[Common and rare variants, polygenic traits and missing heritability]

Recently, a systematic (but limited) search for rare variants implicated in adult height, a highly polygenic trait, has uncovered a number of new variants for which the effect size is inversely correlated with the minor allele frequency. This opens interesting perspectives on the genetic architecture of complex traits and on the vexing problem of "missing heritability".

Genome editing: the road of CRISPR/Cas9 from bench to clinic

Molecular scissors engineered for site-specific modification of the genome hold great promise for effective functional analyses of genes, genomes and epigenomes and could improve our understanding of the molecular underpinnings of disease states and facilitate novel therapeutic applications. Several platforms for molecular scissors that enable targeted genome engineering have been developed, including zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and, most recently, clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated-9 (Cas9). The CRISPR/Cas9 system's simplicity, facile engineering and amenability to multiplexing make it the system of choice for many applications. CRISPR/Cas9 has been used to generate disease models to study genetic diseases. Improvements are urgently needed for various aspects of the CRISPR/Cas9 system, including the system's precision, delivery and control over the outcome of the repair process. Here, we discuss the current status of genome engineering and its implications for the future of biological research and gene therapy.

[The CRISPR case, « ready-made » mutations and Lamarckian evolution of an adaptive immunity system]

Since genetics has shown that mutation predates selection, biology has developed within the Darwinian paradigm framework. However, a mechanism that produces favorable mutations preferentially in response to adaptive constraints has been recently identified. This mechanism, the CRISPR-Cas adaptive immunity system, is considered as a bona fide example of Lamarckian evolution, even if it only reflects loosely Lamarck's ideas. This unusual evolutionary process is made possible by two prokaryotic properties: i) somatic and germinal cells are not distinct sets of cells; ii) Archae and Bacteria very frequently integrate DNA fragments from the environment, and they therefore have access to a source of "ready-made" useful genetic information. The CRISPR-Cas is a defense system against viruses and plasmids that is based on the integration of genomic fragments of these infectious agents into the host genome, and that protects the host against subsequent infections. Therefore, this mechanism does produce advantageous mutations by integrating DNA from the environment and allowing its transmission to descendants. In conclusion, most of the time evolution relies on purely Darwinian processes, i.e. mutations occurring at random, but in a small minority of cases the occurrence of mutations is more or less biased, and is therefore more or less Lamarckian. Although they are rare, such processes are nevertheless important to our understanding of the plurality of modes of evolution.

[The CRISPR system can correct or modify the expression of genes responsible for hereditary diseases]

A new technology, called CRISPR, derived from the immune system of bacteria, uses a Cas9 nuclease and a guided RNA complementary to a 20 nucleotides sequence of a gene to induce double strand DNA breaks. This permits to modify specifically the targeted gene in plant, animal and human cells. Variants of the technique also permit to reduce or increase the expression of a selected gene. This technology may thus be used not only to understand the role of a gene but also to develop therapies for hereditary and acquired diseases.