HCP is caused by changes in the CPOX gene, which controls the CPOX enzyme in the heme pathway. Without enough of this enzyme, there is a buildup of compounds called ALA and PBG, as well as other porphyrins. Most (>90%) people with changes in the CPOX gene never develop acute attacks or skin symptoms.
The acute attack symptoms of HCP are the same as AIP, including the triggers for acute attacks. However, people with HCP can also have blistering on their skin in areas that are exposed to sunlight, like in VP. Other skin symptoms include fragile skin and milia (tiny, white bumps or cysts).
Someone with HCP can have acute attacks alone, with blistering skin symptoms, or have skin symptoms alone.
The long-term complications of HCP are the same as AIP. The skin symptoms may result in scarring.
The treatments for acute attacks in HCP are the same with AIP. People with blistering from sun exposure need to protect themselves from sunlight by using sun protective clothing and avoiding prolonged sun exposure.
HCP is an autosomal dominant condition. Autosomal means that the defect is not on the chromosomes that determine sex, and dominant means that you only need to inherit one changed gene to have the disease.
Most HCP patients have one changed copy of the CPOX gene and one normal copy. It is random which of these two copies are inherited. Each child of someone with VP will have a 50% chance of inheriting the changed copy and 50% chance of inheriting the normal copy.