Keloids are a type of raised scar that can form after skin injury. Unlike typical scars, keloids grow beyond the original wound boundaries, often appearing as firm, rubbery growths. This article explores the nature of keloids and delves into the extent to which genetics influence their development, alongside other contributing factors.
Understanding Keloids
Keloids are characterized by their distinct appearance, presenting as shiny, firm masses of scar tissue. For instance, a hypertrophic scar, while thickened, remains confined to the original wound area and may flatten or regress over time. In contrast, keloids rarely regress on their own and can continue to grow and thicken for months or even years after the initial trauma.
These scars can vary in color, appearing pink, red, purple, or darker than the surrounding skin, and may have a lumpy or ridged texture. They can also cause discomfort, often being itchy, painful, or tender, especially during their early stages. Common areas for keloid development include the chest, shoulders, back, earlobes, and cheeks, though they can form anywhere on the body where skin has been injured.
The Biological Basis of Keloid Formation
Keloid development stems from a dysregulation in the body’s natural wound healing process. Normally, when skin is damaged, the body initiates a series of steps involving hemostasis, inflammation, proliferation, and remodeling to repair the injury. During the proliferative phase, specialized cells called fibroblasts produce collagen, a structural protein that provides strength and support to the new tissue.
In individuals prone to keloids, this process goes into overdrive, leading to excessive collagen production by fibroblasts. This overabundance of collagen, particularly type I and type III collagen, accumulates in a disorganized pattern, forming the dense, fibrotic tissue characteristic of keloids. The dysregulation also involves an imbalance between collagen synthesis and degradation, with increased levels of certain matrix metalloproteinases (MMPs) and their inhibitors (TIMPs) contributing to the excessive deposition of collagen. This abnormal remodeling results in the persistent scar that invades surrounding healthy tissue.
The Genetic Link
A strong genetic predisposition for keloid formation exists. Studies on family history show that keloids tend to run in families, with individuals having a first-degree relative with keloids being up to five times more likely to develop them. Twin studies have also demonstrated a higher concordance for keloids in identical twins compared to fraternal twins.
The prevalence of keloids varies among ethnic groups, indicating a genetic influence. People of African, Asian, and Hispanic descent, particularly those with darker skin tones, have a notably higher incidence of keloids compared to individuals with lighter skin. For instance, keloids are the fifth most common skin disease among adult Black patients in the United Kingdom and the most common among ethnic Chinese patients in Asia. Research identifies genes involved in keloid susceptibility. While no single gene mutation has been definitively identified as the sole cause, studies have implicated genes related to collagen synthesis, inflammation, and cell growth regulation, such as those within the SMAD, COL1A1, and TNF-α pathways. The inheritance pattern is believed to be complex and polygenic, meaning multiple genes contribute to the risk rather than a single gene. Genome-wide linkage studies have identified linkage intervals on chromosomal regions like 2q23 and 7p11 in different populations, though these findings can vary across ethnic groups, highlighting the complex genetic landscape.
Non-Genetic Risk Factors
While genetics play a significant role, several non-genetic factors also influence their formation. Skin type, often categorized by the Fitzpatrick scale, is a notable contributor; individuals with darker skin tones (Fitzpatrick types V and VI) are disproportionately affected due to higher melanin levels, which may correlate with a heightened inflammatory response to skin trauma. Conversely, individuals at either end of the Fitzpatrick scale, including those with very fair skin (types I and II), can also be at increased risk of excessive scarring.
The anatomical location of the injury also impacts keloid susceptibility, with areas of higher skin tension, such as the chest, shoulders, earlobes, and upper back, being more prone to keloid formation. The type of skin injury is another contributing factor; keloids can arise from various traumas, including surgical incisions, burns, piercings, acne, folliculitis, and even vaccinations, particularly the BCG vaccination. Age also plays a role, with keloids being more common in younger individuals, typically between 10 and 30 years old, which might be attributed to more active skin regenerative processes during these years. These environmental and physiological factors interact with an individual’s genetic predisposition, influencing whether a keloid develops and its severity.