Prospective screening for subtelomeric rearrangements in children with mental retardation of unknown aetiology: the Amsterdam experience

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.

Molecular genetic testing for familial hypercholesterolemia: spectrum of LDL receptor gene mutations in The Netherlands

Mutations in the LDL receptor are responsible for familial hypercholesterolemia (FH). At present, more than 600 mutations of the LDL receptor gene are known to underlie FH. However, the array of mutations varies considerably in different populations. Therefore, the delineation of essentially all LDL receptor gene mutations in a population represents a prerequisite for the implementation of nation-wide genetic testing for FH. In this study, the frequency and geographical distribution of 13 known mutations were evaluated in a cohort of 1223 FH patients. We identified 358 mutation carriers, representing 29% of the FH cohort. Four mutations (N543H-2393de19, 1359--1G-->A, 313 + 1 G-->A and W23X) occurred with a relatively high frequency, accounting for 22.4% of the entire study cohort. Two of these common FH mutations (N543H-2393de19 and 1359 - 1G-->A) showed a preferential geographic distribution. Second, to further expand the array of LDL receptor gene mutations, we conducted mutation analysis by denaturing gradient gel electrophoresis (DGGE) in 141 children with definite FH. A mutation was identified in 111 patients, involving 16 new single base substitutions and four small deletions and insertions, which brings the number of different FH-causing mutations in our country up to 61. Our data indicate that an estimate of the prevalence of specific mutations, as well as the compilation of a database of all FH-causing mutations in a given country, can facilitate selection of the most appropriate molecular diagnostic approach.

DNA diagnosis in a family with autosomal dominant aniridia

A large family with autosomal dominant aniridia is described. One of the family members presented with reduced visual acuity, nystagmus, slightly distorted macular reflex, but normal irides and clear media. Because of the high variability in expression of aniridia, even within family, a diagnosis of aniridia could not be excluded. However linkage analysis using tightly linked chromosome 11p13 markers flanking the aniridia locus (catalase, D11S151, and D11S325) made it highly unlikely that this patient inherited the aniridia gene from his affected mother.

Synaptonemal complex proteins

Synaptonemal complexes were isolated from rate spermatocytes for the purpose of biochemical and morphological analysis. Several monoclonal antibodies were elicited against purified synaptonemal complexes to study the composition and assembly of these structures. Four classes of antibodies could be discriminated according to the polypeptides that they recognize on Western blots of purified synaptonemal complexes, namely antibodies recognizing (i) a 190-kDa polypeptide; (ii) a 30- and a 33-kDa polypeptide; (iii) two polypeptides with molecular weights of about 120 kDa; and (iv) polypeptides with molecular weights of 66-55 kDa. The localization of these antigens within spermatocytes was analyzed light microscopically, by means of the immunoperoxidase technique and ultrastructurally, by immunogold labelling of surface-spread spermatocytes. The 66- to 55-kDa polypeptides are not confined to synaptonemal complexes; rather, these polypeptides appear to be chromosomal components. The 190-, 30-, and 33-kDa polypeptides make part of the lateral elements of paired as well as unpaired segments of synaptonemal complexes. The 120-kDa polypeptides were localized on the inner edge of the lateral elements, specifically in paired segments of synaptonemal complexes. The distribution of the 190-, 120-, 30-, and 33-kDa polypeptides within the testis was analyzed by immunofluorescence staining of cryostat sections. All these polypeptides turned out to be specific for nuclei of zygotene up to and including diplotene spermatocytes. Only in some early spermatids could the 190-, 30-, and 33-kDa polypeptides be detected, presumably in remnants of synaptonemal complexes.(ABSTRACT TRUNCATED AT 250 WORDS)

Two major components of synaptonemal complexes are specific for meiotic prophase nuclei

