is the most common enzyme affected in albinism.
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A six-year-old boy comes to the pediatrician clinic for a scheduled check-up. He and his parents recently immigrated to the U.S. from Africa. His parents report that he frequently gets sunburned when playing outside. The parents also say that their child is otherwise healthy with no other medical issues. Physical examination shows a well-appearing boy with extremely pale skin, white hair, and blue eyes. Both his parents are dark skinned. Genetic testing indicates that he has a disorder associated with an increased risk of developing squamous cell carcinoma. Which of the following best describes the disease pathophysiology in this patient?
Content Reviewers:Rishi Desai, MD, MPH
Albinism, meaning white, is a non-contagious, congenital condition that is defined by a drastic reduction or complete lack of pigmentation in the hair, skin and eyes.
Albinism is often portrayed negatively, like Silas the antagonist in the book “The Da Vinci Code”, which contributes to his diminished quality of life with the disease.
The skin is divided into three layers--the epidermis, dermis, and hypodermis.
The hypodermis is made of fat and connective tissue that anchors the skin to the underlying muscle.
Just above is the dermis, which contains hair follicles, nerves and blood vessels.
And just above, the outermost layer of skin, is the epidermis.
The epidermis itself has multiple cell layers that are mostly keratinocytes - which are named for the keratin protein that they’re filled with.
Keratin is a strong, fibrous protein that allows keratinocytes to protect themselves from getting destroyed when you rub your hands through the sand at the beach.
Keratinocytes start their life at the deepest layer of the epidermis called the stratum basale, or basal layer, which is made of a single layer of small, cuboidal to low columnar stem cells that continually divide and produce new keratinocytes that continue to mature as they migrate up through the epidermal layers.
But the stratum basale also contains another group of cells - melanocytes, which secrete a protein pigment, or coloring substance, called melanin.
Melanin is actually a broad term that constitutes several types of melanin found in people of differing skin color.
These subtypes of melanin range in color from black to reddish yellow and their relative quantity and rate at which they are metabolized define a person’s skin color.
When keratinocytes are exposed to the sun, they send a chemical signal to the melanocytes, which stimulates the melanocytes to produce melanin through a multistep enzymatic reaction that begins with tyrosine.
Once it’s made, the melanocytes move the melanin into small sacs called melanosomes, and these get taken up by newly formed keratinocytes, which will later metabolize the melanin as they migrate into higher layers of the epidermis.
Melanin can also be found at the base of hair follicles to color hair; and in the eyes, including the iris where it contributes to eye color, and the choroid tissue layer of the eye where it helps prevent light reflection.
Albinism is caused by a recessive gene mutation encoding any one of the enzymes needed to produce melanin.