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Condition report · Cancer

Bloom Syndrome

Whole genome sequencing reads the BLM gene for inherited changes in both copies that cause Bloom syndrome, a condition marked by small size, sun sensitivity and an early risk of many cancers.

A condition report looks for rare variants known to cause the conditions it covers.

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About this condition

A rare inherited condition causing small body size, sun sensitivity and a greatly increased risk of cancer from an early age. It is caused by changes in both copies of the BLM gene, which helps keep DNA stable when it is copied.

Bloom syndrome is a rare inherited condition that affects growth, the immune system and the body's ability to keep its DNA stable. People with it are very small from before birth and usually have a red, sun-sensitive rash across the nose and cheeks. Most have normal intellectual ability, despite a small head.

The condition is caused by changes in both copies of the BLM gene. BLM makes a DNA helicase, an enzyme that unwinds DNA so that it can be copied and repaired. Without a working version, cells show greatly increased exchanges between chromosomes, and cancers of many types arise more often and at younger ages than usual. Reduced immunity with frequent infections and type 2 diabetes are also common.

Bloom syndrome is inherited in an autosomal recessive way. Both parents of an affected child are usually carriers, and each further child has a 1 in 4 chance of being affected. Knowing the cause allows cancer surveillance suited to the condition, sun protection, and early evaluation of brothers and sisters. Genetic counselling and carrier testing of relatives are available.

The condition we analyse

Autosomal recessive

Caused by loss-of-function changes in both copies of BLM, which encodes a DNA helicase that helps repair and stabilise DNA. Without it, cells show greatly increased exchanges between chromosomes, which causes genomic instability.

Signs and symptoms

Small size before and after birth, with a narrow face and a red, sun-sensitive rash on the cheeks and nose. Cancers of many types, including leukaemia, lymphoma and colorectal cancer, occur far more often and at younger ages than usual. Reduced immunity with frequent infections, type 2 diabetes, reduced fertility in men and early menopause in women are also common.

What a diagnosis changes

Cancer surveillance tailored to the condition, sun protection and regular screening for diabetes are recommended. Because cells are highly sensitive to DNA damage, exposure to X-rays and other ionising radiation is kept low and chemotherapy doses may need adjusting. Both parents of an affected child are usually carriers, so each further child has a 1 in 4 chance of being affected; genetic counselling and carrier testing of relatives are available.

Genes analysed

Also known as

BLM; Bloom-Torre-Machacek syndrome; BLS; BS; BSyn; congenital telangiectatic erythema; congenital telangiectatic erythema syndrome; growth deficiency, sun-sensitive, telangiectatic, hypo and hyperpigmented skin, predisposition to malignancy and chromosomal instability; MGRISCE1

MONDO:0008876OMIM 210900Orphanet 125MedGen 2685Sources: [1][2][3]

Why whole genome sequencing

Bloom syndrome is caused by changes in both copies of one gene, and knowing the cause shapes cancer checks and family planning.

Earlier cancer checks for relatives

Relatives who may be affected can be identified early, so they can avoid sun exposure and start surveillance sooner. Chemotherapy doses may need adjusting for people with the condition, and exposure to radiation is kept as low as possible. Knowing the cause makes these steps clearer.

Two carrier parents and a 1 in 4 chance

Both parents of an affected child are usually carriers. Each further child has a 1 in 4 chance of being affected and a 1 in 2 chance of being a carrier. Once the BLM changes are known in a family, prenatal and preimplantation genetic testing are possible.

References

  1. Langer K, Cunniff CM, Kucine N, 2006. Bloom Syndrome. GeneReviews
  2. Bloom syndrome. MedlinePlus Genetics
  3. Gönenc II, Elcioglu NH, Martinez Grijalva C, Aras S, Großmann N, Praulich I et al., 2022. Phenotypic spectrum of BLM- and RMI1-related Bloom syndrome. Clinical genetics

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