Holoprosencephaly due to mutations in ZIC2: alanine tract expansion mutations may be caused by parental somatic recombination.

We report on the prevalence of mutations in the zinc finger transcription factor gene, ZIC2, in a group of 509 unrelated individuals with isolated holoprosencephaly (HPE) and normal chromosomes. Overall, we encountered 16 HPE patients (from 15 unrelated families) with ZIC2 mutations. Thus, ZIC2 mutation was the apparent cause of HPE in 3-4% of cases. Seven mutations were frameshifts that were predicted to result in loss of function, further supporting the idea that ZIC2 haploinsufficiency can result in HPE. One mutation, an alanine tract expansion which is caused by the expansion of an imperfect trinucleotide repeat, occurred in seven patients from six different families. In three of those families, the father was found to be apparently mosaic for the mutation. We hypothesize that this mutation can arise through errors in somatic recombination, an extremely unusual mutation mechanism. In addition, one mutation resulted in a single amino acid change and one mutation was an in-frame deletion of 12 amino acids. The central nervous system malformations seen in patients with ZIC2 mutations ranged from alobar HPE (most common) to middle interhemispheric fusion defect (one case). Although severe facial anomalies are common in HPE, all of the patients with ZIC2 mutations had relatively normal faces, suggesting that ZIC2 mutations represent a large proportion of HPE cases without facial malformation.

Genotypic and phenotypic spectrum in tricho-rhino-phalangeal syndrome types I and III.

Tricho-rhino-phalangeal syndrome (TRPS) is characterized by craniofacial and skeletal abnormalities. Three subtypes have been described: TRPS I, caused by mutations in the TRPS1 gene on chromosome 8; TRPS II, a microdeletion syndrome affecting the TRPS1 and EXT1 genes; and TRPS III, a form with severe brachydactyly, due to short metacarpals, and severe short stature, but without exostoses. To investigate whether TRPS III is caused by TRPS1 mutations and to establish a genotype-phenotype correlation in TRPS, we performed extensive mutation analysis and evaluated the height and degree of brachydactyly in patients with TRPS I or TRPS III. We found 35 different mutations in 44 of 51 unrelated patients. The detection rate (86%) indicates that TRPS1 is the major locus for TRPS I and TRPS III. We did not find any mutation in the parents of sporadic patients or in apparently healthy relatives of familial patients, indicating complete penetrance of TRPS1 mutations. Evaluation of skeletal abnormalities of patients with TRPS1 mutations revealed a wide clinical spectrum. The phenotype was variable in unrelated, age- and sex-matched patients with identical mutations, as well as in families. Four of the five missense mutations alter the GATA DNA-binding zinc finger, and six of the seven unrelated patients with these mutations may be classified as having TRPS III. Our data indicate that TRPS III is at the severe end of the TRPS spectrum and that it is most often caused by a specific class of mutations in the TRPS1 gene.

Formation of supernumerary euchromatic short arm isochromosomes: parent and cell stage of origin in new cases and review of the literature.

In order to get insight in the formation of isochromosomes we analysed different supernumerary euchromatic short arm isochromosomes for the parent and cell stage of origin. After cytogenetic detection and confirmation by fluorescence-in-situ hybridization we performed short tandem repeat typing in a child with i(9p), three with i(12p) and three with i(18p). The extra chromosomes were monocentric in each case, the i(9p) and i(12p) constitutions were found in mosaic with normal cell lines. Our results and those of other groups indicate a strong role of maternal meiosis in isochromosome formation: in one i(8p), 4 out of 5 i(9p), 7 out of 12 i(12p) and 18 out of 23 i(18p) families a maternal meiotic nondisjunction had occurred prior to the centromere misdivision. For chromosome 18, the majority of isochromosomes originated from a maternal meiosis II error (16/18). For the other tetrasomic constitutions the isochromosomes could be delineated from paternal as well as from maternal origin, the short tandem repeat typing patterns being consistent with meiotic or mitotic cell stages of formation. Thus, independently of the chromosomal origin, in the majority of cases with additional euchromatic isochromosomes maternal meiosis nondisjunction is the initial step followed by centromeric misdivision. Postzygotic nondisjunction as suggested previously due to mosaics observed in tetrasomies 9p and 12p seems to be of minor importance. The observed origin of isochromosomes 18 corresponds to that of trisomy 18, where the majority of cases can be delineated from maternal meiosis II errors.

