actin, gamma 2, smooth muscle, enteric
The ACTG2 gene provides instructions for making a protein called gamma (γ)-2 actin, which is part of the actin protein family. Actin proteins are organized into filaments, which are important for the tensing of muscle fibers (muscle contraction) and cell movement. These filaments also help maintain the cytoskeleton, which is the structural framework that determines cell shape and organizes cell contents.
The γ-2 actin protein is found in smooth muscle cells of the urinary and intestinal tracts. Smooth muscles line the internal organs; they contract and relax without being consciously controlled. The γ-2 actin protein is necessary for contraction of the smooth muscles in the bladder and intestines. These contractions empty urine from the bladder and move food through the intestines as part of the digestive process.
At least 11 ACTG2 gene mutations have been found to cause megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS), which is characterized by impairment of the smooth muscle contractions that move food through the digestive tract (peristalsis) and empty the bladder.
The ACTG2 gene mutations that cause MMIHS are not inherited; rather they occur as a random (de novo) event during the formation of reproductive cells (eggs or sperm) or in early embryonic development. The alterations change single protein building blocks (amino acids) in the γ-2 actin protein. These changes hinder the formation of actin filaments and reduce the ability of smooth muscles in the bladder and intestines to contract. As a result, the bladder cannot empty normally, leading to an enlarged bladder (megacystis) and painful abdominal swelling (distention). In addition partially digested food can build up in the intestines, which also contributes to distention. Poor digestion may lead to malnutrition and the need for supplemental nutrition in people with MMIHS.
ACTG2 gene mutations cause a spectrum of disorders (sometimes referred to as ACTG2-related disorders), with MMIHS (described above) at the severe end. As in MMIHS, most of these mutations change single amino acids in the γ-2 actin protein. However, in less severely affected individuals, the mutations are usually inherited. These mutations often cause intestinal pseudo-obstruction, in which impaired intestinal smooth muscle contractions prevent peristalsis. (This condition mimics a physical blockage of the intestines without an actual obstruction.) In some affected individuals, the smooth muscle problems come and go throughout life. Intestinal malrotation can also occur in people with ACTG2 gene mutations. This condition occurs when the intestines do not fold properly; instead, they twist abnormally, which can impede the movement of food. Effects on the urinary tract include recurrent urinary tract infections and impaired bladder function. Individuals with inherited ACTG2 gene mutations can have one or more of these intestinal or urinary tract abnormalities; they are usually milder than in MMIHS, or they begin later in life. It is unclear why inherited and de novo mutations result in conditions with different severities.
- actin-like protein
- actin, gamma-enteric smooth muscle isoform 1 precursor
- actin, gamma-enteric smooth muscle isoform 2 precursor