Whitehead researchers have developed a new type of genetic² screen for human cells to pinpoint³ specific genes⁴ and proteins used by pathogens（病原体）, according to their paper in Science. In most human cell cultures genes are present in two copies: one inherited from the father and one from the mother. Gene¹ inactivation⁵（钝化作用） by mutation⁶ is therefore inefficient⁷ because when one copy is inactivated⁸, the second copy usually remains⁹ active and takes over.

In yeast¹⁰（酵母）, researchers have it easier: they use yeast cells in which all genes are present in only one copy (haploid单倍体 yeast). Now Carette and co-workers have used a similar approach and used a human cell line, in which nearly all human chromosomes¹¹ are present in a single copy.

In this rare cell line, Carette and co-workers generated mutations in almost all human genes and used this collection to screen for the host genes used by pathogens. By exposing those cells to influenza¹²（流感） or to various bacterial¹³ toxins¹⁵（毒素）, the authors isolated¹⁶ mutants that were resistant¹⁷ to them. Carette then identified the mutated genes in the surviving cells, which code for a transporter molecule¹⁸ and an enzyme¹⁹（酶） that the influenza virus hijacks²⁰ to take over cells.

Working with Carla Guimaraes from Whitehead Member Hidde Ploegh's lab, Carette subjected knockout cells to several bacterial toxins to identify resistant cells and therefore the genes responsible.

The experiments identified a previously²¹ uncharacterized gene as essential for intoxication²² by diphtheria toxin¹⁴（白喉毒素） and exotoxin（外毒素） A toxicity²³, and a cell surface protein needed for cytolethal distending²⁴ toxin toxicity.

"We were surprised by the clarity of the results," says Jan Carette, a postdoctoral（博士后的） researcher in the Brummelkamp lab and first author on the Science article. "They allowed us to identify new genes and proteins involved in infectious processes that have been studied for decades, like diphtheria and the flu. In addition we found the first human genes essential for host-pathogen interactions where few details are known, as is the case for cytolethal distending toxin secreted²⁵ by certain strains of E. coli（大肠杆菌）. This could be important for rapidly responding to newly emerging pathogens or to study pathogen biology that has been difficult to study experimentally."

Brummelkamp sees the work as only the beginning.

"Having knockout cells for almost all human genes in our freezer opens up a wealth of biological questions that we can look at," he says. "In addition to many aspects of cell biology that can be studied, knockout screens could also be used to unravel²⁶ molecular²⁷ networks that are exploited by a battery of different viruses and bacteria."