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Summary: Coexistent mosaic monosomy 21 and fragile X syndrome in a mentally retarded male patient: Fragile X syndrome (FXS) is a well-recognized mental retardation syndrome with characteristic facial features and behavioural phenotype. Monosomy 21 is a rare cytogenetic aberration for which clinical features were incompletely defined since full monosomy 21 is incompatible with life. A 5-year-old male patient with FXS and low-grade mosaicism for full monosomy 21 (46,XY[96%]/45,XY,-21[4%)) is presented. He had lack of speech and severely impaired social skills, hyperactivity, stereotypical hand movements, a special interest towards moving colourful items and a short attention span for other objects around. He had macrocephaly, a rather long face, prominent occiput and prominent midface, retrognathia, down-slanting palpebral fissures, hypertelorism and cup-shaped, posteriorly rotated and low-set ears. Full monosomy in the aberrant cell line was proven by whole chromosome painting. FXS was previously reported to accompany sex chromosome aneuploidies; however, to the best of our knowledge, the present patient is the first FXS patient with an aberration involving autosomes. He contributes to the current knowledge on monosomy 21 phenotype, having dysmorphic facial findings despite the concurrent phenotypic expression of the FXS. As a last conclusion, cytogenetic analysis must be done to all mentally retarded patients with minor dysmorphic features.
Keywords: Monosomy 21 - Autosomal mosaicism - Fragile X syndrome
INTRODUCTION
Monosomy 21 has been a rare cytogenetic finding in human malformations (23). Total absence of one of the chromosomes 21 (full monosomy) was confirmed with fluorescence in situ hybridisation (FISH) a few times (11,17,20). Full monosomy 21 is accepted to be incompatible with in utero survival (10,11,21), and most of the patients untested with FISH for cryptic translocations may be expected to have partial monosomy of chromosome 21 (21, 23). However, monosomy 21 is a rare cause of abortions as well (10). As a consequence, phenotypic features caused by monosomy 21 could only be characterised with the aid of partially monosomic patients evaluated for phenotype-genotype correlations. Mosaic patients for monosomy 21 have also been reported infrequently (23). Expectedly, phenotypic features are milder in these patients; however, presence of hidden unbalanced translocations in the aberrant cell line was tested in none of these patients (23).
Fragile X syndrome (FXS) is a well-delineated mental retardation syndrome. Well-defined facial...