Breakage Syndromes Flashcards
What are chromosome instability disorders
They are a collection of Mendelian autosomal recessive disorders majority of which have defects in genes associated with DNA repair.
Instability: the predisposition of the chromosomes to undergo rearrangement or to display abnormal chromosomal behaviour
Additionally they all,have in common: predisposition to cancer, immune deficiency, growth retardation, premature ageing.
What are the classic breakage syndromes
Fanconi anaemia.
Bloom syndrome.
Ataxia telangiectasia.
What are the mutagen hyper-sensitivity syndromes
Nijmegen breakage syndrome.
Immunodeficiency; centromere can instability; facial anomalies (ICF).
Seckel syndrome.
What ‘non-breakage’ syndromes have been reported with an occasional observation of chromosomal instability.
Xeroderma pigmentosum.
Cockayne syndrome.
Rothmund- Thompson syndrome
What disorders are classed as cohesinopathies and why are they called that
Roberts syndrome.
Cornelia de Lange syndrome.
Variegated aneuploidy syndrome.
Warsaw breakage syndrome.
Premature sister chromatid separation.
Genes that underly these disorders code for cohesins/associated with cohesins. They contribute to the control of sister chromatid segregation at cell division.
What are the features of fanconi anaemia
70% people have major congenital anomalies:
Pre & postnatal growth retardation. Skeletal anomalies including hypoplasia of the radius & absent or low thumbs. Hypogonadism. Hyper pigmentation.
generalised, progressive bone marrow failure (starts with neutropenia and thrombocytopenia and progresses to pancytopenia) progression to AML.
30% people have no major anomalies:
Skin pigmentary abnormalities. Micropthalmia. Growth below the 5th centile.
Discuss fanconi anaemia and cancer
Fanconi is associated with generalised and progressive bone marrow failure.
This is strongly associated with a susceptibility to haematological malignancies, in particular AML 10%. These patients have a POOR PROGNOSIS. With survival at ~6months.
Average age of death is 16yrs from marrow aplasia (sepsis) or from malignancies.
Also susceptibility to solid tumours: head & neck, skin, GI tract, genital tract (25-30%)
What causes fanconi anaemia
Genetically heterogeneous disease with 15 different complementation groups listed on OMIM.
FANCA: most common 2/3 cases.
FANCC & FANCG: 10% each.
Other 12 : 10% total.
Are any of the FANC genes of particular interest
FANCD1 (BRCA2): biallelic deletions are particularly severe form of FA with very high cancer risk: early onset AML & solid tumour.
FANCA: pts homozygous for null mutations have earlier onset anaemia and higher incidence of leukaemia compared to mutations that alter the protein.
FANCG: more severe cytopenias and high incidence of leukaemia.
How do you test for Fanconi
Screen 80-100 metaphase (PHA stimulated and exposed to agent) for breakages. Compare against a normal controls (ideally sex and age matched).
Count average breaks/cell across all cells and absolute breaks/cell. This count and the high numbers of cells should increase the likelihood of detecting mosaics.
What happens to FA patient chromosomes
They have an increase in chromosome breakages, both spontaneously and when exposed to DNA cross linking agents
What are the two DNA cross-linking agents used to test for FA
Mitomycin C MMC.
Diepoxybutane DEB.
What samples can you use to test for FA
Adults: blood (or fibroblasts if still suspect FA on a normal blood result).
Prenatal: AF, CVS.
Known mutation: sequencing.
What are the features of Bloom’s syndrome
IUGR. postnatal growth retardation & short stature. Sun sensitive skin rash. Erythematous rash across cheek and nose. Hypo- &mhyper pigmentation. Immunodeficiency. Infertility in males. Females can sometimes conceive. Facies: triangular shaped face, prominent nose and ears.
Marked predisposition to malignancy, especially acute leuks. Wilms, carcinomas, GI tract and breast reported.
What age do BS develop cancer
Leukaemia: 22yrs.
Solid tumours: 35 yrs.
How do you test for Blooms
Harlequin staining of 20 metaphase looking for sister chromatid exchange.
Normal: 6-10/ cell
BS: >50/cell
Compare results to laboratories record of a range of normal controls tested using the same method.
What causes Blooms
BLM gene 15q26.1. DNA helicase RecQ protein like 3.
Helicase winds and unwinds replicating DNA. Absence likely destablises other enzymes at replication causing replication fork to stall. Results in instability and increased recombination.
