No abstract

Small supernumerary marker chromosomes (SMCs) are present in about 0.05% of the human population. In approximately 30% of SMC carriers (excluding the approximately 60% SMC derived from one of the acrocentric chromosomes), an abnormal phenotype is observed. The clinical outcome of an SMC is difficult to predict as they can have different phenotypic consequences because of (1). differences in euchromatic DNA-content, (2). different degrees of mosaicism, and/or (3). uniparental disomy (UPD) of the chromosomes homologous to the SMC. Here, we present 35 SMCs, which are derived from all human chromosomes, apart from chromosome 6, as demonstrated by the appropriate molecular cytogenetic approaches, such as centromere-specific multicolor fluoresence in situ hybridization (cenM-FISH), multicolor banding (MCB), and subcentromere-specific multicolor FISH (subcenM-FISH). In nine cases without an aberrant phenotype, neither partial proximal trisomies nor UPD could be detected. Abnormal clinical findings, such as psychomotoric retardation and/or craniofacial dysmorphisms, were associated with seven of the cases in which subcentromeric single-copy probes were proven to be present in three copies. Conversely, in eight cases with a normal phenotype, proximal euchromatic material was detected as partial trisomy. UPD was studied in 12 cases and subsequently detected in two of the cases with SMC (partial UPD 4p and maternal UPD 22 in a der(22)-syndrome patient), indicating that SMC carriers have an enhanced risk for UPD. At present, small proximal trisomies of 1p, 1q, 2p, 6p, 6q, 7q, 9p, and 12q seem to lead to clinical manifestations, whereas partial proximal trisomies of 2q, 3p, 3q, 5q, 7p, 8p, 17p, and 18p may not be associated with significant clinical symptoms. With respect to clinical outcome, a classification of SMCs is proposed that considers molecular genetic and molecular cytogenetic characteristics as demonstrated by presently available methods.

It has previously been shown that, in the heterozygous state, mutations in the SOX9 gene cause campomelic dysplasia (CD) and the often associated autosomal XY sex reversal. In 12 CD patients, 10 novel mutations and one recurrent mutation were characterized in one SOX9 allele each, and in one case, no mutation was found. Four missense mutations are all located within the high mobility group (HMG) domain. They either reduce or abolish the DNA-binding ability of the mutant SOX9 proteins. Among the five nonsense and three frameshift mutations identified, two leave the C-terminal transactivation (TA) domain encompassing residues 402-509 of SOX9 partly or almost completely intact. When tested in cell transfection experiments, the recurrent nonsense mutation Y440X, found in two patients who survived for four and more than 9 years, respectively, exhibits some residual transactivation ability. In contrast, a frameshift mutation extending the protein by 70 residues at codon 507, found in a patient who died shortly after birth, showed no transactivation. This is apparently due to instability of the mutant SOX9 protein as demonstrated by Western blotting. Amino acid substitutions and nonsense mutations are found in patients with and without XY sex reversal, indicating that sex reversal in CD is subject to variable penetrance. Finally, none of 18 female patients with XY gonadal dysgenesis (Swyer syndrome) showed an altered SOX9 banding pattern in SSCP assays, providing evidence that SOX9 mutations do not usually result in XY sex reversal without skeletal malformations.

Townes-Brocks syndrome (TBS) is an autosomal dominantly inherited malformation syndrome characterized by anal, renal, limb, and ear anomalies. Recently, we showed that mutations in the putative zinc finger transcription factor gene SALL1 cause TBS. To determine the spectrum of SALL1 mutations and to investigate the genotype-phenotype correlations in TBS, we examined 23 additional families with TBS or similar phenotypes for SALL1 mutations. In 9 of these families mutations were identified. None of the mutations has previously been described. Two of these mutations are nonsense mutations, one of which occurred in three unrelated families. Five of the mutations are short deletions. All of the mutations are located 5' of the first double zinc finger (DZF) encoding region and are therefore predicted to result in putative prematurely terminated proteins lacking all DZF domains. This suggests that only SALL1 mutations that remove the DZF domains result in TBS. We also present evidence that in rare cases SALL1 mutations can lead to phenotypes similar to Goldenhar syndrome. However, phenotypic differences in TBS do not seem to depend on the site of mutation.

Recently mutations in the gene ZFHX1B (SIP1) were shown in patients with “syndromic Hirschsprung disease” with mental retardation (MR) and multiple congenital anomalies (MCA), but it was unclear if Hirschsprung disease is an obligate symptom of these mutations and if the distinct facial phenotype delineated by Mowat et al. [1998: J Med Genet 35: 617–623] is specific for ZFHX1B mutations. In order to address these open questions we analyzed the ZFHX1B gene in five patients, three of whom had “syndromic Hirschsprung disease” two with and one without the facial phenotype described by Mowat et al. [1998], and two of whom had the distinct facial gestalt without Hirschsprung disease. Analyses of microsatellite markers and newly identified SNPs, and/or FISH with BACs from the ZFHX1B region excluded large deletions in all five patients. Direct sequencing demonstrated truncating ZFHX1B mutations in all four patients with the characteristic facial phenotype, but not in the patient with syndromic Hirschsprung disease without the distinct facial appearance. We demonstrate that there is a specific clinical entity with a recognizable facial gestalt, mental retardation and variable MCAs which we propose be called the “Mowat‐Wilson syndrome.” © 2002 Wiley‐Liss, Inc.