Bloom Syndrome: BLM Gene Sequencing
Test Code: 9522
Turnaround time: 5 weeks
Bloom syndrome should be considered in the following individuals:
- An individual with unexplained, severe intrauterine growth retardation that persists into infancy and childhood
- An unusually small, well-proportioned individual with a sun-sensitive erythematous skin lesion in a “butterfly distribution” on the face
- An unusually small individual who develops cancer
Bloom syndrome is an autosomal recessive disorder characterized by severe prenatal and postnatal growth retardation and a sun-sensitive erythematous skin lesion that occurs most commonly on the face. Recurrent infections (otitis media and pneumonia), chronic pulmonary disease, and diabetes mellitus are common. Many affected individuals have learning disabilities. Males are infertile; females enter menopause prematurely. The most common cause of death is cancer (epithelial, hematopoietic, lymphoid, connective tissue, germ cell, nervous system, and kidney), which occurs at younger-than-usual ages.
The diagnosis of Bloom syndrome can be confirmed or ruled out by chromosome analysis of any cell type that can be cultured. The most commonly used cells are blood lymphocytes, but cultures of skin fibroblasts and amniocytes also can be studied. The cytogenetic features of Bloom syndrome cells in mitosis are increased numbers of chromatid gaps, breaks, and rearrangements and increased numbers of quadriradial configurations. A greatly increased frequency (e.g., greater than 50 per metaphase) of sister chromatid exchanges is demonstrable in Bloom syndrome cells exposed to bromodeoxyuridine (BrdU). BLM (15q26.1) is the only gene known to be mutated in Bloom syndrome.
Fewer than 300 cases of Bloom syndrome have been reported. Although very rare in all populations, Bloom syndrome is less rare in Ashkenazi Jews. Approximately 25% of persons in the Bloom Syndrome Registry have Ashkenazi Jewish ancestry. The carrier frequency in the Ashkenazi Jewish population is estimated to be approximately 1%.
For patients with suspected Bloom syndrome, sequence analysis is recommended as the first step in mutation identification. For patients in whom mutations are not identified by full gene sequencing, deletion/duplication analysis is appropriate.
This test is indicated for:
- Confirmation of a clinical diagnosis of Bloom syndrome
Next Generation Sequencing: In-solution hybridization of all coding exons is performed on the patient’s genomic DNA. Although some deep intronic regions may also be analyzed, this assay is not mean to interrogate most promoter regions, deep intronic regions, or other regulatory elements, and does not detect single or multi-exon deletions or duplications. Direct sequencing of the captured regions is performed using next generation sequencing. The patient’s gene sequences are then compared to a standard reference sequence. Potentially causative variants and areas of low coverage are Sanger-sequenced. Sequence variations are classified as pathogenic, likely pathogenic, benign, likely benign, or variants of unknown significance. Variants of unknown significance may require further studies of the patient and/or family members.
Clinical Sensitivity: Unknown. Mutations in the promoter region, some mutations in the introns and other regulatory element mutations cannot be detected by this analysis. Large deletions will not be detected by this analysis. Results of molecular analysis should be interpreted in the context of the patient’s biochemical phenotype.
Analytical Sensitivity: ~99%
Submit only 1 of the following specimen types
* Preferred specimen type: Whole Blood
Type: Whole Blood
In EDTA (purple top) or ACD (yellow top) tube: Infants (2 years): 3-5 ml
Older Children & Adults: 5-10 ml
Specimen Collection and Shipping: Refrigerate until time of shipment. Ship sample within 5 days of collection at room temperature with overnight delivery.
OrageneTM Saliva Collection kit (available through CEN4GEN) used according to manufacturer instructions.
Specimen Collection and Shipping: Store sample at room temperature. Ship sample within 5 days of collection at room temperature with overnight delivery.
Submit copies of diagnostic biochemical test results with the sample, if appropriate.
Sequence analysis is required before deletion/duplication analysis by targeted CGH array. If sequencing is performed by another third party provider, please submit a copy of the sequencing report with the test requisition.
• Deletion/duplication analysis of the BLM gene by CGH array is available for those individuals in whom sequence analysis is negative.
• Custom diagnostic mutation analysis (test code: 6875) is available to family members if mutations are identified by targeted mutation testing or sequencing analysis.