Unbalanced submicroscopic subtelomeric chromosomal rearrangements represent a significant cause of unexplained moderate to severe mental retardation with and without phenotypic abnormalities. We investigated 254 patients (102 from Zürich, 152 from Liège) for unbalanced subtelomeric rearrangements by using fluorescence in situ hybridisation with probes mapping to 41 subtelomeric regions. Mental retardation combined with a pattern of dysmorphic features, with or without major malformations, and growth retardation and a normal karyotype by conventional G-banding were the criteria of inclusion. Selection criteria were more restrictive for the Zürich series in terms of clinical and cytogenetic pre-investigation. We found 13 unbalanced rearrangements and two further aberrations, which, following the investigation of other family members, had to be considered as variants without influence on the phenotype. The significant aberrations included three de novo deletions (two of 1pter, one of 5pter), three de novo duplications (8pter, 9pter, Xpter), one de novo deletion 13qter-duplication 4qter, and five familial submicroscopic translocations [(1q;18p), (2q;4p), (2p;7q), (3p;22q), (4q;10q), (12p;22q)], most of them with several unbalanced offspring with deletion-duplication. Although the incidence of abnormal results was higher (10/152) in the Liège versus the Zürich series (3/102), similar selection criteria in Zürich as in Liège would have resulted in an incidence of 7/106 and thus similar figures. In our series, submicroscopic unbalanced rearrangements explain the phenotype in 13/254 study probands. The most important selection criterion seems to be the presence of more than one affected member in a family. An examination of subtelomeric segments should be included in the diagnostic work-up of patients with unexplained mental retardation combined with physical abnormalities, when a careful conventional examination of banded chromosomes has yielded a normal result and a thorough clinical examination does not lead to another classification. The proportion of abnormal findings depends strongly on selection criteria: more stringent selection can eliminate some examinations but necessitates a high workload for experienced clinical geneticists. Once the costs and workload of screening are reduced, less selective approaches might finally be more cost-effective.

ABSTRACT:The aim of this study was to specify the early setting of the particular craniofacial morphology in Down syndrome during the fetal period from data based on postmortem examinations. The study included 1277 fetuses at 15-38 gestational weeks (GW): 922 control fetuses and 355 fetuses with trisomy 21, selected from fetopathology units in Paris. Body weight (BW) and nine dimensions of the face, skull, and brain were recorded: the outer and inner canthal distances (OCD, ICD), biparietal diameter (BPD), head circumference (HC), brain weight (BrW), occipitofrontal diameters of left and right hemispheres (lOFD, rOFD), weight of the infratentorial part of the brain (IBW), and maximal transversal diameter of the cerebellum (CTD). Four ratios were computed: BPD/HC, OCD/ BPD, BrW/BW, IBW/BrW. Differences between trisomic fetuses and control fetuses were tested by age interval. Results showed that BW, rOFD, and lOFD were lower in trisomic fetuses as early as 15 GW. Cerebellar hypoplasia included lower IBW and CTD in trisomic fetuses. The IBW/BrW ratio was higher in trisomic fetuses, showing that growth restriction affected the infratentorial part of the brain less than the supratentorial part. Early brachycephaly was found in trisomic fetuses, with higher values of BPD and BPD/HC from 15 GW. ICD and OCD were not significantly different in the two groups, but OCD/DBP ratio was lower in trisomic fetuses. These results confirm the early phenotypical expression of trisomy 21 on craniofacial morphology, associated with a marked restriction of brain growth, especially in the supratentorial part. T he distinctive features of craniofacial and brain morphology in trisomy 21 have been well documented in children and adults. Numerous structural abnormalities including reduced BrW, alteration of configuration, and maturation delay were found in the postnatal period. Concerning fetuses, most studies were performed to improve the diagnosis of Down syndrome, and mainly involve ultrasound records. Since cerebellar hypoplasia in trisomy 21 has been found in the postnatal period, much attention has been focused on cerebellum dimensions in fetuses, but the results are contradictory : some studies failed to observe cerebellar hypoplasia (1,2), unlike others (3-5). Other cerebral structures have been studied in fetuses with Down syndrome. Hypoplasia of the frontal lobes has been found in ultrasound studies of trisomy 21 (6,7) but was not confirmed in a postmortem pathologic study (8).Craniofacial dysmorphology was also assessed using skull and face dimensions ratios: brachycephaly was assessed based on the cephalic index (9). In all ultrasonographic studies (10 -12) but one (13), brachycephaly was identical in fetuses with and without trisomy 21. Very few data have been collected concerning the BrW of fetuses with trisomy (8), and no significant difference in BrW was detected before birth.The purpose of this study was to complete the information on the specificities of craniofacial and cerebral growth in fetuses with Down syndrome. It w...

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