HGM2002 Poster Abstracts: 7. Medical Genomics
POSTER NO: 319
22q11 deletion - mini-FISH approach and discovery of further atypical cases
1Jesse Li-Ling, 2C. Paul Daniel, 3Ian Cross, 3John Burn
Heterozygous chromosome microdeletion del(22)(q11.2-q11.2) (Abbreviated below as 22q11 deletion) underlies a wide spectrum of congenital malformations embraced by various labels such as DiGeorge syndrome, Velocardiofacial syndrome, Conotruncal anomaly face syndrome, and CATCH phenotype. It is responsible for a significant proportion of congenital heart defects, cleft palate, velopharyngeal insufficiency, learning disability and psychiatric disorders. Fluorescence in situ hybridisation (FISH) has proved to be an ideal tool for the detection of 22q11 deletion. However, its application for the studying of large number of samples has been limited due to its cost and technique features. We report development of a mini-FISH method, with which up to 28 samples were processed on a single slide. This method can reduce the cost of diagnosis by around 90% owing to minimal probe consumption and can easily be adapted for routine analysis. Using dual-colour labelled probes from the commonly deleted region and a 22q13 marker, we have assessed 22q11 deletion status in over 2,000 samples. The mini-FISH method has attained good sensitivity and specificity, with an overall rate of successful diagnosis exceeding 95%. 92% of samples were scored at first time, among which 91% were diagnosed by metaphase chromosome analysis. Several features of this method, including high efficiency, no affection on cell dispersion or significant increase of the background autofluoresence, minimum cross-contamination, etc. have suggested its suitability for large-scale analysis. Among our identified carriers, most were neonates initially suspected for characteristic cardiovascular malformations or suggestive dysmorphic features. Rare features such as bifid thumb, umbilical and inguinal hernias, and interstitial pulmonary fibrosis were also noted. 22q11 deletion has also been identified among available parents and grandparents, none of which were previously diagnosed. There had been one mother carrier who had no overt clinical problems. One father carrier had no cardiac abnormality, no dysmorphic appearance, and no nasal speech. Most strikingly, we have identified a 22q11 deletion in a patient presenting as 'isolated severe scoliosis'. The patient, a 19-year- old male, was referred to an orthopaedic clinic due to progressive scoliosis accompanied with worsening back pain. Past history revealed bilateral pes cavus, umbilical hernia, unexplained learning difficulties and onset of measles. Other abnormal findings included pain in the right ankle, small hand muscle wasting and very long fingers, lax elbow joints and widespread stretch marks. Initial spinal X-ray and magnetic resonance imaging demonstrated a severe S-shaped thoraco-lumbar kyphoscoliosis and bilateral acetabulae protrusio. A pelvic X-ray further revealed bilateral absence of sacroiliac joints. On the bone scan, radionuclide Tc99m uptake within the spine was surprisingly 'normal' considering the degree of the curvature. Renal retention of Tc99m was noticed incidentally. Retrospectively, the patient was found to be mildly dysmorphic with low front hairline, narrow palpebral fissures, pinched nares, and high palate. Echocardiography revealed a normal heart except for an aortic root of 3.7 cm in diameter (upper limit of normal range). As at least a quarter of 22q11 carriers do not have a cardiovascular abnormality clinically and the characteristic facies can be easily missed in the absence of typical malformations, we call for a yet higher index of suspicion for this important genetic abnormality. FISH for 22q11 deletion should be offered to a yet wider range of patients.
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