Monoclonal antibody II52F10 was elicited against purified synaptonemal complexes (SCs); it recognizes two major components of the lateral elements of SCs, namely an Mr = 30,000 and an Mr = 33,000 protein. We studied the distribution of the antigens of II52F10 within tissues and cells of the male rat by immunoblot analysis and immunocytochemical techniques. Nuclear proteins from various cell types, including spermatogonia and spermatids, did not react with antibody II52F10 on immunoblots; the same holds for proteins from isolated mitotic chromosomes. As expected, an Mr = 30,000 and an Mr = 33,000 protein from spermatocyte nuclei did react with the antibody. In cryostat sections of liver, brain, muscle and gut we could not detect any reaction with II52F10. In the testis the reaction was confined to SCs or SC fragments. Partly on the basis of indirect evidence we identified the antigen-containing cells as zygotene up to and including post-diffuse diplotene spermatocytes. The persistence of some antigen-containing fragments in the earliest stages of spermatids could not be excluded. We conclude that the lateral elements (LEs) of SCs are not assembled by rearrangement of pre-existing components of the nucleus: at least two of their major components are newly synthesized, presumably during zygotene. Furthermore we conclude partly from indirect evidence that the major components of the LEs of SCs are not involved in the chromosome condensation processes that take place during the earliest stages of meiotic prophase.

Identification of two major components of the lateral elements of synaptonemal complexes of the rat

This paper describes the identification of two major components of the lateral elements of synaptonemal complexes of the rat by immunocytochemical techniques. We prepared monoclonal antibodies against synaptonemal complexes (SCs) by immunization of mice with purified SCs. One of these antibodies, II52F10, reacts with a 30 and a 33 kDa polypeptide, which are major components of purified SCs. Using this antibody, we studied the localization of its antigens light microscopically, by means of the indirect immunoperoxidase technique, as well as ultrastructurally, by means of the immunogold labeling technique. The immunolocalization was carried out on whole-mount preparations of lysed spermatocytes. The antibody reacts with paired as well as unpaired segments of zygotene, pachytene and diplotene SCs. In light microscopic preparations, the attachment plaques, particularly those of late pachytene and diplotene SCs, also appear to react strongly. In electron micrographs the lateral elements in paired as well as unpaired segments could be seen to react. No reaction was observed in the attachment plaques; however, in late pachytene and diplotene SCs the swollen terminal segments of the lateral elements did react with the antibody. Thus, we conclude that a 30 and a 33 kDa polypeptide make part of the lateral elements of synaptonemal complexes of the rat.

Amniotic fluid gel acetylcholinesterase determination in prenatal diagnosis: dark field illumination as a method for improving the detection of precipitation bands

This paper describes a sensitive method for the detection of precipitation bands in the qualitative acetylcholinesterase (AChe) gel test. This method--dark field illumination--is compared with two existing methods of detection: epi-illumination of unstained gels and transillumination of dithio-oxamide stained gels. The comparison has been carried out on a selected series of 271 amniotic fluid samples, taken before the 22nd week of gestation. All 96 samples from normal pregnancies were scored as negative with all three visualization methods. For the detection of neural tube defects (NTD) and omphaloceles, dark field illumination proved to be more efficient than epi-illumination or dithio-oxamide staining. In case of another congenital defect, we obtained a positive result with dark field illumination, and a negative one with the other detection methods. It is concluded that dark field illumination was shown to be the most sensitive method for the detection of precipitation bands.

Structure and composition of synaptonemal complexes, isolated from rat spermatocytes

Synaptonemal complexes (SCs) (structures involved in chromosome pairing during meiosis) were isolated and purified from rat spermatocytes for the purpose of biochemical and morphological analysis. Spermatocytes were lysed in a medium, containing Triton X-100, EDTA and DTT; the resulting swollen nuclei were disrupted by DNAse II, and the suspension was centrifuged through 1.5 M sucrose. The resulting preparation consisted for at least 60% of free SCs, as judged from electron micrographs of agar filtrates. The purified SCs still possessed lateral and transversal elements and attachment plaques. A small fraction also contained a central element. Particularly in diplotene SCs, the lateral elements clearly consisted of two subelements, which are connected by thinner fibres. The lateral elements may fall apart into a network of thinner fibres, presumably as a result of degradation during isolation. On SDS-polyacrylamide gels, the major protein components of purified SCs had relative mobilities (Mrs) of 67 to 60 and 57 to 55 kDa; in addition, there were minor proteins with Mrs of 90, 35, 33, 28, and 26 kDa, and varying amounts of histones. The 67 to 60 kDa proteins comigrate with lamins of rat liver pore complexes and laminae. A possible relationship between SCs and pore complexes and laminae is discussed.