Different mutations in the same gene often cause distinct disease phenotypes in humans. Generally, such variations in the clinical phenotypes have been considered to be a consequence of the function or dysfunction of mutant proteins. Thus, a primary emphasis in genotype-phenotype correlation studies has been placed on determining the unique functional properties of encoded mutant proteins. But in vitro functional assays of mutant proteins often show discordance between predicted protein function and clinical outcome. Little is known about the many factors that are potentially involved in this discrepancy, but loss-of-function versus gain-of-function effects are often invoked as a possible mechanism.We previously identified two unrelated individuals with an unusual phenotype that combined four distinct syndromesperipheral demyelinating neuropathy, central dysmyelinating leukodystrophy, Waardenburg syndrome and Hirschsprung disease-that are characterized by deficiencies of Schwann cells, oligodendrocytes, melanocytes and enteric ganglia neurons, respectively 1,2 . Here we describe four more individuals and propose that this complex disorder is a newly described neurocristopathy called PCWH.We previously identified mutations in SOX10 in all affected individuals 1,2 . SOX10 is a transcription factor that contains a central high mobility group (HMG) DNA-binding domain and a transactivation domain at its C terminus 3 . SOX10 is essential for the development of cells in the neural crest lineage, including melanocytes and enteric ganglia neurons 4,5 ; it also controls the proliferation and differentiation of Schwann cells and oligodendrocytes [6][7][8] . Notably, some mutations in SOX10 also cause a distinct and more restricted disease that does not involve either the peripheral (PNS) or the central (CNS) nervous systems 9-11 . This less complicated neurocristopathy, called WS4, combines Waardenburg and Hirschsprung diseases 12 . Most SOX10 disease-associated mutations, regardless of whether they cause PCWH or WS4, result in premature termination codons (PTCs).As in SOX10, different mutations in MPZ are responsible for distinct neurological diseases, which each affect the myelin of the PNS. These neuropathies include early onset congenital hypomyelinating neuropathy (CHN; OMIM 605253), Dejerine-Sottas neuropathy (DSN; OMIM 145900) and the less severe, adult onset Charcot-MarieTooth disease type 1B (CMT1B; OMIM 118200; ref. 13). It has been suggested that the severity of alleles in CHN and DSN is due to dominant-negative effects, whereas the reduced severity of alleles in CMT1B is due to loss of function. But although some nonsense and frameshift alleles cause CMT1B, several truncating mutations have been reported that convey either a CHN or a DSN phenotype.We investigated the molecular mechanisms underlying the neurological phenotypes of the PCWH and WS4 neurocristopathies resulting from allelic SOX10 truncating mutations, as well as those underlying the CHN, DSN and CMT1B myelinopathies caused by allelic MPZ truncatin...

The nonsense-mediated decay (NMD) pathway is an mRNA surveillance system that typically degrades transcripts containing premature termination codons (PTCs) in order to prevent translation of unnecessary or aberrant transcripts. Failure to eliminate these mRNAs with PTCs may result in the synthesis of abnormal proteins that can be toxic to cells through dominant-negative or gain-of-function effects. Recent studies have expanded our understanding of the mechanism by which nonsense transcripts are recognized and targeted for decay. Here, we review the physiological role of this surveillance pathway, its implications for human diseases, and why knowledge of NMD is important to an understanding of genotype -phenotype correlations in various genetic disorders.

Pelizaeus-Merzbacher disease (PMD) and its allelic disorder, spastic paraplegia type 2 (SPG2), are among the best-characterized dysmyelinating leukodystrophies of the central nervous system (CNS). Both PMD and SPG2 are caused by mutations in the proteolipid protein 1 (PLP1) gene, which encodes a major component of CNS myelin proteins. Distinct types of mutations, including point mutations and genomic duplications and deletions, have been identified as causes of PMD/SPG2 that act through different molecular mechanisms. Studies of various PLP1 mutants in humans and animal models have shed light on the genomic, molecular, and cellular pathogeneses of PMD/SPG2. Recent discoveries include complex mutational mechanisms and associated disease phenotypes, novel cellular pathways that lead to the degeneration of oligodendrocytes, and genomic architectural features that result in unique chromosomal rearrangements. Here, I review the previous and current knowledge of the molecular pathogenesis of PMD/SPG2 and delineate future directions for PMD/SPG2 studies.

