van Blitterswijk, Marka

Targeting three defects with one strategy The neurodegenerative diseases amyotrophic lateral sclerosis and frontotemporal dementia are most commonly caused by a mutation in the C9orf72 gene. The mutation is an expanded hexanucleotide repeat in a noncoding region. The expanded repeat produces sense and antisense RNA transcripts, which accumulate in patient cells and appear to sequester RNA-binding proteins. The sense and antisense transcripts are also translated into dipeptide repeat proteins, which are aggregation-prone and accumulate in the brain and spinal cord. Last, loss of function from reduced expression of C9orf72 in neurons and glia could contribute to the disease. Kramer et al. targeted both sense and antisense repeats by blocking a single gene called SPT4 , which mitigated degeneration in human cells by reducing all three types of pathologies. Science , this issue p. 708

Background The GGGGCC-repeat expansion in C9orf72 is the most frequent mutation found in patients with amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Most of the studies on C9orf72 have relied on repeat-primed PCR (RP-PCR) methods for detection of the expansions. To investigate the inherent limitations of this technique, we compared methods and results of 14 laboratories. Methods The 14 laboratories genotyped DNA from 78 individuals (diagnosed with ALS or FTD) in a blinded fashion. Eleven laboratories used a combination of amplicon-length analysis and RP-PCR, whereas three laboratories used RP-PCR alone; Southern blotting techniques were used as a reference. Results Using PCR-based techniques, 5 of the 14 laboratories got results in full accordance with the Southern blotting results. Only 50 of the 78 DNA samples got the same genotype result in all 14 laboratories. There was a high degree of false positive and false negative results, and at least one sample could not be genotyped at all in 9 of the 14 laboratories. The mean sensitivity of a combination of amplicon-length analysis and RP-PCR was 95.0% (73.9–100%), and the mean specificity was 98.0% (87.5–100%). Overall, a sensitivity and specificity of more than 95% was observed in only seven laboratories. Conclusions Because of the wide range seen in genotyping results, we recommend using a combination of amplicon-length analysis and RP-PCR as a minimum in a research setting. We propose that Southern blotting techniques should be the gold standard, and be made obligatory in a clinical diagnostic setting.