Ken-Shiung Chen

Homologous recombination of a flanking repeat gene cluster is a mechanism for a common contiguous gene deletion syndrome

Smith–Magenis syndrome (SMS), caused by del(17)p11.2, represents one of the most frequently observed human microdeletion syndromes. We have identified three copies of a low–copy–number repeat (SMS–REPs) located within and flanking the SMS common deletion region and show that SMS–REP represents a repeated gene cluster. We have isolated a corresponding cDNA clone that identifies a novel junction fragment from 29 unrelated SMS patients and a different–sized junction fragment from a patient with dup(17)p11.2. Our results suggest that homologous recombination of a flanking repeat gene cluster is a mechanism for this common microdeletion syndrome.

Molecular mechanism for duplication 17p11.2 : the homologous recombination reciprocal of the Smith-Magenis microdeletion

Recombination between repeated sequences at various loci of the human genome are known to give rise to DNA rearrangements associated with many genetic disorders. Perhaps the most extensively characterized genomic region prone to rearrangement is 17p12, which is associated with the peripheral neuropathies, hereditary neuropathy with liability to pressure palsies (HNPP) and Charcot-Marie-Tooth disease type 1A (CMT1A;ref. 2). Homologous recombination between 24-kb flanking repeats, termed CMT1A–REPs, results in a 1.5-Mb deletion that is associated with HNPP, and the reciprocal duplication product is associated with CMT1A (ref. 2). Smith-Magenis syndrome (SMS) is a multiple congenital anomalies, mental retardation syndrome associated with a chromosome 17 microdeletion, del(17)(p11.2p11.2) (ref. 3,4). Most patients (>90%) carry deletions of the same genetic markers and define a common deletion. We report seven unrelated patients with de novo duplications of the same region deleted in SMS. A unique junction fragment, of the same apparent size, was identified in each patient by pulsed field gel electrophoresis (PFGE). Further molecular analyses suggest that the de novo17p11.2 duplication is preferentially paternal in origin, arises from unequal crossing over due to homologous recombination between flanking repeat gene clusters and probably represents the reciprocal recombination product of the SMS deletion. The clinical phenotype resulting from duplication [dup(17)(p11.2p11.2)] is milder than that associated with deficiency of this genomic region. This mechanism of reciprocal deletion and duplication via homologous recombination may not only pertain to the 17p11.2 region, but may also be common to other regions of the genome where interstitial microdeletion syndromes have been defined.

Fliih, a Gelsolin-Related Cytoskeletal Regulator Essential for Early Mammalian Embryonic Development

ABSTRACT The Drosophila melanogaster flightless I gene is required for normal cellularization of the syncytial blastoderm. Highly conserved homologues of flightless I are present in Caenorhabditis elegans, mouse, and human. We have disrupted the mouse homologue Fliih by homologous recombination in embryonic stem cells. Heterozygous Fliih mutant mice develop normally, although the level of Fliih protein is reduced. Cultured homozygous Fliih mutant blastocysts hatch, attach, and form an outgrowing trophoblast cell layer, but egg cylinder formation fails and the embryos degenerate. Similarly, Fliih mutant embryos initiate implantation in vivo but then rapidly degenerate. We have constructed a transgenic mouse carrying the complete human FLII gene and shown that the FLII transgene is capable of rescuing the embryonic lethality of the homozygous targeted Fliih mutation. These results confirm the specific inactivation of the Fliih gene and establish that the human FLII gene and its gene product are functional in the mouse. The Fliih mouse mutant phenotype is much more severe than in the case of the related gelsolin family members gelsolin, villin, and CapG, where the homozygous mutant mice are viable and fertile but display alterations in cytoskeletal actin regulation.

