Dominant negative and loss of function mutations of the c-kit (mast/stem cell growth factor receptor) proto-oncogene in human piebaldism.

Piebaldism is an autosomal dominant disorder of melanocyte development and is characterized by congenital white patches of skin and hair from which melanocytes are completely absent. A similar disorder of the mouse, "dominant white spotting" (W), results from mutations of the c-kit proto-oncogene, which encodes the cellular tyrosine kinase receptor for the mast/stem cell growth factor. We have identified c-kit gene mutations in three patients with piebaldism. A missense substitution (Phe----Leu) at codon 584, within the tyrosine kinase domain, is associated with a severe piebald phenotype, whereas two different frameshifts, within codons 561 and 642, are both associated with a variable and relatively mild piebald phenotype. This is consistent with a possible "dominant negative" effect of missense c-kit polypeptides on the function of the dimeric receptor.     

Novel Mutations and Deletions of the KIT (Steel Factor Receptor) Gene in Human Piebaldism

Piebaldism is an autosomal dominant genetic disorder of pigmentation characterized by white patches of skin and hair. Melanocytes are lacking in these hypopigmented regions, the result of mutations of the KIT gene, which encodes the cell surface receptor for steel factor (SLF). We describe the analysis of 26 unrelated patients with piebaldism-like hypopigmentation—17 typical patients, 5 with atypical clinical features or family histories, and 4 with other disorders that involve white spotting. We identified novel pathologic mutations or deletions of the KIT gene in 10 (59%) of the typical patients, and in 2 (40%) of the atypical patients. Overall, we have identified pathologic KIT gene mutations in 21 (75%) of 28 unrelated patients with typical piebaldism we have studied. Of the patients without apparent KIT mutations, none have apparent abnormalities of the gene encoding SLF itself (MGF), and genetic linkage analyses in two of these families are suggestive of linkage of the piebald phenotype to KIT. Thus, most patients with typical piebaldism appear to have abnormalities of the KIT gene.     

毛腿鼠耳蝠、亚洲长翼蝠和栗鼠耳蝠斑驳白化现象

斑驳白化是一种先天性颜色失调表型,在翼手目中较为罕见。2016年5月至8月期间,分别在3个省份共发现3种存在斑驳白化现象蝙蝠。在河南省内乡县七里坪乡北湾村,发现1只怀孕雌性斑驳白化毛腿鼠耳蝠(Myotis fimbriatus),斑驳区域包括背部左侧区域及腹部左侧边缘一带。在陕西省柞水县下梁镇,发现1只怀孕雌性斑驳白化亚洲长翼蝠(Miniopterus fuliginosus),斑驳部位包括背部下端、腹部底端股骨边缘、左右股骨、尾骨及小部分尾膜区域。在云南省晋宁县双河乡法古甸村,发现2只斑驳白化雄性栗鼠耳蝠(Myotis badius),斑驳区域分别为背部左下侧和背颈部一侧,区域面积较小。其中,毛腿鼠耳蝠和栗鼠耳蝠的斑驳白化现象为国内首次报道。此外,通过测量蝙蝠体重、前臂长等体型参数,并与同种正常个体进行比较,发现斑驳白化亚洲长翼蝠体重偏大,1只斑驳白化栗鼠耳蝠前臂长偏大,其余斑驳白化蝙蝠体型数据均在正常范围内,表明斑驳白化现象对蝙蝠的正常生存与繁殖未造成严重影响。这为将来斑驳白化蝙蝠的生理生态研究提供科学依据。     

Seven novel KIT mutations in horses with white coat colour phenotypes

White coat colour in horses is inherited as a monogenic autosomal dominant trait showing a variable expression of coat depigmentation. Mutations in the KIT gene have previously been shown to cause white coat colour phenotypes in pigs, mice and humans. We recently also demonstrated that four independent mutations in the equine KIT gene are responsible for the dominant white coat colour phenotype in various horse breeds. We have now analysed additional horse families segregating for white coat colour phenotypes and report seven new KIT mutations in independent Thoroughbred, Icelandic Horse, German Holstein, Quarter Horse and South German Draft Horse families. In four of the seven families, only one single white horse, presumably representing the founder for each of the four respective mutations, was available for genotyping. The newly reported mutations comprise two frameshift mutations (c.1126_1129delGAAC; c.2193delG), two missense mutations (c.856G>A; c.1789G>A) and three splice site mutations (c.338-1G>C; c.2222-1G>A; c.2684+1G>A). White phenotypes in horses show a remarkable allelic heterogeneity. In fact, a higher number of alleles are molecularly characterized at the equine KIT gene than for any other known gene in livestock species.     

