We describe the range of retinal, peripheral and central nervous system disease expression in a single family with NARP syndrome from the ATPase 6 m.8993T>C mtDNA point mutation. Even amongst family members with the greatest degrees of ectodermal and mesodermal heteroplasmy, there was great variability in tissue types affected and severity of injury within those tissues.
All subjects in our series had greater degrees of hair-bulb heteroplasmy than blood leukocyte heteroplasmy, but the overall trends were similar. Family members with the greatest degrees of hair-bulb and leukocyte heteroplasmy suffered the severest neurological and ophthalmological deficits, but neither hair-bulb nor leukocyte heteroplasmy uniformly predicted which tissues would be affected in a given individual or the severity of deficits within a given tissue (Table ).
There were analogous patterns of tissue injury on high-resolution retinal and brain imaging characterized by heterogeneous, patchy areas of neuronal loss (Figs. c, d, and d, e). With 3-T MRI, we were able to appreciate that the basal ganglia hyperintensities typical of NARP-MILS spectrum disease [12
] are not homogeneous, but rather consist of multiple, small cystic and cavitary lesions. These basal ganglia hyperintensities likely reflect discrete foci of tissue loss from mitochondria-related energy failure [13
]. Since basal ganglia necrosis is also a defining feature of MILS, the prominence of this finding in our study reinforces the concept that NARP and MILS reflect overlapping phenotypes of ATPase 6 dysfunction [3
We observed similar patterns of heterogeneous, focal and destructive injury to neuronal layers of the inner and outer retina. On SD-OCT, the most prominent abnormality was thinning of the photoreceptor (RPE) layer with scattered areas of severe neuronal loss causing a disruption to normal retinal architecture. There was also associated thinning of the retinal nerve fiber and ganglion cell layers in proportion to the degree of RPE injury. Since RNFL thinning is common in other types of retinitis pigmentosa [25
] and the severest injury was to the outer retina, we suspect that the primary retinal injury in NARP syndrome is to energy-dependent photoreceptors followed by secondary transsynaptic degeneration of neighboring retinal layers [26
]. The abnormalities observed on SD-OCT and AOSLO are also consistent with findings on retinal pathology from a child who died from MILS from the m.8993T>G mutation [27
], and illustrate how high-resolution retinal imaging can detect neurodegenerative injury in a mitochondrial disorder.
Our study demonstrates that cognitive impairment in NARP-MILS is characterized by selective dysfunction of specific functional domains, especially in information processing speed, visual-spatial copy and memory and verbal fluency. The visual-spatial deficits were independent of the degree of primary visual loss from RP (see Fig. ). Visual-spatial and executive function impairment are characteristic of other mitochondrial cytopathies, such as chronic progressive external ophthalmoplegia and Kearns-Sayre syndrome, and the localization of these deficits has been attributed to the prefrontal, parietal and occipital cortex [28
]. However, cerebellar dysfunction also causes deficits in executive functioning, verbal fluency, memory and visual-spatial processing [30
], and striatal networks also modulate many cognitive processes, including language, and executive and visual-spatial functioning [31
]. Given the extensive cerebellar and basal ganglia injury seen in NARP syndrome with relative sparing of cortical structures, we propose that some of the cognitive deficits characteristic of NARP-MILS spectrum disease may be due to cerebellar and basal ganglia dysfunction. Our results also suggest that the mini-mental state examination is not an adequate screening test for cognitive dysfunction in NARP syndrome, and that clinicians should pursue more sensitive evaluations focused on information processing speed, verbal fluency and visual-spatial function.
Two main factors argue against depression confounding the neuropsychological testing results [33
]. First, all subjects showed similar patterns of deficits, despite variability in affective symptoms. Second, verbal and working memory tend to be impaired when depression causes cognitive deficits [34
], but these domains were not significantly affected in this family, even in those subjects with the severest depression. Depression has not traditionally been recognized as part of the NARP-MILS phenotypic spectrum, but the prominence of affective symptoms in this family raises the question as to whether mitochondrial dysfunction may be involved in its pathogenesis [35
]. Depression is common in mitochondrial cytopathies [36
], and is also prominent in other neurodegenerative disorders that affect striatal networks, including Huntington’s and Parkinson’s diseases [38
]. In our analysis, however, we were not able to account for other factors that might have influenced depression risk in this family, including the history of bipolar disease in the father of D1–D4 or shared environmental exposures. More research is needed to explore this possible association between depression and mitochondrial disease further.
There are several limitations to this study. We characterized disease expression in a single family, which risks overemphasizing shared autosomal or environmental factors that can influence mitochondrial disease expression or confound phenotypes. It is also difficult in this kind of analysis to account for the effects of age on mitochondrial disease progression. We also did not measure heteroplasmy using the urinary epithelium, which has been recently reported to predict neurological involvement in MELAS [40
]. Nevertheless, we believe that this study adds to our understanding of the range of phenotypic expression seen in this prototypical mitochondrial neurodegenerative disorder.
In summary, this study characterized patterns of disease expression in NARP syndrome from the m.8993T>C ATPase 6 mtDNA mutation and illustrated how neurodegeneration in the retina, brain and peripheral nervous system can share common mechanisms.