What referrals should you consider breakage syndromes
TAR syndrome. Limb reduction. VACTERAL syndrome. Microcephaly with severe IUGR.
How do correcting events occur
Due to the increased rate of chromosome recombination. An intragenic recombination event between two mutations that result in a functionally normal allele and one with a double mutation.
What are the features of ataxia telangiectasia
Progressive cerebellar ataxia in infancy/ early childhood (diminished, absent deep reflexes, flexor, plantar responses). Failure to thrive. Truncated ataxia progressing to peripheral ataxia. Ocular motor apraxia. Slurred speech/ can’t swallow or eat well. Recurrent infections.ocular telangiectasias. Immunodeficiency.
What the incidence and survival for ataxia telangiectasia
1/100,000.
No cure: 50% die by 25yrs. Usually from pulmonary failure
What causes ataxia telangiectasia
ATM. 11q22.3.
Mostly null mutations (missense, frameshift have milder phenotypes).
Large serine-threonine kinase. Signals existence of DSB. Delays G1-S and G2-M. Upstream of BRCA inDSB repair .
Discuss AT and cancer
Predisposition to malignancy. (Susceptible to X-Ray radiation 1000x more likely to get cancer).
35% pts get lymphoid tumour, lymphoma, lymphocytic leukaemia, ALL (high proportion are T-cell).
A 4 fold increase in risk of breast cancer.
What would you expect to see cytogenetically in AT pts
Recurrent rearrangements (inv, t) involving 7, 14, X.
T-cell receptors and immunoglobulin loci.
14q11: TCR A/D. 14q32: IGH.
7p13: TCR G. 7q35: TCR B.
inv(7), t(7;7), t(7;14), t(14;14), inv(14), t(X;14).
Large abnormal clones often see telomeric dicentrics
How do you test for AT
There’s about 600 mutations in ATM (14 large del/dup). Can sequence gene or MLPA for large del/dups.
Cytogenetically isn’t diagnostic of AT.
What are the no classical forms of AT and differential diagnosis
Adult onset: milder: slower rate of neurological determination and little/ no cancer (5762ins137).
A-T Fresno: combined features of AT and Nijmegen.
Freidreich ataxia. Cerebral palsy. Cohen oculomotor apraxia.
What are the features of Nijmegen syndrome
IUGR. postnatal GR: short stature. Progressive microcephaly. Speech, development and ID decline over time. POI. recurrent infections, combined immunodeficiency. Predisposition to malignancy. Bird like facies (prominent midface/ nose, slopping forehead, large ears, small jaw, up slanting palpebral fissures).
What causes Nijmegen
NBN 8q21. Majority cases: 657_661del5 (5bp deletion in exon6. typically truncating mutations)
Repairs damaged DNA. Regulated cell division and proliferation. LoF results in reduced cell proliferation.
Discuss Nijmegen and cancer
40% patients develop malignancy by 20yrs. Major cause of death.
Majority are lymphomas: 55% are T-ALL, 45% are B-cell NHL (DLBCL, LB) usually by 15yrs.
Also have solid tumours: rhabdomyosarcoma, gliomas.
Discuss cyto and Nijmegen
Spontaneous open chromatids/ chromosome breaks, aneuploidies, markers, partial endoreduplication.
T cell receptor rearrangements 7, 14.
14q11, 14q32, 7p13, 7q35.
Differential diagnosis for Nijmegen
Rubinstein taybi: microcephaly, prominent midface, ID
What are the features of ICF
ICF: Immunodeficiency, Centromere instability, Facial,anomalies.
Immunodeficiency. Opportunistic infections (esp respiratory). Despite presence of B-cells, T-cell dysfunction.
Centromere instability. Pericentromeric instability. Caused by extensive hypomethylation of satellite DNA at centromeres.
Facial dysmorphism. Mild: epicanthic folds, hypertelorism, flat nasal bridge, low set ears, macroglossia.
Discuss ICF and methylation
The physical and cytogenetic phenotype can be considered secondary to a failure of methylation.
Cytogenetic phenotype is due to extensive hypomethylation of satellite 2 repeats at 1qh and 16qh, and satellite 1 repeats at 9qh.
Hypomethylation causes by LoF of DNMT3B
What causes ICF
DNMT3B 20q11.2. DNA methyltransferase 3B (50% patients) ICF1.
ZBTB24 6q24 (50% patients) ICF2
What’s the incidence and life expectancy of ICF patients
1/100,000.
Life expectancy: POOR: severe infections, chronic gastrointestinal problems, failure to thrive.
