Reversal of C9orf72 mutation-induced transcriptional dysregulation and pathology in cultured human neurons by allele-specific excision.

Efforts to genetically reverse C9orf72 pathology have been hampered by our incomplete understanding of the regulation of this complex locus. We generated five different genomic excisions at the C9orf72 locus in a patient-derived induced pluripotent stem cell (iPSC) line and a non-diseased wild-type (WT) line (11 total isogenic lines), and examined gene expression and pathological hallmarks of C9 frontotemporal dementia/amyotrophic lateral sclerosis in motor neurons differentiated from these lines. Comparing the excisions in these isogenic series removed the confounding effects of different genomic backgrounds and allowed us to probe the effects of specific genomic changes. A coding single nucleotide polymorphism in the patient cell line allowed us to distinguish transcripts from the normal vs. mutant allele. Using digital droplet PCR (ddPCR), we determined that transcription from the mutant allele is upregulated at least 10-fold, and that sense transcription is independently regulated from each allele. Surprisingly, excision of the WT allele increased pathologic dipeptide repeat poly-GP expression from the mutant allele. Importantly, a single allele was sufficient to supply a normal amount of protein, suggesting that the C9orf72 gene is haplo-sufficient in induced motor neurons. Excision of the mutant repeat expansion reverted all pathology (RNA abnormalities, dipeptide repeat production, and TDP-43 pathology) and improved electrophysiological function, whereas silencing sense expression did not eliminate all dipeptide repeat proteins, presumably because of the antisense expression. These data increase our understanding of C9orf72 gene regulation and inform gene therapy approaches, including antisense oligonucleotides (ASOs) and CRISPR gene editing.

Single-Molecule Sequencing of the C9orf72 Repeat Expansion in Patient iPSCs.

A hexanucleotide GGGGCC repeat expansion in the C9orf72 gene is the most frequent genetic cause of amyotrophic lateral sclerosis (ALS) and frontal temporal dementia (FTD). C9orf72 repeat expansions are currently identified with long-range PCR or Southern blot for clinical and research purposes, but these methods lack accuracy and sensitivity. The GC-rich and repetitive content of the region cannot be amplified by PCR, which leads traditional sequencing approaches to fail. We turned instead to PacBio single-molecule sequencing to detect and size the C9orf72 repeat expansion without amplification. We isolated high molecular weight genomic DNA from patient-derived iPSCs of varying repeat lengths and then excised the region containing the C9orf72 repeat expansion from naked DNA with a CRISPR/Cas9 system. We added adapters to the cut ends, capturing the target region for sequencing on PacBio's Sequel, Sequel II, or Sequel IIe. This approach enriches the C9orf72 repeat region without amplification and allows the repeat expansion to be consistently and accurately sized, even for repeats in the thousands. Key features • This protocol is adapted from PacBio's previous "no-amp targeted sequencing utilizing the CRISPR-Cas9 system." • Optimized for sizing C9orf72 repeat expansions in patient-derived iPSCs and applicable to DNA from any cell type, blood, or tissue. • Requires high molecular weight naked DNA. • Compatible with Sequel I and II but not Revio.