BLOOM SYNDROME

Bloom Syndrome — the highest cancer predisposition of any known genetic disorder, with mean cancer onset at age 24, where carrier screening in Ashkenazi Jewish populations can prevent affected births through informed reproductive planning.

Whole genome sequencing reads the complete BLM gene — identifying the Ashkenazi founder mutation (BLMAsh) and all rare non-Ashkenazi alleles — for accurate carrier screening regardless of ancestry.

About this condition

Bloom Syndrome

Bloom syndrome (BS) is an autosomal recessive chromosomal instability disorder caused by pathogenic variants in BLM (chromosome 15q26.1), encoding BLM RecQ-like helicase — a DNA helicase essential for maintaining genomic stability during DNA replication. BLM deficiency leads to dramatically elevated rates of sister chromatid exchange (SCE), a cytogenetic hallmark that is diagnostic when present at 10x normal levels. The increased chromosomal instability produces proportionally elevated somatic mutation rates across all tissues, resulting in the highest cancer predisposition of any known single-gene disorder.

Bloom syndrome is characterized by prenatal and postnatal growth deficiency (adult height typically <150cm), sun-sensitive facial erythema (often misdiagnosed as lupus), immunodeficiency with recurrent infections, and infertility in males. The defining clinical feature is dramatically elevated cancer risk across virtually all tissue types — leukemia and lymphoma in childhood, and carcinomas of the breast, colon, and skin in adulthood. The mean age of first cancer diagnosis is approximately 24 years. Approximately 50% of affected individuals develop cancer, and many develop multiple independent primary cancers.

In the Ashkenazi Jewish population, a single founder mutation — c.2207_2212delinsTAGATTC (BLMAsh) — accounts for nearly all BS alleles. The carrier frequency is approximately 1 in 100-110 Ashkenazi Jews, making BLM one of the high-priority genes in Ashkenazi carrier screening panels alongside HEXA (Tay-Sachs), ASPA (Canavan), FANCC (Fanconi anemia), GBA (Gaucher), and others. Outside the Ashkenazi population, BS is rare (approximately 1 in 50,000-100,000), with a heterogeneous spectrum of BLM variants.

Gene locus: BLM (15q26.1)

Single-variant Ashkenazi panels detect BLMAsh but miss the rare BLM alleles that cause Bloom syndrome in non-Ashkenazi populations. Complete BLM gene sequencing is required for accurate pan-ethnic carrier screening.

Cancer surveillance in Bloom syndrome requires a unique protocol — no other condition has this breadth of cancer risk

Bloom syndrome produces cancer risk across essentially all tissue types — leukemia, lymphoma, carcinoma of the breast, colon, skin, lung, cervix, and others. No existing cancer screening protocol was designed for this breadth of risk. The Bloom Syndrome Registry at Weill Cornell Medical College maintains the world's longitudinal data and recommends intensive multi-cancer surveillance beginning in early childhood. Confirming the BLM molecular diagnosis is required to enroll in the Registry and to access the condition-specific surveillance recommendations that may detect cancers at earlier, more treatable stages.

Non-Ashkenazi carriers exist but are invisible to standard Ashkenazi screening panels

Standard Ashkenazi carrier screening tests for the single BLMAsh founder mutation. In mixed-ancestry couples — one Ashkenazi partner, one non-Ashkenazi — this approach has a blind spot: the non-Ashkenazi partner may carry a rare BLM variant not included on the panel, testing falsely negative. The couple receives incorrect reassurance. Whole genome sequencing reads the complete BLM coding sequence in both partners regardless of ancestry, providing equivalent sensitivity for all BLM pathogenic variants and eliminating the ancestry-dependent gap in standard screening panels.

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