Bloom syndrome is a rare inherited disorder characterized primarily by short stature and a predisposition to various types of cancer. It is always associated with a decreased stability in the chromosomes that can be seen by cytogenetic laboratory techniques.
Description
Bloom syndrome (BS) was first described by D. Bloom in 1954. The clinical symptoms of BS include small body size, sun-sensitive skin that is prone to a reddish rash, patchy spots on the skin that are either lighter or darker than the expected skin color, severe immune deficiency, and an enormous predisposition to various types of cancer. The hallmark of the disorder is genetic instability that manifests itself in chromosomes that tend to exchange material with one another.
Genetic profile
BS is inherited in an autosomal recessive manner. The gene responsible for this disorder is known as BLM and it is located on chromosome 15, in band q26.1. Changes or mutations in the BLM gene lead to decreased stability in the chromosomes. Chromosomes of people with BS will show an increased amount of gaps, breaks, and structural rearrangements.
The most characteristic chromosomal abnormality in BS involves the tendency for deoxyribonucleic acid (DNA) strands to exchange material, most likely during replication. DNA is the molecule that encodes the genetic information and determines the structure, function, and behavior of a cell. The exchange of DNA may occur between a chromatid of each of the two homologues of a chromosome pair, forming a unique structure called a quadriradial, or between the two sister chromatids of one chromosome, known as sister-chromatid exchange (SCE).
The BLM gene produces the BLM protein. The BLM protein is a member of the helicase family and is thus capable of unwinding DNA and RNA. This unwinding process provides single stranded templates for replication, repair, recombination, and transcription. Additionally, the BLM protein may function in a post-replication recombination process that resolves errors generated during replication. Mutations (changes) prevent the BLM gene from making BLM protein. Without adequate amounts of this protein, errors are likely to occur in these important processes and these errors are less likely to be repaired.
As of 2001, it is known that mutations in the BLM gene lead to the symptoms of BS. However, the precise relationship between these mutations and the symptoms seen in BS is still unknown.
Additionally, the DNA of individuals affected with BS is much more prone to spontaneous mutations, perhaps because the inadequate amount of BLM hinders the correction of these errors.
Demographics
BS is a very rare condition, thought to affect a very small proportion of the general population (approximately 1/6,330,000). However, in the Ashkenazi Jewish population, approximately 1/60,000 people are affected with BS. Approximately 1/100 people of this ethnic group are carriers of a mutation in the BLM gene. These carriers do not have BS but are capable of passing it on to
their children if the other parent is also a carrier. If both parents are carriers, each pregnancy will have a 25% chance of being affected with the disorder. Carriers, or individuals with only one copy of the abnormal gene, do not appear to have an increased risk for cancer or other symptoms associated with BS. They have near normal or normal genetic stability.
Signs and symptoms
There are two characteristic signs that are seen in nearly all individuals with BS. The first is an overall small body size, which is usually noted at birth and continues throughout the person's lifetime. The growth deficiency is often accompanied by a small brain and head. The head may be dolichocephalic as well, meaning that is it elongated from the front to the back of the head. The average height for an adult with BS is 147.5 cm for males and 138.6 cm for females.
The second characteristic that is very common in individuals with this disorder is an enormous predisposition to cancer. Both benign (non-cancerous) and malignant (cancerous) tumors arise at an early age and with great frequency in a wide variety of body locations and cell types. Thirty-seven percent of patients have malignant tumors. The mean age at diagnosis of a cancer is 24 years with a range of 2–46 years. Lymphomas and leukemias are common and generally appear before the age of 25. Carcinomas are common as well, usually appearing after the age of 20, most often in the colon, skin, breast, or cervix. Cancer is the most common cause of death for individuals with BS. Radiation treatment or chemotherapy can lead to further complications in these patients due to the increased sensitivity to exposures that may damage their fragile chromosomes.
There are additional features that may or may not be present in individuals with BS and they vary in severity from person to person. In some cases of BS, the person may have some unique facial features, including a narrow, triangular face shape, a prominent nose, a small jaw, and protuberant ears. The voice may be high pitched and somewhat squeaky in tone.
Infants may experience repeated respiratory tract infections, ear infections, and vomiting and diarrhea that can lead to a life-threatening loss of body water (dehydration). Additionally, after the first significant exposure to sunlight, an infant may develop a reddish "butterfly rash" on the cheeks and nose described as erythematous or telangiectatic. The severity of the rash can vary from a faint blush during the summertime to a severely disfiguring, flaming red lesion. Rarely, other areas of the body that are exposed to sunlight can show a similar rash. In childhood, the skin may begin to appear "patchy" showing some spots with less pigment than the rest of the skin (hypopigmentation) and some with more pigment than the rest of the skin (hyperpigmentation).
Men diagnosed with this disorder may have abnormally small testes and might be unable to produce sperm, making them infertile. Women can have early menopause and often have reduced fertility.
Individuals with BS have a higher incidence of diabetes mellitus when compared to the general population. The average age of onset of diabetes is 25 years, earlier than the usual age of onset of type II diabetes and later than that of type I. Additionally, this disorder can lead to a compromised immune system, resulting in an increased susceptibility to bacterial infections. Infections of the respiratory tract and ears are seen most commonly.
