Autosomal dominant optic atrophy type 1 (ADOA1, MIM#165500), also known as Kjer-type optic atrophy [
1,
2], is the most frequent form of inherited optic neuropathies (ION), with a prevalence ranging from 1:10,000 [
3,
4] in Denmark to 1:50,000–30,000 [
3‐
6] worldwide. ADOA is typically characterized by childhood-onset insidious moderate to severe progressive bilateral visual loss, color vision deficits, centrocecal scotoma, temporal optic nerve pallor and macular ganglion cell loss [
7], which often leads to legal blindness [
8‐
10]. Although ADOA is genetically heterogeneous [
11‐
13], the vast majority of ADOA patients (approx. 75%) harbor a mutation in the
OPA1 gene, less frequent mutations in
OPA3 and
WFS1 are observed in ADOA patients.
OPA1 codes for a mitochondrial dynamin-like GTPase [
14‐
16], which is an inner membrane protein targeted to mitochondria by a highly basic N-terminal import sequence and is anchored to mitochondrial cristae where it faces the intermembrane space [
17].
OPA1 mRNA is 6492 bp (largest isoform) in length and has been mapped to chromosome 3q28-q29 [
15,
18,
19].
OPA1 consists of 31 exons of which the last one is non-coding [
15,
19]. Through alternative splicing of exon 4, 4b and 5b, eight different mRNA isoforms with different expression patterns in different tissues can be produced [
20]. The OPA1 protein of the main isoform counts 960 amino acids and contains 5 different domains, the N-terminal hydrophobic mitochondrial targeting domain, a N-terminal coiled-coil domain, a GTPase domain, a dynamin central region and a proposed GTPase effector domain and the C-terminal coiled-coil domain [
14,
21].
OPA1 is ubiquitously expressed in human tissues with the highest levels found in retinal ganglion cells and the brain [
20,
21]. OPA1 is involved in many different processes. It is the key player during mitochondrial inner membrane fusion, it plays a role in mitochondrial DNA, respiratory chain and membrane potential maintenance, cristae organization by controlling their shape and structure and it is further involved in cytochrome c mediated apoptosis [
22‐
25]. More than 340 unique DNA variants have been allocated to
OPA1 (
http://mitodyn.org/home.php?select_db=OPA1) [
26]. Half of these mutations result in haploinsufficiency of
OPA1 by premature termination of translation leading to a truncated protein. Most of these mutations are located in the GTPase domain, whereas no mutation was found in the alternatively spliced exons (4, 4b and 5b). The phenotypic expression of
OPA1 mutations in ADOA shows a high variability between families as well as members of the same family [
27], often with an incomplete penetrance ranging from asymptomatic carriers and mild visual impairment to legally blind individuals [
28]. ADOA can be classified either as a non-syndromic or isolated form, presenting with pure ophthalmologic features or as syndromic form associated with extra-ocular neurological manifestations, known as ADOA plus syndrome [
5,
29]. Around 20% of ADOA patients develop additional neuro-muscular features, often in the third decade of life, which are sometimes present at a subclinical level. These include sensorineural deafness, ataxia, axonal sensory-motor polyneuropathy, mitochondrial myopathy and external ophthalmoplegia, whereas in one case parkinsonism and dementia were reported [
30]. Many different mutations have been identified in
OPA1 responsible for ADOA, including nonsense mutations, missense mutations, frameshift mutations, in-frame deletions and splice site or splice region mutations [
14,
28,
31]. Since the vast majority of
OPA1 mutations are predicted to result in a premature stop codon, mainly caused by splice site or splice region, nonsense or frameshift mutations, the generation of a truncated protein leading to haploinsufficiency is proposed to be the major cause for the pathogenesis of ADOA [
28]. The majority of transcripts containing a premature termination codon (PTC) are degraded by a mechanism called nonsense-mediated decay (NMD), which is used to protect the cell from translation of potentially harmful proteins [
32]. In this study we analyzed all coding exons including the intron-exon boundaries of
OPA1 by DNA sequencing of two unrelated families presenting with clinical features of ADOA. We report two novel splice region mutations in
OPA1, which we characterized at RNA level, thereby expanding the mutational spectrum of
OPA1.