A patient with hypotonia, blepharophimosis, ptosis, a bulbous nose, a long philtrum, upturned corners of the mouth, and mild developmental delay was found to have a small subtelomeric deletion of the long arm of chromosome 14 (q32.31-qter). In comparing her phenotype with previously reported patients with similar 14q deletions, due to either a linear deletion or to a ring chromosome 14, a clinically recognizable terminal 14q microdeletion syndrome was evident. Due to the limited number of cases reported, it was not possible to assign specific features to specific regions of terminal 14q. The comparison of features in cases with a linear deletion of 14qter (n = 19) to those in cases with a deletion due to a ring chromosome 14 (n = 23), both with the same breakpoint in 14q, showed that seizures and retinitis pigmentosa have been found only in patients with ring chromosomes. Several hypotheses are put forward to explain this difference: mitotic instability of ring chromosomes; a telomere position effect in ring chromosomes in which the 14p telomere silences nearby gene(s) on the q-arm; and dose-dependent gene(s) involved in seizures and retinitis pigmentosa located on the short arm of chromosome 14. In our opinion, only seizures may be explained by the mitotic instability of ring chromosomes, while both seizures and retinitis pigmentosa may be explained by silencing of gene(s) on 14q by the 14p telomere; the third hypothesis seems unlikely to explain either symptom.

Objective: The frequency of subtelomeric rearrangements in patients with unexplained mental retardation (MR) is uncertain, as most studies have been retrospective and case retrieval may have been biased towards cases more likely to have a chromosome anomaly. To ascertain the frequency of cytogenetic anomalies, including subtelomeric rearrangements, we prospectively screened a consecutive cohort of cases with unexplained MR in an academic tertiary centre. Methods: Inclusion criteria were: age <18 years at referral, IQ<85, no aetiological diagnosis after complete examination, which included karyotyping with high resolution banding (HRB). Results: In 266 karyotyped children, anomalies were detected in 20 (7.5%, seven numerical, 13 structural); 39 cases were analysed by FISH for specific interstitial microdeletions, and anomalies were found in nine (23%). FISH analyses for subtelomeric microdeletions were performed in 184 children (44% moderate-profound MR, 51% familial MR), and one rearrangement (0.5%) was identified in a non-familial MR female with mild MR (de novo deletion 12q24.33-qter). The number of probable polymorphisms was considerable: 2qter (n=7), Xpter (n=3), and Ypter (n=1). A significantly higher total number of malformations and minor anomalies was present in the cytogenetic anomaly group compared to the group without cytogenetic anomalies. Conclusions: The total frequency of cytogenetic anomalies in this prospective study was high (1:10), but the frequency of subtelomeric rearrangements was low. The most likely explanations are the high quality of HRB cytogenetic studies and the lack of clinical selection bias. Conventional cytogenetic analyses, combined with targeted microdeletion testing, remain the single most effective way of additional investigation in mentally retarded children, also in a tertiary centre.

We report on a girl with a duplication of chromosome band 11p12-->13, which includes the Wilms tumor gene (WT1) and the aniridia gene (PAX6). The girl had borderline developmental delay, mild facial anomalies, and eye abnormalities. Eye findings were also present in most of the 11 other published cases with partial trisomy 11p, including 11p12-->13. Recently, it was shown that introduction of additional copies of the PAX6 gene into mice caused very variable eye abnormalities. Therefore, a PAX6 gene dosage effect is likely to be present in mice and humans. The central nervous system may be less sensitive to an altered PAX6 gene dosage, which is consistent with the borderline developmental delay in the present patient. Urogenital abnormalities were absent in this patient and in most of the other patients with partial trisomy of 11p. Therefore, the effect of a WT1 gene duplication on the embryological development of the urogenital tract remains uncertain.

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