Intelligence in individuals with BS seems to be average to low average. When they exist, limitations in intellectual abilities range from minimal to severe. Even when intelligence is normal in these individuals, there tends to be a poorly defined and unexplained learning disability
that is often accompanied by a short attention span. BS is often accompanied by a persistent optimistic attitude.
Diagnosis
BS can be suspected by the doctor but is generally confirmed by a cytogenetic study known as sister chromatid exchange (SCE) analysis. This disorder is the only one that features an increased risk of SCE. This analysis is indicated in any child or adult with unexplained growth deficiency regardless of whether or not other features of the BS are present.
SCE analysis involves taking a blood sample, treating it with a special process in the laboratory, and examining the chromosomes. In individuals with BS, the chromosomes will show an approximately 10-fold increased rate of sister chromatid exchange. Most likely, unique chromosome structures called quadriradials will also be visible in a higher frequency than expected. SCE and quadiradials are present in untreated cells from individuals without BS, although much less frequently.
In addition to examining the chromosomes, it is also possible to look for specific changes in the BLM gene. This type of evaluation is generally used only for those who may be carriers of the gene mutation rather than those who are suspected to have the disorder. Carriers cannot be identified by SCE analysis because they do not show an increased rate of SCE.
Carrier testing is available for the Ashkenazi Jewish population. In these individuals, there is one particular mutation in the BLM gene that is responsible for most cases of BS. A blood sample can be tested for the presence of this mutation. Almost all Ashkenazi Jewish carriers of the BS gene can be identified in this manner. The great majority of carriers of the mutation causing BS are of Ashkenazi Jewish descent and, thus, this test is designed for that high-risk population. The test is not accurate for people from other ethnic populations in whom the specific changes of the BLM gene are not so well understood.
Prenatal diagnosis is available for carrier couples with previously identified mutations in the BLM gene.
It is thought that BS is highly underdiagnosed. Many affected individuals are treated for a symptom or are mistakenly considered to have another rare disorder.
Treatment and management
There is no treatment for BS—the underlying genetic defect cannot be repaired. However, early diagnosis and management can increase the life span of these individuals.
Babies and young children with BS are often poor eaters. Thus, nutritious food and multivitamins may help improve growth. Treatment with growth hormone has been attempted in several cases but has been generally unsuccessful. Further investigation into this possibility has been limited due to reports that cancer has developed in conjunction with growth hormone treatment.
The reddish skin lesions can be controlled by avoiding the sun, wearing a hat or bonnet, and by using a sunscreen. Avoidance of sun exposure is most critical in the first few years of life, since the severity of the skin lesion appears to be established at that time.
Cancer surveillance is of utmost importance in BS. After the age of 20, annual sigmoidoscopy and fecal blood testing are recommended, as well as breast self-examinations and pap smears for women. It is suggested that the individual be followed closely by a specialist or clinic knowledgeable about BS so that any subtle symptoms of carcinomas can be treated. Early surgical removal of these tumors provides the best chance of a cure. Individuals may wish to store their bone marrow early in life in case a later treatment diminishes their existing bone marrow. Unfortunately, early diagnosis of leukemia is not known to improve the chances of curative therapy; thus, surveillance of the blood and blood-forming tissues in children with BS is not recommended as a part of the cancer surveillance.
Additionally, individuals with this disorder are instructed to avoid x rays, chemotherapeutic drugs and other environmental exposures that may damage their unusually fragile chromosomes. Due to the immunodeficiencies often associated with BS, it is important to treat any bacterial infections promptly.
Prognosis
The mean age at death is 23 years with a range from 1–48 years. Cancer is the most common cause of fatalities in individuals with BS and is thought to be responsible for approximately 80% of deaths. Chronic respiratory infection is the next most common cause of death.
PERIODICALS
Gennery, A. R., et al. "Immunodeficiency Associated With DNA Repair Defects." Clinical and Experimental Immunology 121 (2000): 1-7.
Meyn, M. S. "Chromosome Instability Syndromes: Lessons for Carcinogenesis." Current Topics in Microbiology and Immunology 221 (1997): 71-148.
Nakura, J., et al. "Helicases and Aging." Cellular and Molecular Life Sciences 57 (2000): 716-730.
Rong, Suo-Bao, Jouni, Valiaho, and Mauno Vihinen. "Structural Basis of Bloom Syndrome (BS) Causing Mutations in the BLM Helicase Domain." Molecular Medicine 6 (2000): 155-164.
Watt, Paul M., and Ian D. Hickson. "Genome Stability: Failure to Unwind Causes Cancer." Current Biology 6 (1996): 265-267.
Woods, C. Geoffrey. "DNA Repair Disorders." Archives of Disease in Childhood 78 (1998): 178-184.
WEBSITES
"Bloom Syndrome." OMIM—Online Mendelian Inheritance in Man. National Center for Biotechnology Information. <http://www3.ncbi.nlm.nih.gov/omim/>.
