An international group of scientists meeting in Washington called on Thursday for what would, in effect, be a moratorium on making inheritable changes to the human genome.  The group said it would be “irresponsible to proceed” until the risks could be better assessed and until there was “broad societal consensus about the appropriateness” of any proposed change. The group also held open the possibility for such work to proceed in the future by saying that as knowledge advances, the issue of making permanent changes to the human genome “should be revisited on a regular basis.”  The meeting was convened by the National Academy of Sciences of the United States, the Institute of Medicine, the Chinese Academy of Sciences and the Royal Society of London. The academies have no regulatory power, but their moral authority on this issue seems very likely to be accepted by scientists in most or all countries. Similar restraints proposed in 1975 on an earlier form of gene manipulation by an international scientific meeting in California were observed by the world’s scientists.  “The overriding question is when, if ever, we will want to use gene editing to change human inheritance,” David Baltimore said in opening the conference this week. The participation of the Chinese Academy of Sciences is a notable achievement for the organizers of the meeting, led by Dr. Baltimore, former president of the California Institute of Technology, given that earlier in the year Chinese scientists seemed to be racing ahead independently toward clinical alterations to the human germline.  The meeting was prompted by a new genetic technique, invented three years ago, that enables DNA to be edited with unprecedented ease and precision. The technique, known as Crispr-Cas9 and now widely accessible, would allow physicians to alter the human germline, which includes the eggs and the sperm, to cure genetic disease or even enhance desirable physical or mental traits.  Unlike gene therapy, an accepted medical technique that alters the body’s ordinary tissues, editorial changes made to the human germline would be inherited by the patient’s children and thus contribute permanent changes to the human gene pool. These, if sufficiently extensive, might, in principle, alter the nature of the human species.  For decades, the ability to make changes that could be inherited in the human genome has been viewed as a fateful decision — but one that could be postponed because there was no safe and efficient way to edit the genome. The Crispr-Cas9 technique has suddenly made it possible to cross this Rubicon, and the long theoretical issue now requires practical decisions.  Some biologists think inheritable alterations to the human genome should be indefinitely prohibited. Others believe the science behind the technology should be pursued as vigorously as possible.  Scientists first raised the issue for public discussion in March this year, calling for an international meeting to recommend appropriate policies.  So far only one such experiment on the human germline has been reported by researchers at Sun Yat-sen University in Guangzhou, China. The experiment, to alter human embryos, was ethically defensible, given that the embryos all carried a chromosomal defect that made them unviable, and it provided a salutary caution in that almost everything went wrong that could have gone wrong. In particular, the gene-editing technique cut the genome at many unintended sites.  Several speakers at the conference laid out plans for correcting various hereditary diseases in the egg or sperm by cutting and pasting the correct DNA sequence into errant genes.  But others noted there was no pressing medical demand now for making heritable changes to the human genome because diseases caused by a single errant gene were rare. Inheritable gene editing is inapplicable to the common diseases like cancer or diabetes in which the hereditary component is caused by many different genes.  Even in the case of single-gene defects, known as Mendelian diseases, germline editing is unnecessary in many cases because parents can produce an unaffected child through in vitro fertilization in which, after genetic screening, only healthy embryos are implanted in the womb.  But in some cases, such as when a parent has two copies of a genetically dominant disease like Huntington’s, editing the human germline may be the only way to ensure that a child will not also have the disease. Dr. George Q. Daley, of Boston Children’s Hospital, described several such conditions in which germline alteration would be medically effective, while casting doubt on the feasibility of designer babies, a proposed outcome of germline editing.  Many desired human traits, such as intelligence, are governed by a chorus of genes acting in concert. Often genes have more than one effect, and these secondary effects are in most cases unknown, so that altering a large number of genes would incur undetermined hazards, Dr. Daley noted.  “If we are going to view certain applications of human genome editing as permissible, can we draw a line and not throw out legitimate medical applications in order to stave off those that are less palatable to most of us?” Dr. Daley asked.  Other scientists suggested that the possible risks of human genome editing would be rapidly reduced as the techniques were refined. “Many of these technologies are improving so fast it’s hard to measure,” said George Church, a professor of genetics at the Harvard Medical School. Erroneous cuts made by Crispr-Cas9 can now be reduced to less than one per three trillion base pairs of DNA; the human genome is three billion base pairs in length.  “The concept of a ban on gene editing does not make sense,” Dr. Church wrote in the current issue of the journal Nature. “Banning human germline editing could put a damper on the best medical research and instead drive the practice underground to black markets and uncontrolled medical tourism.”  The three national academies also called for a continuing forum in which potential uses of germline editing can be discussed, with the hope of helping nations coordinate their policies.  “Everything I’ve learned here says we’re not ready to be doing this yet, and that we have to perfect the technologies and get a better consensus,” Dr. Baltimore said after the conference. “But I certainly don’t want to close down any avenues of research. The door is open but we don’t want to make any decisions now.”  Dr. Baltimore said he hoped that in reviewing the academies’ decision, “I very much hope that people will understand that this is a major step forward, but like any new technology, it has to be approached with care and thoughtfulness.”  A version of this article appears in print on December 4, 2015, on page A1 of the New York edition with the headline: Scientists Call for Moratorium on Editing of Human Genome. Order Reprints| Today's Paper|Subscribe
The call for a moratorium by China, Britain and the United States comes after the invention of a new technique that eases editing of the human genome.