Improving CRISPR-Cas9 directed faithful transgene integration outcomes by reducing unwanted random DNA integration

BACKGROUND

The field of genome editing has been revolutionized by the development of an easily programmable editing tool, the CRISPR-Cas9. Despite its promise, off-target activity of Cas9 posed a great disadvantage for genome editing purposes by causing DNA double strand breaks at off-target locations and causing unwanted editing outcomes. Furthermore, for gene integration applications, which introduce transgene sequences, integration of transgenes to off-target sites could be harmful, hard to detect, and reduce faithful genome editing efficiency.

METHOD

Here we report the development of a multicolour fluorescence assay for studying CRISPR-Cas9-directed gene integration at an endogenous locus in human cell lines. We examine genetic integration of reporter genes in transiently transfected cells as well as puromycin-selected stable cell lines to determine the fidelity of multiple CRISPR-Cas9 strategies.

RESULT

We found that there is a high occurrence of unwanted DNA integration which tarnished faithful knock-in efficiency. Integration outcomes are influenced by the type of DNA DSBs, donor design, the use of enhanced specificity Cas9 variants, with S-phase regulated Cas9 activity. Moreover, restricting Cas9 expression with a self-cleaving system greatly improves knock-in outcomes by substantially reducing the percentage of cells with unwanted DNA integration.

CONCLUSION

Our results highlight the need for a more stringent assessment of CRISPR-Cas9-mediated knock-in outcomes, and the importance of careful strategy design to maximise efficient and faithful transgene integration.

Molecular and functional correction of a deep intronic splicing mutation in CFTR by CRISPR-Cas9 gene editing

Cystic fibrosis (CF) is an autosomal recessive disorder caused by mutations in the CFTR gene. The 10th most common mutation, c.3178-2477C>T (3849+10kb C>T), involves a cryptic, intronic splice site. This mutation was corrected in CF primary cells homozygous for this mutation by delivering pairs of guide RNAs (gRNAs) with Cas9 protein in ribonucleoprotein (RNP) complexes that introduce double-strand breaks to flanking sites to excise the 3849+10kb C>T mutation, followed by DNA repair by the non-homologous end-joining pathway, which functions in all cells of the airway epithelium. RNP complexes were delivered to CF basal epithelial cell by a non-viral, receptor-targeted nanocomplex comprising a formulation of targeting peptides and lipids. Canonical CFTR mRNA splicing was, thus, restored leading to the restoration of CFTR protein expression with concomitant restoration of electrophysiological function in airway epithelial air-liquid interface cultures. Off-target editing was not detected by Sanger sequencing of in silico-selected genomic sites with the highest sequence similarities to the gRNAs, although more sensitive unbiased whole genome sequencing methods would be required for possible translational developments. This approach could potentially be used to correct aberrant splicing signals in several other CF mutations and other genetic disorders where deep-intronic mutations are pathogenic.

Genetic association study of GABRA2 single nucleotide polymorphisms and electroencephalography in alcohol dependence

The gamma aminobutyric acid (GABA) system has been implicated in the susceptibility to develop alcohol dependence and in determining electroencephalogram (EEG) beta activity. The role of the GABA receptor alpha-2 gene (GABRA2) in human alcohol dependence was determined in a genetic and electrophysiological study. The study population comprised 586 white UK individuals with alcohol dependence but a very low prevalence of co-morbid drug dependence, and 603 ancestrally matched healthy controls. Genotyping for seven GABRA2 single nucleotide polymorphisms (SNPs), identified from the literature as positively associated with alcohol dependence, was performed with success rates of 90% or greater. EEGs were available in 32 selected patients who had been abstinent from alcohol for a minimum of 24 months and in 138 ancestrally matched healthy controls. None of the SNPs showed allelic or haplotypic association with alcohol dependence. All markers were in Hardy Weinberg equilibrium (HWE) in the controls. HWE for marker rs279841 in the alcohol dependent sample was p=0.0199 and combined p=0.0166. Linkage disequilibrium patterns appear to be very similar to that observed in the HapMap CEU data. A significantly higher prevalence of excess EEG fast activity was found in the patients (31 vs. 14%, p=0.018). A significant relationship was found between the presence of excess EEG fast activity and GABRA2 SNPs rs548583, rs279871 and rs279841. This allelic association study provides no evidence for an association between GABRA2 polymorphisms and alcohol dependence. However, a significant relationship was identified between GABRA2 and excess EEG fast activity. This dissociation of effect may reflect the fact that the EEG is a more direct marker of phenotypic GABRA2 expression than the more heterogeneous alcohol dependence phenotype.

Alcohol consumption and heavy drinking: a survey in three Italian villages

AIMS

We investigated drinking habits, and heavy and problem drinking prevalence in a sample of individuals attending the consulting rooms of local General Practitioners in three Italian villages.

METHODS

The samples were selected to be representative of the entire population of the three villages. Information on alcohol-drinking patterns was collected using a questionnaire that included a masked form of the CAGE rating scale. According to the frequency of alcohol intake, subjects were grouped in three categories: abstainers, occasional drinkers, and daily drinkers. In agreement with WHO guidelines, 40 g/day for males and 20 g/day for females were taken as cut-off for 'heavy drinking' and consumptions of > 80 g/day for males and 40 g/day for females were used to define 'problem drinking'.

RESULTS

A total of 2972 individuals were included in the survey (19% of the population). Of these, 44% were abstinent, 20% occasional drinkers, and 36% daily drinkers. Daily drinking was found to be more common in males than in females but heavy drinking was significantly higher in females compared with males (P > or = 0.0001). The problem drinkers were 12% of the entire population and the CAGE-positive subjects (> or = 2 positive answers) were 3.5%.

CONCLUSIONS

Our data indicate that alcohol drinking is widely diffused in the three communities. A large layer of the population drinks above the WHO-established cut-off. The incidence of heavy and problem drinking seems to have significant gender and regional differences that are important to consider when planning effective prevention programmes.