The HLA-B27 gene is a component of the human immune system. It helps the immune system distinguish between the body’s own healthy cells and foreign invaders like bacteria or viruses. Its presence is consistently linked to an increased risk for developing certain inflammatory health conditions. Understanding how this gene is inherited is important for individuals and their families.
Understanding Genetic Inheritance
Genes are the basic units of heredity, guiding the development and functioning of all living organisms. They are organized into structures called chromosomes, found within the nucleus of every cell. Humans inherit 23 chromosomes from each parent, totaling 46 chromosomes in each cell.
Genetic traits pass from parents to children through different patterns. In autosomal dominant inheritance, only one copy of a gene is needed for an associated trait or condition to be expressed. In autosomal recessive inheritance, an individual must inherit two copies of a specific gene, one from each parent, for the trait to appear.
How HLA-B27 is Passed Down
The HLA-B27 gene is inherited in an autosomal dominant pattern. If one parent carries the HLA-B27 gene, each child has a 50% chance of inheriting it. This probability applies to each pregnancy independently.
Inheriting the HLA-B27 gene does not automatically mean a person will develop an associated health condition. This is known as incomplete penetrance, where the gene’s presence increases susceptibility but does not guarantee disease manifestation. Only a fraction of individuals who inherit the gene will develop symptoms. For instance, only about 5% to 10% of children who inherit HLA-B27 will develop ankylosing spondylitis, even with a family history of the disease.
Conditions Linked to HLA-B27
The HLA-B27 gene is strongly associated with a group of autoimmune diseases called spondyloarthropathies. These conditions involve inflammation, particularly in the spine, other joints, and where tendons and ligaments attach to bone. The gene is a significant genetic risk factor for these disorders.
Conditions commonly linked to HLA-B27 include:
Ankylosing spondylitis, a chronic inflammatory disease primarily affecting the spine.
Reactive arthritis, which can develop after certain infections.
Psoriatic arthritis, an inflammatory arthritis affecting some individuals with psoriasis, especially with spinal inflammation.
Inflammatory bowel disease-associated arthritis, seen in conditions like Crohn’s disease and ulcerative colitis, particularly when involving spinal inflammation.
Many individuals who carry the HLA-B27 gene never develop any associated conditions, indicating other genetic and environmental factors also play a role. This underscores that the gene is a risk factor, not a guarantee of disease.
What a Positive Test Means for Families
For individuals who test positive for the HLA-B27 gene, it signifies an increased genetic predisposition to certain inflammatory conditions, but it is not a disease diagnosis. Routine testing for HLA-B27 in the general population without symptoms or family history is not recommended, as many carriers remain healthy. The test is typically used to support a suspected diagnosis in individuals already experiencing symptoms consistent with an associated condition.
If a family member tests positive, this information can prompt discussions about genetic counseling. Genetic counseling offers guidance on understanding inheritance patterns and potential risks for other family members. While testing asymptomatic family members is not usually advised, awareness of symptoms can be beneficial for early detection and management. Monitoring for early signs like chronic back pain, joint stiffness, or eye inflammation can facilitate timely medical evaluation. Lifestyle factors and the interplay of other genes also contribute to whether a condition develops. Consulting with healthcare professionals is the recommended step to discuss individual risk, symptoms, and appropriate management strategies.