By David N. Leff
When you bite into your morning Danish, a banana-shaped gland located behind your stomach - the pancreas - swings into action. This multipurpose organ is riddled with a million or so barely visible particles called Islets of Langerhans. Each of these tiny globs is home to the pancreatic beta cells, which manufacture insulin on demand. That demand kicks in when the carbohydrate in your breakfast pastry turns into a sudden surge of glucose - the blood sugar that fuels the body's energy source.
In response to that red alert, those beta cells churn out enough insulin to metabolize the glucose into fat, the body's energy-storage fuel. If, for one reason or another, those beta cells aren't up to their glucose-metabolizing task, hyperglycemia - excess glucose - results, and diabetes mellitus can ensue, with all its horrendous complications.
Something like 14 million Americans suffer from "adult onset" diabetes mellitus Type II, which means they make insulin all right, but that hormone doesn't get through to its target glucose. Type II patients tend to obesity, and can usually be treated with strict diet and exercise regimens. Type I ("juvenile onset") diabetics aren't so lucky. Their beta cells are destroyed by autoimmune attack - for reasons not fully understood - and the insulin they produce is zero.
They have to depend for life - indeed, for life itself - on external supplies of insulin, delivered to their blood by injection. The trouble is that their daily glucose demand is not as regular as clockwork, but as variable as the weather. So in between two or three injections a day, they may suffer the disagreeable symptoms of too much or too little glucose.
Among the many devices aimed at improving on the conventional, decades-old subcutaneous insulin injection route are indwelling pumps and nasal sprays. But all of these methods suffer from inconvenience, interference with lifestyle and, most importantly, the near-impossibility of matching insulin administration to the body's minute-to-minute needs.
One modern, experimental recourse is to transplant healthy Islets of Langerhans into the bodies of Type I patients. But despite numerous attempts by many scientists, the recipients' immune systems still have the last word, and almost invariably reject the foreign donor cells. (See BioWorld Today, June 10, 1999, p. 1.)
Gene Therapy's Turn At Cracking Type I
The current issue of Nature, dated Nov. 23, 2000, announces a new and different ploy. Its title: "Remission in models of type 1 diabetes by gene therapy using a single-chain insulin analog [SIA]." Its senior author is pioneer molecular diabetologist Ji-Won Yoon, who chairs the Julia McFarlane Diabetes Research Center at the University of Calgary, Alberta, Canada.
His point of departure is the native insulin hormone's 51-amino-acid molecule, which consists of two chains, A and B, bridged by a 35-amino-acid sequence, the "C peptide." When insulin answers the call of glucose, it sheds that C sequence - a cleavage job handled by enzymes in the beta cells. This process unlocks the A and B chains, which then connect and fold to create native insulin.
Yoon and his co-authors genetically engineered a single-chain insulin analogue that retained 20 percent to 40 percent of native insulin's activity. They linked the DNA encoding this payload protein to a liver cell-specific promoter, which controls SIA expression in response to blood glucose levels. Then they inserted this construct into the genome of a modified, recombinant adeno-associated virus vector, and introduced their package into rat models of Type I diabetes.
Turning a healthy rat into a Type I diabetic is an age-old technique based on an anticancer drug called streptozocin. The trick is that streptozocin is specific to treating metastatic islet-cell carcinoma of the pancreas.
The Calgary team injected one trillion (1011) SIA particles through the portal veins of their Type I surrogate diabetic rats. (That vein feeds blood to the liver, whose cells convert carbohydrates into glucose.) One week later, the animals' hyperglycemia had declined to healthy, normoglycemia levels and stayed normal for the entire eight-month study period, with no symptoms of disease. Lower doses proved insufficient for such complete remission, but higher ones did not result in hypoglycemia - dangerously low blood glucose. They found that the viral genome became incorporated exclusively in liver cells.
"Until now," Yoon observed in a press statement, "the insertion of an insulin-producing gene into the body has been plagued with problems: the failure of the inserted gene to function reliably over time, the failure of the gene to regulate blood glucose levels, and the failure of the gene product to be effectively metabolized into insulin.
"Type I diabetes," he continued, "previously known as juvenile diabetes, is caused by the destruction of insulin-producing beta cells in the pancreas. This destruction occurs when the immune system mistakenly attacks the beta cells. The absence of insulin means that people with diabetes have high blood glucose and associated complications - kidney, eye and nerve conditions as well as heart and vascular disease."
Streptozocin-dosed rats are not the only rodents that stand in for Type I diabetes. The nonobese diabetic (NOD) mouse spontaneously loses its insulin to autoimmune, anti-beta-cell cytotoxic T lymphocytes.
Hopes For Curing Diabetes In Folks
When the co-authors administered 1012 virus particles per mouse to NOD mice, blood glucose levels reached normoglycemia seven days after treatment, and remained in that state for more than five months. In contrast, control NOD mice given the construct minus its promoter died within three weeks.
"After 25 years of trying to find causes and cures for diabetes," Yoon said, "this discovery is one of the high points of my scientific career. We expect this breakthrough," he concluded, "to lay the groundwork for clinical trials on the use of gene therapy which, we hope, will cure autoimmune Type I diabetes in people."
Apropos, a News&Views commentary accompanying Yoon's paper in Nature, made the point, "Rodents are quite different from humans with respect to maintaining glucose levels, and extending these results to human physiology may prove a challenge. Rodent livers have much higher basal rates of glucose production than human livers. So small effects of insulin on the liver may be able to control post-eating glucose levels in rodents but may be less effective in humans."