The body of an average, healthy adult produces more than two hundred billion new platelets every day. The small cell fragments circulate through the blood stream, helping form blood clots and release growth hormones, for only a matter of days before they’re replaced. If a scientist wants to distribute a molecule quickly through the body, there are few speedier vehicles than platelets to carry their cargo. With that in mind, one team of researchers has successfully used platelets to produce, store, and dispense an enzyme that’s missing in people with Hurler Syndrome, a genetically inherited metabolic disease. They describe their success with the technique in a PNAS Early Edition paper this week.
“Theoretically it should be quite straight-forward to translate this to humans,” says Dao Pan, an experimental hematologist at Cincinnati Children’s Hospital and an author of the new paper.
Pan and collaborators at the National Institutes of Health had been studying ways to make mice with Hurler Syndrome produce the missing enzyme—α-L-iduronidase (IDUA). In 2009, they reported in PNAS that they’d engineered red blood cells to churn out the enzyme. During that research, they noticed that levels of lysosome enzymes seemed to be dependent not only on red blood cell levels, but platelets as well.
Platelets form when megakaryocytes—large bone marrow cells—bud off sections of their internal contents. The platelets don’t have their own nuclei or DNA, but contain proteins from their parent megakaryocyte. Since platelets aren’t normal, complete cells, researchers weren’t sure, for many years, whether they contained lysosomes at all. But Pan’s observation suggested that at least platelets were able to carry lysosomal enzyme in their organelles.
“We spent some time looking into that,” Pan says, “and started to pay attention to platelets.”
If platelets contained lysosomes, that made them an ideal delivery vehicle for lysosomal enzymes like IDUA. But first, the researchers had to get the IDUA gene into megakaryocytes. And that meant inserting the gene into the hematopoietic stem cells that give rise to megakaryocytes.
Pan’s team inserted the IDUA gene into the stem cells, but added a special DNA sequence to the gene that prevented it from being turned on in blood cells other than megakaryocytes. Adding a gene to the genome and having it immediately turned on in all cells, Pan explains, might lead to the activation of other, unwanted genes that promote cancer.
“That’s why we’re trying to develop strategies that only drive expression in the select cells that we want,” she says.
In the new paper, Pan’s team reports that the inserted IDUA gene is successfully copied into megakaryocytes, where the IDUA enzyme is then produced. When the megakaryocytes form platelets, the enzyme is shuttled into their lysosomes. Finally, when platelets are broken down, just a few days later, the IDUA is released into the body. Pan confirmed that the final enzyme was fully functional and, in mice with Hurler Syndrome, the new approach restored normal levels of IDUA in their bodies and reversed the symptoms of disease.
Pan and her collaborators are now working to improve the efficiency of the IDUA production, test out the method with other lysosomal enzymes, and begin the steps necessary to translate the technique to human medicine. But she also says that platelets could be a useful production and delivery vehicle for other types of molecules.
“It has the potential to work for other proteins,” she says. “It’s efficient and it’s also protective. It’s not that you just secrete an enzyme into the blood. The enzyme can be packed away within the platelet.”