EDSS score of 3.10 ± 0.45 at entry, which was not significantly different from the remaining 37 patients (2.62 ± 0.25). During the observation period, the EDSS score of the APOE ϵ4 carriers deteriorated to 4.00 ± 0.63 while the other patients remained stable with an EDSS score of 2.74 ± 0.31. The interaction of genotype with disability over time was significant (P = .02 by repeated-measures analysis of variance). There were no differences in the number of relapses occurring in the 2 groups.

Conclusions  These preliminary observations suggest that APOE genotype may influence disease progression in MS. The APOE ϵ4 allele was not associated with an increased risk of MS or relapses.

APOLIPOPROTEIN E (apoE) is involved in the transport of lipids in the serum. Several lines of evidence suggest that it is also involved in the regeneration of axons and myelin following lesions of central and peripheral nervous tissue, including a significant increase of apoE in regenerating peripheral nerve¹ and central nervous system tissue.² In in vitro studies, apoE acts as a nerve growth factor³ or cofactor.⁴

In humans, there are 3 common alleles of APOE, the gene coding for apoE, ϵ2, ϵ3, and ϵ4. The APOE ϵ4 allele has been consistently associated with an increased risk of Alzheimer disease.^5,6 The proposed mechanisms for this association include inadequate repair,⁷ which may be relevant to other neurologic illnesses since repair determines to a large extent the development of disability. Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system that is characterized mainly by exacerbations followed by remissions. Although some patients with MS have a relentlessly progressive course of the disease from onset, the majority recover from early relapses, some of whom remain stable for many years, while others develop progressive disability. Since repair is essential for the restoration of central nervous system function following MS relapses, APOE genotype may influence clinical progression of the disease. In the present study we examined the association of APOE genotype with disease susceptibility and progression in MS.

The study group included 47 patients with MS who had a clinically definite⁸ relapsing-remitting course. They were enrolled by informed consent in an open-label trial with copolymer 1 (glatiramer acetate [Copaxone], Teva Pharmaceuticals, Kfar Saba, Israel) from 1990 to 1997. The clinical inclusion and exclusion criteria used in this study are detailed elsewhere,⁹ and in principle included patients with mild to moderate relapsing-remitting disease. Since interferon treatment was not available at that time, most, if not all, eligible patients were included without a selection bias. Patients were examined every 3 months according to a protocol that included a detailed neurologic examination and assessment of the Kurtzke functional systems Expanded Disability Status Scale (EDSS).¹⁰ During and following all exacerbations, a full examination including the EDSS was performed. For all missing data, the last observation was carried forward in the statistical analysis. Progression was defined as a deterioration in the EDSS score maintained over 2 consecutive examinations 3 months apart.

DNA was isolated from routine blood samples obtained as approved by the institutional review board. Identification of APOE ϵ4 alleles was carried out as previously described.¹¹ The genotyping was performed at the end of the follow-up period and the physicians were blinded to these data when evaluating the patients.

For variables with a single measurement per patient, the groups with or without APOE ϵ4 alleles were compared by means of independent t tests or the Fisher exact test. For comparison of EDSS measures over time, an analysis of variance (ANOVA) with repeated measures was used. Significance was set at P<.05, and data are reported as mean ± SE.

We found that 9 patients were heterozygous and 1 patient was homozygous for the APOE ϵ4 allele. The ϵ4 allele frequency of 12% (11/94) in these patients with MS was similar to that of the general Israeli population.⁶ In the subsequent analysis, the homozygous patient was included in the APOE ϵ4 carrier group. Excluding the 1 patient homozygous for the APOE ϵ4 allele does not alter significantly any of the subsequent results.

Table 1 summarizes the characteristics of patients with and without APOE ϵ4 alleles at entry into the study. Mean age and male-to-female ratio were similar in both groups. The APOE ϵ4 carriers had a shorter duration of illness at entry (48 ± 12 months vs 57 ± 10 months), a similar exacerbation rate over the previous 2 years (1.05 ± 0.05 per year vs 1.12 ± 0.06), and a higher EDSS score (3.10 ± 0.45 vs 2.62 ± 0.25). None of these differences, however, was statistically significant (P>.35 for all comparisons).