Discuss cytogenetically and ICF
Mutli-radial chromosome: up to 40% mets. Contains 3 or more chromosomes of 1 and 16 (sometimes 9). Caused by interchange within the HC regions.
Whole chromosome area deletions and pericentromeric breaks (chr 1 and 16) .
Isochromosome and t’s with breaks near centromeres.
Prominent stretching of HC regions of 1 and 16.
Examination of 20 metaphase
What are the features of Roberts syndrome
Pre and postnatal GR (moderate- severe). Limb malformations, craniofacial anomalies. ID (majority patients, but normal ID reported in some cases). CHD. renal problems. Large genitialia.
Discuss limb malformations in Roberts
Symmetrical tetraphocomelia (limb reduction). upper limbs more severely affected than lower limbs.
Oligodactyly. Syndactyly. Clinodactyly. Elbow and knee flex on contractures.
Discuss craniofacial anomalies of Roberts
Microcephaly. Cleft lip and palate. Cloudy cornea.
Micrognathia. Hypertelorism. Down slanting palpebral fissures. Dysplastic ears. Malar hypoplasia. Hypoplastic nasal alae.
What causes Roberts
ESCO2 8p21.1. Acetyltransferase.
Proteins involved in establishment of sister chromatid cohesin.
Mutations result in delayed division, increased cell death, impaired proliferation. (Loss of progenitor cells at embryogenesis likely to be responsible for the developmental defects)
Roberts is an abnormality of sister chromatid apposition around the centromeres.
Discuss cytogenetics and Roberts
C-Banding as G-Banding can obscure the phenomenon
PCS: PATHOGNOMIC sign of RBS and is seen in patients with mild to severe phenotypes.
PCS: puffing/ repulsion of the HC regions around the centromere. And sister chromatids bulging away from each other. Gives a railroad track appearance. Due to a lack of constriction around the centromere.
Puffing: chr 1, 9, 16. Repulsion: Yq and acrocentrics.
Aneuploidy is seen in 10-20% mets
When would Roberts be susceptible prenatally
When the scan shows symmetrical tetrphocomelia and IUGR
Mortality is high in severely affected pregnancies and newborns (mildly affected pts may survive to adulthood)
Differential diagnosis of Roberts
TAR: thrombocytopenia - absent radius. Bilateral absence of the radii with presence of both thumbs and thrombocytopenia.
Presence of cleft suggests RBS over TAR.
What’s are the features of mosaic variegated anueploidy
Pre and postnatal GR. microcephaly. Dev del/ID. brain and ophthalmological anomalies. Dysmorphic facies (triangular facies, micrgnathia, epicanthic folds)
What causes MVA
BUB1B encodes BUBR a key protein in MITOTIC spindle checkpoint. Mutations increase the incidence of cancer (75%).
CEP57 a centrosomal protein involved in nuclear in and stabilising micro tubules (so far no link with cancer).
MVA caused by defective cell division resulting in nondisjunction at mitosis.
Discuss PCS in MVA
PCS: premature chromatid seperation. Split chromosomes, splayed chromatids in all/most chromosomes.
Results in many cells showing aneuploidy, double aneuploidy, monosomies.
Chromosome breakage often observed.
What are the features of Xermoderma Pigmentosum
Extreme sensitivity to UV (eyes and exposed skin, nervous system in some pts) results in blistering, extreme freckling of exposed skin. Onset about 2 years (can be aggressive and earlier).
Increased risk of skin cancer (by 10years with poor sun protection).
30% develop progressive neurological problems: hearing loss, poor coordinate, difficultly walking, loss of intellectual function, seizures, problems talking and swallowing.
What causes Xermoderma Pigmentosum
Build up of unrepaired DNA damage. All genes involved in DNA damage repair (most: nucleotide excision repair).
At least 8 inherited forms of XP: 1-8. Mutations of at least 8 genes cause XP.
Over 50% XPC, ERCC2, POLH.
What are the features of PCC Premature Chromosome Condensation.
Now called Microcephaly Primary Hereditary HCPH1.
Microcephaly (2sd below mean for see, age and ethnicity at birth and 3sd below at 6months).
No gross anomalies of brain archetecture- variable degrees of ID.
Testing of HCPH1/ PCC
G-banding: over 10% prophase-like cells vs less than 2% in a normal control.
If suspect MCPH don’t use colcemid as the spindle inhibitor attenuated the condensation defect by masking the postmitotic effect in early G1.
Sequencing of ASPM: greatest number of MCPH cases.
