Sequence Homology at the Breakpoint and Clinical Phenotype of Mitochondrial DNA Deletion Syndromes

Abstract

Mitochondrial DNA (mtDNA) deletions are a common cause of mitochondrial disorders. Large mtDNA deletions can lead to a broad spectrum of clinical features with different age of onset, ranging from mild mitochondrial myopathies (MM), progressive external ophthalmoplegia (PEO), and Kearns-Sayre syndrome (KSS), to severe Pearson syndrome. The aim of this study is to investigate the molecular signatures surrounding the deletion breakpoints and their association with the clinical phenotype and age at onset. MtDNA deletions in 67 patients were characterized using array comparative genomic hybridization (aCGH) followed by PCR-sequencing of the deletion junctions. Sequence homology including both perfect and imperfect short repeats flanking the deletion regions were analyzed and correlated with clinical features and patients' age group. In all age groups, there was a significant increase in sequence homology flanking the deletion compared to mtDNA background. The youngest patient group (<6 years old) showed a diffused pattern of deletion distribution in size and locations, with a significantly lower sequence homology flanking the deletion, and the highest percentage of deletion mutant heteroplasmy. The older age groups showed rather discrete pattern of deletions with 44% of all patients over 6 years old carrying the most common 5 kb mtDNA deletion, which was found mostly in muscle specimens (22/41). Only 15% (3/20) of the young patients (<6 years old) carry the 5 kb common deletion, which is usually present in blood rather than muscle. This group of patients predominantly (16 out of 17) exhibit multisystem disorder and/or Pearson syndrome, while older patients had predominantly neuromuscular manifestations including KSS, PEO, and MM. In conclusion, sequence homology at the deletion flanking regions is a consistent feature of mtDNA deletions. Decreased levels of sequence homology and increased levels of deletion mutant heteroplasmy appear to correlate with earlier onset and more severe disease with multisystem involvement.

Mitochondrial Disease in Childhood: mtDNA Encoded

“…In a large referral center, 67 patients with mtDNA deletions and disease were studied. Approximately 40 % of patients had the 5 kilobase deletion, were all older than 6 years, and were mostly only detected from muscle samples [66]. In this same study, in the group <6 year of age, only 15 % had the 5 kilobase deletion.…”

“…In contrast, patients with adult onset mtDNA deletion syndrome, chronic progressive external ophthalmoplegia, are limited phenotypically to neuromuscular and ophthalmologic symptoms [87]. As described previously, the older patients usually have a 3-fold increase in the common 5-kilobase mtDNA deletion involving 13 perfect repeat regions within the mtDNA [66]. It is tempting to think that disease severity and onset is related to deletion size or genes deletion, but this has not been found.…”

“…Large deletions, 5 kilobases in size, have been mapped to identical regions spanning the ATPase8 gene to ND5 gene, typically flanked by a perfect 13-base pair direct nucleotide repeat [66]. In a large referral center, 67 patients with mtDNA deletions and disease were studied.…”

“…The mechanism is unclear, but one clue is that almost all deletions are somatic and not maternally inherited [86]. One hypothesis is that deletions found in younger patients arise from maternal germ line mutational events or during early embryogenesis, where the deletion enters into all 3 germ layers; in others, deletions occur in later random somatic events [66]. This would be consistent with the severe multisystemic findings of Pearson syndrome and later-aged onset of KSS, and adult onset of chronic progressive external ophthalmoplegia.…”

“…The problem of variation of heteroplasmy levels between tissues, as well as changes within particular tissues over time, can make diagnosis difficult [118]. As noted previously, invasively obtained muscle tissue may be superior in consistency level of heteroplasmy compared to blood, which is especially true for KSS compared to Pearson syndrome [66]. The investigation of the m.3243A > G mutation has demonstrated that this mutation decreases in heteroplasmy over time, but clinical symptoms and heteroplasmy levels are most accurately measured in urinary epithelial cells compared to leukocytes and buccal mucosa cells [119].…”