A Chinese scientist, Jiankui He, last month announced the birth of twin babies whose genes he claims to have modified while they were embryos. On that day, the world woke up to the realization that the age of GMO humans is upon us, with all its troubling ethical implications and complications. The truth is, the science is complicated but the ethics are simple.  
While committed to pressing ahead on this kind of research, the scientific community has denounced Dr. He and declared itself horrified by his recklessness in crossing what has been, up to now, a bioethical bright line. Laypeople are also concerned, and wondering how human dignity and human rights are to be kept at the center of mankind's new ability to permanently alter the human genome in the laboratory. Guidelines must be formulated and laid down in law, and all of us need to be prepared to voice our opinion and demand adherence to the highest principles as this unfolds.
Catholics, thankfully, can turn to the Church for a deeply reasoned approach to the bioethics of human genetic manipulation, one whose "fundamental principle expresses a great "yes" to human life." Around the time that the CRISPR gene-editing technique used by Dr. He appeared on the scientific horizon, the Vatican released Dignitas personae in 2008. This document evaluates, in detail, the ethics of genetic modification, and explains how a technique developed in hopes of relieving human suffering can be used morally. And also how it needs to be prevented from turning into a tool for scientists with a eugenic perspective.
It's important to understand that genetic engineering is aimed at curing genetically-based diseases, and that there are two types of engineering: somatic and germ line. Somatic cell gene therapy seeks to eliminate a genetic defect on cells that are not reproductive, for instance the cells of the pancreas in a person with diabetes. The change would only affect the person treated, not their offspring. Germ line therapy involves changing the DNA of reproductive cells, meaning the changes will be passed down from generation to generation. Somatic therapy is morally licit, but germ line therapy (used by Dr. He to create twin girls) is not. 
First, the effect of gene modification in embryos--both on the subject themselves and their descendants--are completely unknown at this time. For instance, one dream of scientists is to find a way to edit out the gene that causes sickle cell from embryos. But this a gene that is thought to have evolved as a protection against malaria. Removing it may leave the subject, and his or her descendants, more susceptible to infections or blood disorders. There is simply no way of knowing. This coupled with the fact that all embryo modifications are done without the subjects' consent (and the consent of future generations) make germ cell editing a grave abuse of human rights. From Dignitas personae: "..in the present state of research, it is not morally permissible to act in a way that may cause possible harm to the resulting progeny." 
Second, although genetic engineering is touted as a way to eliminate disease, it is also proposed for enhancement purposes. Dr. He, for instance, does not claim to have cured the girls of a disease, but believes he made them resistant to HIV--an enhancement. This application clears the way for a market-based form of eugenics aimed at improving the gene pool. Our modern culture is already too comfortable with eugenics as it is practiced through ultrasound prenatal diagnosis. Today, fetuses found to be "defective" are routinely aborted. Using genetic modification, wealthy parents would have the ability to "enhance" their children with favorable traits. Again from Dignitas personae: "..such manipulation would…lead to indirect social stigma with regard to people who lack certain qualities." The vision of a world divided into biologically superior and biologically average humans is a chilling one. But it's no more chilling than a world in which man claims all-encompassing dominion over human life, choosing and deleting, enhancing and rejecting on the way to "perfection". This is a mad kind of hubris. It refuses to accept human life in its finite nature and rejects an attitude of respect for all people.
Thirdly, genetic modification of germ lines can be done only by creating multiple embryos in a laboratory and discarding most of them. The idea of creating multiple human beings in order to genetically edit them, hoping for one or two successes and subsequently destroying the failures is amazing in its cruelty.  Even those who may be used to the idea of in vitro fertilization as a therapy for infertility are shocked by the process whereby a couple would request the creation of embryo sons and daughters in order to give life to the "best" edited version. Whether done for infertility or genetic enhancement, this is a grave assault on human dignity.
Sounds complicated, and it is. But it's the science that's complicated, not the ethics. And although it's a new and brave world we find ourselves in, we do have, thanks to the Church, a principled way forward on gene therapy. We also have the right, and duty, to demand from lawmakers and scientists that something as significant and morally delicate as the manipulation of the human genome be done using standards that have human dignity and human rights at their very core.