Genomic rearrangements play a major role in the pathogenesis of human genetic diseases. Nonallelic homologous recombination (NAHR) between low-copy repeats (LCRs) that flank unique genomic segments results in changes of genome organization and can cause a loss or gain of genomic segments. These LCRs appear to have arisen recently during primate speciation via paralogous segmental duplication, thus making the human species particularly susceptible to genomic rearrangements. Genomic disorders are defined as a group of diseases that result from genomic rearrangements, mostly mediated by NAHR. Molecular investigations of genomic disorders have revealed genome architectural features associated with susceptibility to rearrangements and the recombination mechanisms responsible for such rearrangements. The human genome sequence project reveals that LCRs may account for 5% of the genome, suggesting that many novel genomic disorders might still remain to be recognized.

Tyrosyl-DNA phosphodiesterase 1 (Tdp1) cleaves the phosphodiester bond between a covalently stalled topoisomerase I (Topo I) and the 3 0 end of DNA. Stalling of Topo I at DNA strand breaks is induced by endogenous DNA damage and the Topo I-specific anticancer drug camptothecin (CPT). The H493R mutation of Tdp1 causes the neurodegenerative disorder spinocerebellar ataxia with axonal neuropathy (SCAN1). Contrary to the hypothesis that SCAN1 arises from catalytically inactive Tdp1, Tdp1 À/À mice are indistinguishable from wild-type mice, physically, histologically, behaviorally, and electrophysiologically. However, compared to wild-type mice, Tdp1 À/À mice are hypersensitive to CPT and bleomycin but not to etoposide. Consistent with earlier in vitro studies, we show that the H493R Tdp1 mutant protein retains residual activity and becomes covalently trapped on the DNA after CPT treatment of SCAN1 cells. This result provides a direct demonstration that Tdp1 repairs Topo I covalent lesions in vivo and suggests that SCAN1 arises from the recessive neomorphic mutation H493R. This is a novel mechanism for disease since neomorphic mutations are generally dominant.

In the majority of patients with Pelizaeus-Merzbacher disease, duplication of the proteolipid protein gene PLP1 is responsible, whereas deletion of PLP1 is infrequent. Genomic mechanisms for these submicroscopic chromosomal rearrangements remain unknown. We identified three families with PLP1 deletions (including one family described elsewhere) that arose by three distinct processes. In one family, PLP1 deletion resulted from a maternal balanced submicroscopic insertional translocation of the entire PLP1 gene to the telomere of chromosome 19. PLP1 on the 19qtel is probably inactive by virtue of a position effect, because a healthy male sibling carries the same der(19) chromosome along with a normal X chromosome. Genomic mapping of the deleted segments revealed that the deletions are smaller than most of the PLP1 duplications and involve only two other genes. We hypothesize that the deletion is infrequent, because only the smaller deletions can avoid causing either infertility or lethality. Analyses of the DNA sequence flanking the deletion breakpoints revealed Alu-Alu recombination in the family with translocation. In the other two families, no homologous sequence flanking the breakpoints was found, but the distal breakpoints were embedded in novel low-copy repeats, suggesting the potential involvement of genome architecture in stimulating these rearrangements. In one family, junction sequences revealed a complex recombination event. Our data suggest that PLP1 deletions are likely caused by nonhomologous end joining.

Congenital hypomyelinating disorders are a heterogeneous group of inherited leukoencephalopathies characterized by abnormal myelin formation. We have recently reported a hypomyelinating syndrome characterized by diffuse cerebral hypomyelination with cerebellar atrophy and hypoplasia of the corpus callosum (HCAHC). We performed whole-exome sequencing of three unrelated individuals with HCAHC and identified compound heterozygous mutations in POLR3B in two individuals. The mutations include a nonsense mutation, a splice-site mutation, and two missense mutations at evolutionally conserved amino acids. Using reverse transcription-PCR and sequencing, we demonstrated that the splice-site mutation caused deletion of exon 18 from POLR3B mRNA and that the transcript harboring the nonsense mutation underwent nonsense-mediated mRNA decay. We also identified compound heterozygous missense mutations in POLR3A in the remaining individual. POLR3A and POLR3B encode the largest and second largest subunits of RNA Polymerase III (Pol III), RPC1 and RPC2, respectively. RPC1 and RPC2 together form the active center of the polymerase and contribute to the catalytic activity of the polymerase. Pol III is involved in the transcription of small noncoding RNAs, such as 5S ribosomal RNA and all transfer RNAs (tRNA). We hypothesize that perturbation of Pol III target transcription, especially of tRNAs, could be a common pathological mechanism underlying POLR3A and POLR3B mutations.

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