The Subtypes & Genetics of VDDR

Understanding the genetics of Vitamin D-Dependent Rickets (VDDR) is like looking at a blueprint for a manufacturing plant. For the body to build strong bones, vitamin D must go through a precise multi-step “assembly line” to become a key that can unlock doors in the intestines and bones. When a child has VDDR, a genetic mutation has broken one of the machines on that assembly line, jammed the locks on the doors, or destroyed the keys prematurely ^[1][2].

The Assembly Line: Type 1 VDDR (Enzyme Deficiency)

In Type 1 VDDR, the body has trouble making the active hormone. Think of this as a factory where the product is built in stages.

• Type 1B (The Liver Machine): The mutation is in the CYP2R1 gene. This gene is responsible for the first step of processing vitamin D in the liver ^[3]. Without it, the body can’t create the precursor needed for the next step.
• Type 1A (The Kidney Machine): The mutation is in the CYP27B1 gene. This machine is in the kidneys and performs the final, most important conversion: turning “inactive” vitamin D into the “active” hormone (1,25-dihydroxyvitamin D) ^[1]. This is the most common form of VDDR.
  • Analogy: In Type 1, the factory is broken. If the doctor gives the child the finished product (active vitamin D like calcitriol), the body can use it perfectly because the “locks” (receptors) are still working ^[4].

The Locked Doors: Type 2 VDDR (Receptor Resistance)

In Type 2 VDDR, the body can make the “key” (active vitamin D) just fine—in fact, the blood levels of active vitamin D are often extremely high as the body tries to overcompensate ^[5]. The problem is that the “locks” (the receptors) are broken.

• Type 2A (The Broken Lock): The mutation is in the VDR (Vitamin D Receptor) gene ^[6]. The active hormone tries to enter the cell to do its job, but the receptor cannot “hear” the signal. This often results in hair loss (alopecia) because the receptor is also needed for hair growth ^[6].
• Type 2B (The Interference): Here, the lock and key work, but another protein (often called hnRNP) is physically blocking the keyhole or interfering with the door’s ability to open ^[4].
  • Analogy: In Type 2, the keys are plentiful, but the doors are jammed. Treatment is much harder and often requires massive doses of calcium or specialized IV therapy to “force” the doors open ^[7].

The “Speed Trap”: Type 3 VDDR

• Type 3 (Rapid Degradation): This is a rare “gain-of-function” mutation in the CYP3A4 gene. Instead of a machine being broken, the enzyme is working too fast ^[8]. It destroys active vitamin D before the body can even use it.
  • Analogy: It’s like having a hole in the gas tank; no matter how much fuel you put in, it disappears before it reaches the engine. Treatment requires out-pacing the destruction with high doses of active vitamin D ^[8].

How VDDR is Inherited: Autosomal Recessive

Most forms of VDDR are autosomal recessive ^[4]. This means that for a child to have the condition, they must inherit one copy of the mutated gene from each parent.

• The Parents (Carriers): Both parents typically have one healthy gene and one mutated gene. They do not have symptoms because their one healthy gene provides enough instructions for the body to function.
• The Odds for Each Pregnancy:
  • 25% Chance: The child inherits two mutated genes and has VDDR ^[6].
  • 50% Chance: The child inherits one mutated gene and is a healthy “carrier” just like the parents.
  • 25% Chance: The child inherits two healthy genes and is not a carrier.

This inheritance pattern is why VDDR is often seen in families where the parents are related (consanguinity), as they are more likely to carry the same rare genetic mutation ^[9].