19.03.18 Chromosome breakage syndromes

Question 1

What are chromosome breakage syndromes (CBSs)?

Answer 1

• A group of genetic disorders that are typically transmitted in an autosomal recessive mode of inheritance
• In culture, cells from patients have high levels of chromosomal breakage, leading to rearrangements.
• Disorders are characterized by a defect in DNA repair mechanisms
• Also get increased predisposition to cancer

Question 2

What are the main examples of CBSs? (4 of them)

Answer 2

• Fanconi anaemia
• Bloom syndrome
• Nijmegen breakage syndrome
• Ataxia telangiectasia

Additional ones:
- Xeroderma pigementosum (sun sensitivity, high risk of skin cancer, common genes are XPC, ERCC2 and POLH, get build up of unrepaired DNA damage, get UV induced uncontrolled growth of cells and development of tumours)

• Cockayne syndrome (AR, genes are ERCC6 and ERCC8, NOT associated with higher risk of cancer
• Trichothiodystrophy (AR, short brittle hair, nail abn, ID, congenital ichthyosis, photo-sensitivity, genes are ERCC2, ERCC3, MPLKIP)
• Diagnosis is initially made on clinical symptoms but definitively made following lab testing
• If normal result but still suggestive of a CBS, then test a second sample (i.e. skin) due to mosaicism risk

Question 3

Example 1 - Fanconi Anaemia
What is it?
Symptoms?
Genes involved?

Answer 3

• Most common breakage syndrome (1 in 350,000)
• incidence higher in Ashkenazi Jews and the Afrikaans
• mostly recessive (can be X-linked with FANCB)
• Pre & postnatal growth retardation (starts prenatally)
• Microcephaly, developmental delay
• Skeletal malformations (radial defects and hypoplastic or absent thumbs)
• Hypogonadism, hyper/hypopigmentation
• Pancytopenia, progressive bone marrow failure and increased susceptibility to leukaemia and other malignancies
• Av death 16 yrs (from marrow aplasia)
• 15 genes involved - all linked to recognition and repair of damaged DNA - hence susceptible to haematological malignancy
• Cells have reduced ability to excise UV-induced pyrimidine dimers from the cellular DNA,
• Makes them sensitive to small concs of DNA crosslinking agents - leading to double-strand breaks in the S phase of the cell cycle
• Most common gene is FANCA (66% of cases)
• FANCC and FANCG account for 10% (12 more genes make up 10% combined)
• Testing involves culturing blood sample with DNA interstrand crosslinking agent (e,g, DEB, mitomycin C)
• This arrests cells in late S phase - Get increased breakage of chr. N.B. this only identifies affected HOMs and won’t detect carrier HETs
• Common breakages are monosomy 7, -5q, -7q, -20q, trisomy 8
• Seq then carried out to detect familial mutation (this is used to test carrier HETs)
• Also get somatic mosaicism - due to back conversion KO - WT - so need to look at lots of cells in culture (80-100 metaphases is recommended)

Question 4

Example 2 - Bloom syndrome (BS)
What is it?
Symptoms?
Genes involved?

Answer 4

• Second most common CBS
• incidence higher in Ashkenazi Jews
• Autosomal recessive
• severe pre- and postnatal growth deficiency, immune abnormalities, sensitivity to sunlight, insulin resistance, and a high risk for many cancers that occur at an early age
• leukaemia’s develop around 22yrs, solid cancers at 35yrs
• Caused by mutations in BLM (encodes DNA helicase at 15q26.1)
• Normally, BLM acts as TSG - essential for genome stability by suppressing inappropriate recombination.
• It forms part of a multi-enzyme complex which acts to dissolve double holliday junctions at stalled replication forks, and maintains telomeres
• Lack of BLM leads to hyper-recombination and telomere association, genomic instability and cancer predisposition
• In BS commonly see quadriradial configuration (caused by chromatid rearrangements) - made up of 2 homologous chr forming a 4 armed structure due to breaks and rearrangements
• Also get high sister chromatid exchanges (SCE )levels in BS - normal is 6-10 in a cell, in BS get >50
• can get mosaicism due to chr recombination - whereby a compound het changes to a normal allele and double mutated allele
• Seq of BLM gene is then recommended

Question 5

Example 3 - Nijmegen Breakage syndrome (NBS)
What is it?
Symptoms?
Genes involved?

Answer 5

• Also known as Berlin BS
• incidence higher in Slavic populations
• Autosomal recessive
• short stature, microcephaly, DD, facial features, recurrent RTI, increased rate of POF and susceptibility of cancers
• Immunodeficiency - abn levels of IgG and IgA and low T cells (normally due to rearrangements involving T cell receptors and Ig heavy chain gene)
• increased risk of T cell and B cell lymphoma <20yrs
• sensitive to effects of radiation exposure
• Caused by truncating mutations in NBN (involved in repairing damaged DNA at 8q21.3)
• Majority are HOM for 5bp del in exon 6
• NBN regulates cell division and proliferation, so mutations lead to immunodeficiency (due to reduced immune cell proliferation)
• Cytogenetically, you see spontaneous open chromatid and chr breaks, aneuplodies, marker chr. Also get translocations, and end to end fusions

Question 6

Example 4 - Ataxia Telangiectasia (AT)

Answer 6

• AR - get all the ataxias (celebellar, truncal, peripheral, ocularmotor), immunodeficiency, hypogonadism, predisposition to neoplasms (38% increased risk)
• Mutations in ATM at 11q22.3
• ATM encdes a large serine-threonine kinase - involved in signally the presence of dsDNA breaks (similar to role of BRCA genes). When DNA breakage is present it slows doen G1 to S, and G2 to M stages of mitosis. Patients develop ataxia due to brain cells dying because of defective DNA damage repair of neurons
• 600 unique mutations, 14 large scale deletions
• Diagnose by seq ATM - normally patients are compound HETs.
• some ATM missense have dominant negative effects - so a HET mutation can also cause disease
• Can expose dividing cells to irradiation and look at breakage to detect affected patients
• Don’t get increased SCE, but get gaps and breaks