Figure 1 summarizes the mean EDSS scores of both groups during the 2 years of observation. During the first year, a single (non–APOE ϵ4 carrier) patient was lost to follow-up. Only this non–APOE ϵ4 carrier was not observed for the 2 full years. During this 2-year observation period, the APOE ϵ4 carriers' disease course deteriorated to a mean EDSS score of 4.00 ± 0.63 while the other patients remained stable with a score of 2.74 ± 0.31. The interaction of genotype with disability over time was significant (P = .02 by repeated-measures ANOVA). Analysis of the cases by the criteria of sustained change in EDSS revealed that 7 (70%) of the 10 APOE ϵ4 carriers compared with 13 (35%) of the 37 non–APOE ϵ4 carriers had progressed in their disability (P = .03, Fisher exact test). Since some patients were under observation for longer periods, we performed a similar analysis over 4 years. Only 4 APOE ϵ4 carriers and 13 noncarriers were observed to the end of this period. However, if the missing data are completed by carrying the last observation forward, the trend to progression continued in the APOE ϵ4 carriers (EDSS score = 4.30 ± 0.64) whereas the non–APOE ϵ4 group remained relatively stable (EDSS score = 2.91 ± 0.33).

During the first 2 years of follow-up, the non–APOE ϵ4 carriers had 12 exacerbations (2 of which were recurrent), which was proportionally similar to the 3 exacerbations observed in carriers. Mean EDSS scores before, at the peak, and at the resolution of the exacerbation, respectively, were 3.67 ± 1.30, 4.67 ± 1.30, and 4.50 ± 1.26 in the ϵ4 group compared with 2.00 ± 0.54, 3.37 ± 0.44, and 2.04 ± 0.52 in the non-ϵ4 group. Analysis by repeated-measures ANOVA of the EDSS scores before and at the peak of the exacerbation did not reveal any significant difference between the groups. However, similar analysis of the EDSS scores during the peak of the exacerbation and at resolution, revealed a borderline significant interaction of group with the repeated measure (P = .049, 1 tailed). Thus, although the groups did not differ in the severity of the relapses, there was some indication of impaired recovery in the APOE ϵ4 carriers.

Our results implicate the APOE ϵ4 allele as a factor determining the accumulation of disability in MS, although it is not associated with a higher risk of developing the disease itself. The latter finding is in accord with previous studies, which did not find this allele to be a risk factor for the development of MS.^12,13 To our knowledge, the present study is the first to link the APOE ϵ4 allele and progression of disability in MS. Although the assay used in the present study did not identify ϵ2 alleles, the expected number of these alleles is less than 5% and there is no evidence that they influence neuronal repair.

The effect of the APOE ϵ4 allele was statistically significant despite the groups being relatively small. This effect was consistently found regardless of whether data were analyzed by comparing group means or by a nonparametric analysis of disease progression. The slightly shorter duration of illness with higher EDSS scores at entry in the APOE ϵ4 group are also compatible with a worse prognosis in this group. To offset the effect of higher EDSS scores of the APOE ϵ4 group at entry into the study we analyzed the results omitting the APOE ϵ4 patient with the highest EDSS score at entry (6.0) and the 6 non-ϵ4 patients with the lowest EDSS score at entry (1.0). This resulted in slightly higher entry mean EDSS scores in the non-ϵ4 group (2.94 ± 0.26 vs 2.78 ± 0.34); nevertheless, the ϵ4 patients were still found to progress significantly more over time (P = .02 by repeated-measures ANOVA). If only patients with EDSS scores between 1.5 and 5.5 are included in an alternate analysis, the resulting mean EDSS scores at entry were 2.78 ± 0.34 in the ϵ4 group (n = 9) and 2.61 ± 0.20 in the non-ϵ4 group (n = 28), and the ϵ4 carriers were still found to progress significantly more over time (P = .04). In addition, what data there are from the few relapses are compatible with impairment in recovery in the APOE ϵ4 carriers. It is important to note that the data set for the smaller group of APOE ϵ4 carriers was complete for the main observation period of 2 years. Interestingly, the single homozygous patient did not experience disease progression during this period, and had she been excluded from the analysis, the statistical significance of the results would have been enhanced.

