CAMBRIDGE, Mass. (AP) - Billionaire philanthropists Eli and Edythe Broad announced Thursday that they're adding $400 million to a joint biomedical venture at Harvard and MIT, saying they want to accelerate research into treatments for a slew of debilitating diseases.

The endowment is on top of $200 million the couple have already given to launch the Broad Institute, which was created in 2004.

The Broads also said that the experimental institute has been working so well they plan to convert it into a permanent research venture.

"Four years later ... we are here to declare that experiment a great success," Eli Broad said. "This country has been good to us. We consider this more than an opportunity, but a responsibility to give back."

The Broads, who live in Los Angeles, made their wealth building two Fortune 500 companies: KB Home, a house building firm, and SunAmerica, Inc., a financial services company that specializes in retirement savings.

The event Thursday brought together the presidents of the rival universities, MIT President Susan Hockfield and Harvard University President Drew Gilpin Faust, both of whom praised the couple and what they said was the spirit of collaboration and innovation fostered by the institute.

"Scientific research must transcend the boundaries of discipline, departments, and even institutions," Faust said.

The Broad Institute was launched in 2004 in the wake the Human Genome Project, which created the first map of the approximately 25,000 human genes in 2003.

The goal of the institute was to bring together scientists and students to take advantage of the information revealed by the Human Genome Project and use it to tackle some of the world's toughest health problems from cancer to infectious, psychiatric, and metabolic diseases.

One of the institute's guiding principles is collaboration. Rather than being established at a single university, the institute spanned both the MIT and Harvard campuses, including the region's 17 Harvard-affiliated teacher hospitals.

The more than 1,200 scientists and staff affiliated with the institute include specialists in a range of disciplines, including chemistry, medicine and computer science.

Gov. Deval Patrick, who has touted the life sciences as a cornerstone of the state's new economy, praised the Broads' investment. Earlier this year Patrick signed a landmark $1 billion, 10-year life sciences bill designed to lure biomedical researchers to the state.

"The life sciences initiatives has helped to spawn interest from all over the world in the supercluster we have here in the commonwealth and that is very, very important for the future of healing and for our own economic growth," he said.

Researchers at the institute have already made significant breakthroughs, including identifying dozens of new genetic risk factors for a diseases like type 2 diabetes, Chrohn's disease, schizophrenia, bipolar disorder, autism, cardiovasular disease and certain cancers.

Institute researchers have combined genomics and chemical biology to uncover potential new therapies for cancer, malaria and other diseases - as well as developing new uses for existing drugs.

The institute is also involved in sequencing the genetic map of more than 20 mammals to help provide new depth to the understanding of evolution.

Institute Director Eric Lander said the Broads made an enormous bet by launching the institute to unleash the creative power of hundreds of researchers.

"Their bet has paid off more handsomely than any of us imagined," Lander said.