HNF4A Gene

Maturity-onset diabetes of the young (MODY) is a group of several conditions characterized by abnormally high blood sugar levels. These forms of diabetes typically begin before age 30, although they can occur later in life. In MODY, elevated blood sugar arises from reduced production of insulin, which is a hormone produced in the pancreas that helps regulate blood sugar levels. Specifically, insulin controls how much glucose (a type of sugar) is passed from the blood into cells, where it is used as an energy source.

The different types of MODY are distinguished by their genetic causes. The most common types are HNF1A-MODY (also known as MODY3), accounting for 50 to 70 percent of cases, and GCK-MODY (MODY2), accounting for 30 to 50 percent of cases. Less frequent types include HNF4A-MODY (MODY1) and renal cysts and diabetes (RCAD) syndrome (also known as HNF1B-MODY or MODY5), which each account for 5 to 10 percent of cases. At least ten other types have been identified, and these are very rare.

HNF1A-MODY and HNF4A-MODY have similar signs and symptoms that develop slowly over time. Early signs and symptoms in these types are caused by high blood sugar and may include frequent urination (polyuria), excessive thirst (polydipsia), fatigue, blurred vision, weight loss, and recurrent skin infections. Over time uncontrolled high blood sugar can damage small blood vessels in the eyes and kidneys. Damage to the light-sensitive tissue at the back of the eye (the retina) causes a condition known as diabetic retinopathy that can lead to vision loss and eventual blindness. Kidney damage (diabetic nephropathy) can lead to kidney failure and end-stage renal disease (ESRD). While these two types of MODY are very similar, certain features are particular to each type. For example, babies with HNF4A-MODY tend to weigh more than average or have abnormally low blood sugar at birth, even though other signs of the condition do not occur until childhood or young adulthood. People with HNF1A-MODY have a higher-than-average risk of developing noncancerous (benign) liver tumors known as hepatocellular adenomas.

GCK-MODY is a very mild type of the condition. People with this type have slightly elevated blood sugar levels, particularly in the morning before eating (fasting blood sugar). However, affected individuals often have no symptoms related to the disorder, and diabetes-related complications are extremely rare.

RCAD is associated with a combination of diabetes and kidney or urinary tract abnormalities (unrelated to the elevated blood sugar), most commonly fluid-filled sacs (cysts) in the kidneys. However, the signs and symptoms are variable, even within families, and not everyone with RCAD has both features. Affected individuals may have other features unrelated to diabetes, such as abnormalities of the pancreas or liver or a form of arthritis called gout.

Congenital hyperinsulinism is a condition that causes individuals to have abnormally high levels of insulin, which is a hormone that helps control blood sugar levels. People with this condition have frequent episodes of low blood sugar (hypoglycemia). In infants and young children, these episodes are characterized by a lack of energy (lethargy), irritability, or difficulty feeding. Repeated episodes of low blood sugar increase the risk for serious complications such as breathing difficulties, seizures, intellectual disability, vision loss, brain damage, and coma.

The severity of congenital hyperinsulinism varies widely among affected individuals, even among members of the same family. About 60 percent of infants with this condition experience a hypoglycemic episode within the first month of life. Other affected children develop hypoglycemia by early childhood. Unlike typical episodes of hypoglycemia, which occur most often after periods without food (fasting) or after exercising, episodes of hypoglycemia in people with congenital hyperinsulinism can also occur after eating.