DNA Rearrangements on Both Homologues of Chromosome 17 in a Mildly Delayed Individual with a Family History of Autosomal Dominant Carpal Tunnel Syndrome

Disorders known to be caused by molecular and cytogenetic abnormalities of the proximal short arm of chromosome 17 include Charcot-Marie-Tooth disease type 1A (CMT1A), hereditary neuropathy with liability to pressure palsies (HNPP), Smith-Magenis syndrome (SMS), and mental retardation and congenital anomalies associated with partial duplication of 17p. We identified a patient with multifocal mononeuropathies and mild distal neuropathy, growth hormone deficiency, and mild mental retardation who was found to have a duplication of the SMS region of 17p11.2 and a deletion of the peripheral myelin protein 22 (PMP22) gene within 17p12 on the homologous chromosome. Further molecular analyses reveal that the dup(17)(p11.2p11.2) is a de novo event but that the PMP22 deletion is familial. The family members with deletions of PMP22 have abnormalities indicative of carpal tunnel syndrome, documented by electrophysiological studies prior to molecular analysis. The chromosomal duplication was shown by interphase FISH analysis to be a tandem duplication. These data indicate that familial entrapment neuropathies, such as carpal tunnel syndrome and focal ulnar neuropathy syndrome, can occur because of deletions of the PMP22 gene. The co-occurrence of the 17p11.2 duplication and the PMP22 deletion in this patient likely reflects the relatively high frequency at which these abnormalities arise and the underlying molecular characteristics of the genome in this region.

The Smith-Magenis syndrome [del(17)p11.2]: Clinical review and molecular advances

The Smith-Magenis syndrome (SMS) is a multiple congenital anomaly, mental retardation syndrome associated with a deletion of chromosome 17p11.2. Since the recognition of this disorder as a clinical entity in 1982, the phenotypic features of SMS have been well described. Unfortunately, the often subtle physical and chromosomal findings of SMS may preclude the diagnosis in some affected individuals. This article offers a comprehensive review of more than 100 SMS patients whose cases have been reported, including details of the more recently studied clinical aspects of SMS. SMS has been postulated to be a contiguous gene deletion syndrome. Currently, only a few genes have been mapped to the SMS critical region. Further research (including genotype-phenotype correlation) is needed to identify the gene or genes that, when deleted, cause this disorder. Although the molecular etiology of SMS is unknown, recent investigations have identified multiple repetitive sequences within the SMS region. As described in other human disorders, repetitive sequences may be involved in homologous recombination and cause deletion. The advances in the molecular dissection of the SMS region are described, and hypotheses regarding the molecular mechanisms of SMS are offered. Parallels are made between the SMS region and other regions in the human genome where the molecular etiologies of diseases have been elucidated.

Genetic Mapping Refines DFNB3 to 17p11.2, Suggests Multiple Alleles of DFNB3, and Supports Homology to the Mouse Model shaker-2

The nonsyndromic congenital recessive deafness gene, DFNB3, first identified in Bengkala, Bali, was mapped to a approximately 12-cM interval on chromosome 17. New short tandem repeats (STRs) and additional DNA samples were used to identify recombinants that constrain the DFNB3 interval to less, similar6 cM on 17p11.2. Affected individuals from Bengkala and affected members of a family with hereditary deafness who were from Bila, a village neighboring Bengkala, were homozygous for the same alleles for six adjacent STRs in the DFNB3 region and were heterozygous for other distal markers, thus limiting DFNB3 to an approximately 3-cM interval. Nonsyndromic deafness segregating in two unrelated consanguineous Indian families, M21 and I-1924, were also linked to the DFNB3 region. Haplotype analysis indicates that the DFNB3 mutations in the three pedigrees most likely arose independently and suggests that DFNB3 makes a significant contribution to hereditary deafness worldwide. On the basis of conserved synteny, mouse deafness mutations shaker-2 (sh2) and sh2J are proposed as models of DFNB3. Genetic mapping has refined sh2 to a 0.6-cM interval of chromosome 11. Three homologous genes map within the sh2 and DFNB3 intervals, suggesting that sh2 is the homologue of DFNB3.

Prenatal Diagnosis of Charcot‐Marie‐Tooth Disease Type 1A

Charcot-Marie-Tooth disease (CMT) is the most common cause of peripheral neuropathy, with an incidence of 1: 2500 persons affected. CMT1A is caused by a submicroscopic duplication in 17p12. Several methods exist for determining a diagnosis in an individual. Many of these methods are not suitable for prenatal diagnosis. Previously, we reported the use of fluorescence in situ hybridization (FISH) to detect the common duplication found in more than 98% of individuals with CMT1A. We also have reported the validation of the FISH assay for amniotic fluid specimens and chorionic villus samples. Herein, we report our experience with testing for CMT1A in prenatal specimens.