What are the genetic changes related to vitamin D-dependent rickets ?
The two types of vitamin D-dependent rickets have different genetic causes: CYP27B1 gene mutations cause VDDR1, and VDR gene mutations cause VDDR2. Both genes are involved in the body's response to vitamin D, an important vitamin that can be can be acquired from foods in the diet or made by the body with the help of sunlight. Vitamin D helps maintain the proper balance of several minerals in the body, including calcium and phosphate. One of vitamin D's major roles is to control the absorption of calcium and phosphate from the intestines into the bloodstream.  The CYP27B1 gene provides instructions for making an enzyme called 1-alpha-hydroxylase (1-hydroxylase). This enzyme carries out the final reaction to convert vitamin D to its active form, calcitriol. Once converted, calcitriol attaches (binds) to a protein called vitamin D receptor (VDR), which is produced from the VDR gene. The resulting calcitriol-VDR complex then binds to particular regions of DNA and regulates the activity of vitamin D-responsive genes. By turning these genes on or off, VDR helps control the absorption of calcium and phosphate and other processes that regulate calcium levels in the body. VDR is also involved in hair growth through a process that does not require calcitriol binding.  Mutations in either of these genes prevent the body from responding to vitamin D. CYP27B1 gene mutations reduce or eliminate 1-hydroxylase activity, which means vitamin D is not converted to its active form. The absence of calcitriol means vitamin D-responsive genes are not turned on (activated). VDR gene mutations alter the vitamin D receptor so that it cannot regulate gene activity, regardless of the presence of calcitriol in the body; often the altered receptor cannot interact with calcitriol or with DNA.  Without activation of vitamin D-responsive genes, absorption of calcium and phosphate falls, leading to hypocalcemia and hypophosphatemia. The lack of calcium and phosphate slows the deposition of these minerals in developing bones (bone mineralization), which leads to soft, weak bones and other features of vitamin D-dependent rickets. Low levels of calcium stimulate production of PTH, resulting in secondary hyperparathyroidism; hypocalcemia can also cause muscle weakness and seizures in individuals with vitamin D-dependent rickets. Certain abnormalities in the VDR protein also impair hair growth, causing alopecia in some people with VDDR2.