Probing Early Mysterious Deaths in Families, Jim Loyd, MD, Helps Solve Riddle of Genetic Mutation in PPH
Citation: Advances in Pulmonary Hypertension 4, 1; 10.21693/1933-088X-4.1.4
Soon after a 30-year-old woman with primary pulmonary hypertension (PPH) was referred to Jim Loyd, MD, the first pieces of a huge and complex puzzle began falling into place. During the last month of his residency at Vanderbilt University School of Medicine in 1980, as his pulmonary fellowship was about to begin, Dr Loyd began to see telltale evidence in this patient's medical history that flew in the face of conventional wisdom at the time about the nature of PPH. “Her family history revealed many mysterious early deaths, including her mother, three maternal aunts, and a cousin who had been a nurse at our hospital.”
Because of pathologic interpretations, the prevailing view was that PPH was many different diseases and that trying to find a genetic link among families was not feasible. In the era before prostacyclin therapy, most investigators thought that a circulating vasoconstrictive substance was responsible for PPH, or alternatively, that microembolic pulmonary embolism was the cause. Following the same patient who initially perked his interest, Dr Loyd began a long thread of investigations that eventually brought about a new basic understanding of the cause of PPH. “That woman's family has now had 22 patients with PPH,” added Dr Loyd. During the same month that this patient's family history became apparent, Dr Loyd was joined at Vanderbilt by John Newman, MD, who was already primed to address the problem after completing a pulmonary fellowship at the University of Colorado.
“We began our studies by locating and contacting the authors of prior reports on PPH families, and in our first paper in 1984, we reported new information from six of those families, including eight new cases; we described inheritance patterns showing vertical transmission that suggested an autosomal dominant pattern.” This information pointed toward the implication of a single gene. At the time, however, the actual identification of the gene remained a fantasy, he said.
Building the Registry of Families with PPH
The pace of research quickened as Drs Loyd and Newman demonstrated that multiple pathologic forms were present in PPH patients within the same families, a finding that validated the pleiotropic effects of the disease and thereby justified grouping all of the PPH families together into the same gene search. Slowly the network of families—and the physicians who followed them—grew into a registry compiled throughout the early and mid-1990s and a turning point was reached when statisticians working with Dr Loyd conducted power calculations that suggested a genome-wide search might succeed. Vital to future progress was an NIH grant and the organization of a specimen bank collected for 15 years.
The work of Bill Nichols, PhD, was instrumental because he conducted a genome-wide microsatellite marker search and soon identified a large, 30-megabase region near the end of the long arm of chromosome 2 that had a highly significant association with the disease. The years following the linkage were very exciting, as the team narrowed the linked region by identifying recombinant individuals. Hope soared and crashed as candidate genes at first looked promising but then had to be abandoned. “Kirk Lane joined our group and after reviewing all of the genes positioned in the linked region, chose bone morphogenetic protein receptor II (BMPR2) as a candidate gene because of its membership in the TGFβ superfamily.” From there the first mutation was identified in a PPH family by DNA fingerprinting and the first report appeared in 2000 describing BMPR2 mutations in seven of eight PPH families. The database of families affected by PPH has grown to 107, with a total of 283 patients.
Risk Stratification: One in Ten or One in a Million?
Although most of the families tested have mutations in BMPR2, about a quarter have not shown evidence of such mutation. Several features have emerged among the families: the disease seems to begin at a younger age in subsequent generations, it also skips generations and is much more prevalent in women. “When we look at individuals who have the BMPR2 mutation, only one in five actually gets the disease, yet four in five people have the mutation throughout their life span and never develop PPH. It is very helpful when we know which mutation exists in a family because family members at risk can request to be tested. If the test is done and they do not have the mutation, it changes their risk of PPH from one in ten to one in a million,” said Dr Loyd.
One of the most intriguing questions that so far has produced only speculative answers is why some patients with the mutation develop the disease while others do not. “We are beginning to look across all the chromosomes using search strategies to identify regions where there might be modifiers,” said Dr Loyd. Yet so far, gender seems to be the only variable that is known to modify clinical expression. In one family from Tennessee, for example, with 22 PPH patients, 19 of them are women. No one has been able to pinpoint why women are at much greater risk than men, however.
When asked if a patient's genetic or familial predisposition could be related to the pathways widely recognized in recent years as contributors to the pathophysiology of PPH, Dr Loyd noted that this is one of the areas investigators are exploring. As he described the potential for possibly associating a genetic link with these pathways, he suggested that the link may be “within a pathway that is not even yet recognized, or in one that is known, but which is not known to be related to pulmonary hypertension.” Such a circumstance, however, is not new to Dr Loyd. This quandary was similar in many respects to the position he was in when he first began searching for the gene itself 25 years ago when so little was known about a familial connection and a comparable mystery was waiting to be solved.