The proband suffered subacute loss of vision bilaterally at age 12, after a febrile illness, without recovery. We observed this patient at age 16, three months after the acute onset of recurrent vomiting and vertigo. Neurological examination at admission showed horizontal nystagmus, segmental and parcellar myoclonic jerks at upper and lower limbs, slight dysmetria at the right upper limb, weak deep tendon reflexes, positive Romberg sign and mild ataxic gait.Brain magnetic resonance imaging (MRI) revealed bilateral lesions of the brainstem involving the vestibular nuclei (Figure
1B, upper row). Moreover, MRI showed atrophy of the optic nerves bilaterally with hyperintense signal in the T2-weighted images with enhancement after gadolinium administration (not shown). Serum lactic acid after standardized exercise was abnormally elevated (3.1 mM, n.v. 1–2 mM). Ophthalmologic evaluation showed bilateral temporal pallor, visual acuity (a measure of central vision function) was 0.2 bilaterally and visual fields demonstrated bilateral cecocentral scotomas, more severe in OD (Figure
1C). Optical coherence tomography evaluating retinal nerve fiber layer thickness showed bilateral diffuse optic atrophy with nasal sparing (Figure
1D). Cognitive functions assessment was normal, as well as audiometry and electroencephalography. A therapy with high dose idebenone (540 mg/day) was started with improvement of vomiting and vertigo and visual function after about two months.Follow-up evaluation after eight months from therapy start showed an improvement of visual acuity in OS (0.4) and a slight improvement of visual field defect in both eyes (mean deviation from −18 to −15 in OD and −16 to −15 in OS) (Figure
1C). Lactic acid after exercise was slightly worse compared to the baseline evaluation (3.9 mM), but the patient reported intense physical training in the previous month. Brain MRI follow-up showed a complete resolution of vestibular nuclei lesions (Figure
1B, middle row), and the appearance of a new lesion affecting the left tuberculus quadrigeminus (Figure
1B, lower row).
We first surveyed the proband for the three common LHON mutations, which were absent, then his entire mtDNA was sequenced (GenBank accession number KF927040). The mitogenome was characterized by the diagnostic mutational motif of haplogroup H (
http://www.phylotree.org/). In addition, it also harbored the m.4171C>A/
MT-ND1 transversion relative to the revised Cambridge Reference Sequence, [
6] which causes the p.L289M amino acid change and was previously reported in association only with a “pure” LHON phenotype, and four transitions: the synonymous m.3798C>T/
MT-ND1 and the three non-synonymous changes m.4705T>C/
MT-ND2, m.5263C>T/
MT-ND2 and m.14180T>C/
MT-ND6, all affecting complex I. Restriction fragment length polymorphism (RFLP) analysis of the proband, one unaffected sister and the mother (Figure
1A) on DNA extracted from both blood cells and urinary epithelium sediment confirmed that the m.4171C>A/
MT-ND1 transversion was consistently homoplasmic. As for the three non-synonymous changes, they cause the amino acid substitutions p.M79T and p.A265V in ND2, and p.Y165C in ND6, but they all have been reported as diagnostic markers for some ancient continent-specific haplogroups or sub-haplogroups: m.4705T>C/
MT-ND2 for sub-haplogroup F1b1a1a1, m.5263C>T/
MT-ND2 for sub-haplogroups V1a1, U5a1d1, F4, B4a1c3a, M29a, M2b1b, G1a1a3, L2a5, and m.14180T>C/
MT-ND6 for sub-haplogroups T2b26, D4b1a1a, L2a1l2a (
http://www.phylotree.org/). Finally, all three amino acid residues are moderately conserved [
7] and
in silico analysis predicted a possibly pathogenic role for the m.14180T>C/
MT-ND6 and m.4705T>C/
MT-ND2 variants (Table
1). RFLP survey of these three mutations confirmed that they were homoplasmic in the mother and one unaffected sister. We also extended the conservation analysis on the protein regions surrounding these polymorphisms, according to a previously established method [
8]. All three changes had an invariant amino acid within four contiguous amino acidic positions (+4/−4 aa) and they hit a protein region with a local conservation higher than global conservation of the protein subunit (Table
1). Finally, we compared the mtDNA background of our proband with those of the previous cases reported with the m.4171C>A/
MT-ND1 transversion (Table
2) [
8‐
11]. Only one pedigree from China, characterized by almost complete penetrance, displayed the co-occurrence of possibly synergistic complex I variants [
11].
Table 1
Conservation analysis of private variants and contiguous amino acid residues
Locus
|
MT-ND2
|
MT-ND2
|
MT-ND6
|
AA change
| p.M79T | p.A265V | p.Y165C |
Conservation (%)
|
Eukaryotes
| 25 (n = 161) | 48 (n = 161) | 36 (n = 124) |
Vertebrates
| 3 (n = 126) | 57 (n = 126) | 54 (n = 83) |
Mammals
| 41 (n = 95) | 75 (n = 95) | 26 (n = 43) |
Prediction
|
Polyphen 2
| Benign | Benign | Probably damaging |
Provean
| Deleterious | Neutral | Deleterious |
Contiguous amino acid conservation
|
Interval (−10/+10)
| 69-89 | 255-275 | 155-174 |
Local conservation (%)
| 68 | 83 | 69 |
Global conservation (%)
| 62 | 62 | 52 |
Nearest invariant position (−/+)
| −1/+2 | −1/+1 | −1/+4 |
Table 2
Non-synonymous nucleotide changes reported in published mtDNAs with m.4171C>A
1 | Korea | A |
4171A, 4824, 8794, 14766 | |
2 | Korea | A |
4171A
| |
3 | China | n.a. | n.a. | |
4 | China | N9a1 |
4171A, 10203
b,,12358, 14564
b, 14766, 14841
c
, 15095 | |
5 | France | J2b1 |
4171A, 4216, 7632
c, 10398, 13708, 14766, 15257, 15452A, 15812 | |
6 | Belarus | H |
4171A, 4705
b, 5263
b, 14180
b
| Present study |