LAKE TAHOE, CALIF. – and during human development, said at the annual meeting of the Society for Pediatric Dermatology.
Dr. Biesecker, senior investigator and head of the clinical genomics section of the National Human Genome Research Institute’s Medical Genomics and Metabolic Genetics Branch, discussed mosaicism and a number of overgrowth syndromes that he and his associates have been studying that have clinical relevance for pediatric dermatologists. He noted that mosaicism can affect any tissue, anywhere, in any pattern. “If an affected cell cannot survive gametogenesis, fertilization, or survive early development, this generates Happle-type mosaicism,” explained Dr. Biesecker, who is trained in pediatrics and in clinical and molecular genetics.
“This is characterized by patchy manifestations, and no parent-to-child transmission or recurrence. You must always be careful here, though, because Mother Nature does what she wants to. Mosaic mutations can happen more than once, but it’s a very unlikely outcome. Happle-type mosaicism is also characterized by discordant monozygotic twins,” he noted.
The prototype for Happle-type mosaicism is Proteus syndrome, formerly known as Elephant Man disease, which is caused by a mutation in the AKT1 gene. Patients with Proteus syndrome undergo severe, relentless overgrowth, and about 25% of them die during childhood. “If you see one of these patients, you have a serious clinical problem on your hands,” he said. “There is enormous individual variability, but it is ultra rare.”
Dermatologic lesions that are characteristic of Proteus syndrome include cerebriform connective tissue nevus, which typically presents on the hands and feet. “A wide range of vascular malformations have also been associated with this, even patients with arteriovenous malformations,” Dr. Biesecker said. “They are a serious problem.” Linear verrucous epidermal nevus is another characteristic lesion of Proteus syndrome. It can present in a number of ways and in various body sites. “The natural history of these lesions is important,” he commented. “Over time, are they stable, or do they spread and expand over time? These lesions do not ever spontaneously regress. This does enable molecular diagnosis, but don’t bother sampling their blood, because it will be negative. You have to have a biopsy sample.”
Overgrowth syndromes that do not meet criteria for Proteus syndrome fall into a category known as PIK3CA-related overgrowth spectrum, which Dr. Biesecker characterized as “a bunch of clinical designations all caused by the same underlying somatic mutation in a gene called PIK3CA. There is an enormous variability in these patients, ranging from those who have profound overgrowth, including malformations, truncal overgrowth, and vascular malformations, and digital overgrowth in all sorts of patterns. We designate this as PIK3CA-related overgrowth spectrum (PROS), because we can’t clinically separate these things from one another.”
These conditions include what used to be called CLOVES syndrome (congenital lipomatous overgrowth, vascular malformations, epidermal nevi, and scoliosis/skeletal/spinal anomalies), facial infiltrating lipomatosis, and megalencephaly-capillary malformation syndrome. PROS is about 100 times more common than Proteus syndrome. “There are no rational boundaries to distinguish these entities,” Dr. Biesecker said. “They are rationalized under a combined clinical-molecular PROS framework, meaning that the molecular diagnosis is absolutely key to correctly diagnosing these patients.”
In this way, mosaicism challenges the traditional concept of diagnosing overgrowth disorders. “What we thought were separate disorders are in fact many manifestations of a single disorder,” he continued. “When I was doing my genetics training, we were taught that it would turn out that there was one gene for every disease, and one disease for every gene. That is completely wrong; it’s much more complicated than that. Mosaicism is also important for us as biologists, because it gives us a window into biology we otherwise would not see. Without a mosaicism, Proteus syndrome cannot exist biologically. So if I want to understand that gene product, I have to study patients who are mosaics. Mosaicism can happen in any tissue, whether it’s visible or not.”
Dr. Biesecker, who has been elected to serve as president of thefor 2019, noted that most of the gene mutations that cause overgrowth disorders are the same ones implicated in cancer. “It makes sense, because cancer is a disorder of uncontrolled proliferation and differentiation,” he said. “These overgrowth disorders are similar but less severe manifestations of the same problem. It turns out that these mosaic patients are single gene model systems of cancer biology.” Therefore, when a drug company develops an anti-cancer drug, he continued, it also can be useful for PROS or Proteus syndrome. It’s much easier to inhibit a protein that’s overactive than it is to replace the activity of a gene that has lost its function.
But in PROS and Proteus, “we have very different treatment objectives than oncologists do,” he said. “Our goal is to reduce the signaling caused by these mutations; we do not want to kill the cells. Some of my patients with these disorders have pretty close to 50% of cells in their body carrying these mutations. If I were thinking like an oncologist, the oncologist wants to kill cancer cells; that’s their objective. If I were to kill all of the mutant cells in my patients, I’m certain that would kill them.”
One promising development is the investigational oral agent, which is an inhibitor of AKT1. Dr. Biesecker and his colleagues at the NIH have been working to figure out what dosing should be used in humans based on mouse data, lab data, and data from cancer patients. They started with about one-twelvth the dose that oncologists use. After treating the first patient with overgrowth syndrome, on day 15 that person’s AKT1 level dropped to about 20% of normal, while on day 75 it moved to around 60% of normal. “We are right in that zone where we want to drive the activity of that protein to about half of what it should be,” Dr. Biesecker said. He and his colleagues also have observed regression of lesions in a patient with cerebriform connective tissue nevus who was treated with ARQ 092. “We’ve never seen this before.”
Dr. Biesecker disclosed that he is a member of the Illumina medical ethics board. He has received royalties from Genentech and in-kind research support from ArQule and Pfizer.