Bloom Syndrome
Description
Bloom syndrome (also known as Bloom-Torre-Machacek syndrome) is a rare genetic disorder that affects multiple body systems, causing severe growth deficiency, sun-sensitive skin changes, and a significantly increased risk of cancer. It is caused by variants in the BLM gene, which normally helps maintain the stability of DNA during cell division. When BLM doesn‘t function properly, cells accumulate genetic damage over time, driving many of the condition‘s features. Bloom syndrome is very rare — fewer than 300 cases have been documented worldwide — but understanding its symptoms, causes, and inheritance pattern is important for at-risk families, particularly those of Ashkenazi Jewish descent, where a specific founder variant is more common.
Genes Involved
Bloom syndrome is caused by variants in a single gene:
- BLM — encodes a RecQ-family DNA helicase, an enzyme that helps unwind and repair DNA during replication. Loss of BLM function leads to widespread chromosomal instability, which underlies the growth, immune, and cancer-related features of the syndrome.
Recognizing the Signs and Symptoms
Bloom syndrome is usually recognized in infancy or early childhood. Common features include:
- Growth Deficiency: Severe, proportionate growth delay starting before birth; most affected individuals remain well below average height and weight throughout life.
- Sun-Sensitive Skin Changes: A butterfly-shaped, reddish skin rash (telangiectatic erythema) across the face, especially after sun exposure.
- Distinctive Facial Features: A narrow face, small jaw, prominent nose and ears, and a long, narrow head shape.
- Immunodeficiency: A weakened immune system leading to frequent ear, sinus, and respiratory infections.
- Increased Cancer Risk: A markedly higher lifetime risk of many cancer types — including leukemia, lymphoma, skin, gastrointestinal, and other early-onset cancers — often occurring at a much younger age than in the general population.
- Insulin Resistance/Type 2 Diabetes: Many individuals develop insulin resistance, sometimes progressing to diabetes.
- Fertility Issues: Reduced fertility is common, particularly in males.
- Other: Learning difficulties, chronic lung disease, and a high-pitched voice have also been reported in some individuals.
Causes
Bloom syndrome is caused by biallelic (two-copy) pathogenic variants in the BLM gene. Without functional BLM protein, cells lose an important DNA-repair safeguard, leading to chromosomal instability. This instability is thought to drive both the growth abnormalities and the strongly elevated cancer risk seen in the condition. A specific founder variant, sometimes called BLM^Ash, is more common in individuals of Ashkenazi Jewish descent.
Inheritance/recurrence risk
Bloom syndrome follows an autosomal recessive pattern of inheritance. A person needs to inherit one altered copy of the BLM gene from each parent to be affected.
- If both parents carry one altered copy of BLM (are carriers), each pregnancy has a 25% chance of resulting in a child with Bloom syndrome, a 50% chance of the child being an unaffected carrier, and a 25% chance of the child inheriting neither altered copy.
- Carriers (heterozygotes) do not develop Bloom syndrome themselves.
- Genetic testing can confirm a diagnosis, identify carrier status in family members, and support reproductive planning, including prenatal or preimplantation testing once the family‘s specific BLM variants are known.