Following acute MS exacerbations, recovery results from several factors, primarily resolution of edema. Over the long term, however, accumulated disability may result from other factors such as residual extensive demyelination after exacerbations and primary axonal damage. Further studies are needed to investigate the correlation of APOE with specific mechanisms involved in MS progression. The available data indicate that the association between APOE and disability in MS could be due to many factors, including modulation of immune processes and preservation of neuronal function and repair capacity following demyelination. Although there is some evidence for a differential immunomodulatory role for the different apoE types, there is far more evidence supporting their differing effects on repair, especially that associated with lipid metabolism.¹⁴ Specifically linking apoE and demyelination are the observations that oligodendroglia are most involved in producing apoE in the injured optic nerve,¹⁵ that apoE is released in segmental demyelination of the sciatic nerve,¹⁶ and that cerebrospinal fluid levels of apoE correlate with MS exacerbations.¹⁷

A significant effect of APOE genotype on functional disability in MS is compatible with its effect in other neurodegenerative diseases. Clinical evidence links the APOE ϵ4 allele with earlier age at onset of frontotemporal dementia,¹⁸ faster decline of cognitive performance in persons with Down syndrome,¹⁹ worse prognosis of motor neuron disease,²⁰ and cognitive decline following cardiac surgery.²¹ Molecular evidence indicates that apoE ϵ4 is associated with impaired regeneration,⁷ is internalized into neurons less than apoE ϵ3,²² and is associated with reduced neurite outgrowth in cultured dorsal root ganglia²³ and neuroblastoma cells.²⁴ These properties are of special relevance in view of the axonal degeneration reported in MS.^25,26 Interestingly, one of the proteins that accumulates during axonal damage in MS is the β-amyloid precursor protein,²⁷ which is known to interact with apoE in Alzheimer disease. The factors that influence the extent of axonal injury in MS may include neurotrophic factors such as apoE.^3,4 The extent of axonal injury may, in turn, determine the progression of disability in the disease.

The immunomodulatory effects of apoE include suppression of tumor necrosis factor α secretion by glial cells²⁸ and genotype-specific modulation of microglial activation.²⁹ In the present study, however, there was no evidence that APOE ϵ4 is associated with a higher incidence of MS, more frequent relapses, or severity of relapses. These findings do not support a major role of APOE genotype in the regulation of the inflammatory immune process in MS. It is interesting to note that the progression of disability in the APOE ϵ4 carriers was not associated with documented exacerbations but rather to insidious deterioration. The APOE ϵ4 allele may therefore be associated with a propensity to transform the course of MS from a relapsing-remitting to a secondary progressive form.

Evidently, the observation period in the present study was relatively short and the data must be viewed in the perspective of a chronic disease lasting for decades. We are currently examining the hypothesis that the proportion of APOE ϵ4 carriers is higher in progressive forms of MS than in patients with a long-standing benign course. All the patients in the present study were white and were receiving copolymer 1 (glatiramer acetate), which raises the possibility of an interaction between APOE and these factors. It is theoretically possible that the results are not due to a direct effect of APOE status on the natural history of the disease but rather are an interaction with the copolymer 1 treatment. However, we consider this possibility unlikely since the effects of copolymer 1 on disability over 2 years in the largest published study were marginal.³⁰ Although of some interest, a comparative study on untreated patients followed up for long periods is both impractical and unethical.

The results of this study are based on a relatively small number of patients over a relatively short period, and should be considered preliminary. If confirmed, there are several potential implications for the effect of APOE genotype on disease progression in MS. It may be useful as a prognostic marker in counseling patients and as a variable in the analysis and comparison of clinical studies. It may explain some of the variance in disease progression between different clinical studies. The data are compatible with an effect of APOE genotype on neuronal protection or repair, thus supporting a possible role for APOE genotypes in central nervous system disease.

Accepted for publication April 16, 1999.

This study was supported by the Sieratzki Chair of Neurology, Tel Aviv University, Tel Aviv, Israel; the Miriam Turjanski de Gold and Dr Roberto Gold Fund for Neurological Research, Buenos Aires, Argentina; and the Streifler Foundation, Tel Aviv.

This study was presented in part at the annual meeting of the American Academy of Neurology, Minneapolis, Minn, April 29, 1998.

Reprints: Amos D. Korczyn, MD, MSc, Sieratzki Chair of Neurology, Tel Aviv University, Ramat Aviv 69978, Israel (e-mail: neuro13@ccsg.tau.ac.il).