Multiplex-FISH for pre- and postnatal diagnostic applications.

For >3 decades, Giemsa banding of metaphase chromosomes has been the standard karyotypic analysis for pre- and postnatal diagnostic applications. However, marker chromosomes or structural abnormalities are often encountered that cannot be deciphered by G-banding alone. Here we describe the use of multiplex-FISH (M-FISH), which allows the visualization of the 22 human autosomes and the 2 sex chromosomes, in 24 different colors. By M-FISH, the euchromatin in marker chromosomes could be readily identified. In cases of structural abnormalities, M-FISH identified translocations and insertions or demonstrated that the rearranged chromosome did not contain DNA material from another chromosome. In these cases, deleted or duplicated regions were discerned either by chromosome-specific multicolor bar codes or by comparative genomic hybridization. In addition, M-FISH was able to identify cryptic abnormalities in patients with a normal G-karyotype. In summary, M-FISH is a reliable tool for diagnostic applications, and results can be obtained in

Two brothers with multiple congenital anomalies and mental retardation due to disomy (X)(q12-->q13.3) inherited from the mother.

We present the phenotypic, cytogenetic and molecular findings in two dysmorphic and mentally retarded brothers with disomy Xq12-->q13.3. The mother and the grandmother carry the same rearrangement of the X chromosome, which was interpreted as an inverted insertion of the segment (X)(q12-->q13.3) into Xq21.2. The X-inactivation-specific-transcript (XIST) is expressed in the probands mother but is absent in her son, confirming the hypothesis that X inactivation is realized only if two X inactivation centers reside on different X-chromosomes (trans-configuration). In the phenotypically normal mother the aberrant X chromosome was late replicating in all cells, indicating functional monosomy of the constitutional segment trisomy. The phenotype of the brothers is considered to be the consequence of a functional disomy Xq12-->q13.3. The trait combination observed in the brothers was compared with the spectrum of clinical and anthropological traits for proximal Xq disomy in males, elaborated by phenotype analyses of the available literature cases.

The fragile-X phenotype. Computer assisted analysis of the dysmorphological features and discrimination from the Sotos phenotype.

Fragile-X and Sotos phenotypes may be difficult to distinguish. This is illustrated with a case report. Computer assisted phenotype analyses (MDDB), using the complete trait list of this patient, suggested the fragile-X diagnosis, which later was confirmed by molecular techniques. The results of corresponding phenotype analyses are summarized for 17 children with proven fragile-X, and 10 children with suggested Sotos syndrome.

Partial trisomy 8q. Two case reports with maternal translocation and inverted insertion: phenotype analyses and reflections on the risk.

Partial trisomy 8qter-->q23 or q24.1 has been reported in 15 literature cases. We add two further case reports here. Patient 1 inherited the derivative (2) of a balanced maternal reciprocal translocation t(2;8)(qter;q2300) after 2:2 disjunction and adjacent-1 segregation, and is trisomic for the segment 8qter-->q2300. Patient 2 inherited a recombinant (8) of a balanced maternal inverted insertion inv ins(8)(q1300;q2300q24.2) and is trisomic for the segment 8q24.2-->q2300. The phenotype of both patients is described and compared to the spectrum of symptoms established from the 15 literature cases. This spectrum contains all features observed with a frequency of > = 50%. Patient 1 had 35% of the features of this spectrum; Patient 2 had 47%. The intrauterine survival probability of unbalanced offspring is assumed to be the same in both cases, as nearly the same segments are trisomic. The pedigrees indicate that the inversion carrier may have a reduced production probability of unbalanced gametes and therefore a reduced risk compared to the translocation carrier.