Chinese white Rongchang pig does not have the dominant white allele of KIT but has the dominant black allele of MC1R

The mast/stem cell growth factor receptor (KIT) and melanocortin receptor 1 (MC1R) mutations are responsible for coat color phenotypes in domestic pigs. Rongchang is a Chinese indigenous pig breed with a white coat color phenotype. To investigate the genetic variability of the KIT and MC1R genes and their possible association with the coat color phenotype in this breed, a gene duplication and splice mutation of KIT were diagnosed in a sample of 93 unrelated Rongchang animals. The results show that Rongchang pigs have a single copy of KIT without the splice mutation at the first nucleotide of intron 17, indicating that the dominant white I allele of KIT is not responsible for their white phenotype. The KIT mRNA and MC1R coding sequences were also determined in this breed. Three putative amino acid substitutions were found in the KIT gene between Rongchang and Western white pigs, their association with the Rongchang white phenotype remains unknown. For the MC1R gene, Rongchang pigs were demonstrated to have the same dominant black allele (E(D1)) as other Chinese breeds, supporting the previous conclusion that Chinese and Western pigs have independent domestication origin. We also clarified that the Rongchang white phenotype was recessive to nonwhite color phenotypes. Our results provide a good starting point for the identification of the mutations underlying the white coat color in Rongchang pigs.     

A de novo germline mutation of KIT in a white-spotted Brown Swiss cow

White-spotting coat colour phenotypes in cattle are either fixed characteristics of specific cattle breeds or occur sporadically owing to germline genetic variation of solid-coloured parents. A Brown Swiss cow showing a piebald pattern resembling colour-sidedness was referred for genetic evaluation. Both parents were normal solid-brown-coloured cattle. The cow was tested negative for the three known DNA variants in KIT, MITF and TWIST2 associated with different depigmentation phenotypes in Brown Swiss cattle. Whole-genome sequencing of the cow was performed and a heterozygous variant affecting the coding sequence of the bovine KIT gene was identified on chromosome 6. The variant is a 40 bp deletion in exon 9, NM_001166484.1:c.1390_1429del, and leads to a frameshift that is predicted to produce a novel 50 amino acid-long C-terminus replacing almost 50% of the wt KIT protein, including the functionally important intracellular tyrosine kinase domain (NP_001159956.1:p.(Asn464AlafsTer50)). Interestingly, among three available offspring, two solid-coloured daughters were genotyped as homozygous wt whereas a single son showing a slightly milder but still obvious depigmentation phenotype inherited a copy of the novel variant allele. The genetic findings provide strong evidence that the identified loss-of-function KIT variant most likely represents a de novo germline mutation that is causative owing to haploinsufficiency.     

Unexpectedly high allelic diversity at the KIT locus causing dominant white color in the domestic pig.

Mutations in KIT encoding the mast/stem cell growth factor receptor (MGF) are responsible for coat color variation in domestic pigs. The dominant white phenotype is caused by two mutations, a gene duplication and a splice mutation in one of the copies leading to skipping of exon 17. Here we applied minisequencing and pyrosequencing for quantitative analysis of the number of copies with the splice form. An unexpectedly high genetic diversity was revealed in white pigs. We found four different KIT alleles in a small sample of eight Large White females used as founder animals in a wild boar intercross. A similar number of KIT alleles was found in commercial populations of white Landrace and Large White pigs. We provide evidence for at least two new KIT alleles in pigs, both with a triplication of the gene. The results imply that KIT alleles with the duplication are genetically unstable and new alleles are most likely generated by unequal crossing over. This study provides an improved method for genotyping the complicated Dominant white/KIT locus in pigs. The results also suggest that some alleles may be associated with negative pleiotropic effects on other traits.     

The c.42_52del11 Mutation in TPRN and Progressive Hearing Loss in a Family from Pakistan

The DFNB79 locus harbors TPRN mutations in which have been reported in a few families with deafness. Four frameshift mutations in TPRN have been described to cause severe or severe-to-profound hearing loss in Moroccan and Pakistani families, and a single frameshift mutation was associated with progressive hearing loss in deaf individuals in a Dutch family. We identified a Pakistani family in which the affected individuals were homozygous for a pathogenic mutation, c.42_52del11, in TPRN (p.G15Afs150X). In contrast to the previously reported individuals affected by the same mutation, hearing loss is likely to be progressive in this family. Thus the same mutation of TPRN can be associated with different thresholds of hearing as well as differences in the stability of the phenotype.     

An essential gene mutagenesis screen across the highly conserved piebald deletion region of mouse chromosome 14

The piebald deletion complex is a set of overlapping chromosomal deficiencies on distal mouse chromosome 14. We surveyed the functional genetic content of the piebald deletion region in an essential gene mutagenesis screen of 952 genomes to recover seven lethal mutants. The ENU‐induced mutations were mapped to define genetic intervals using the piebald deletion panel. Lethal mutations included loci required for establishment of the left‐right embryonic axis and a loss‐of‐function allele of Phr1 resulting in respiratory distress at birth. A functional map of the piebald region integrates experimental genetic data from the deletion panel, mutagenesis screen, and the targeted disruption of specific genes. A comparison of several genomic intervals targeted in regional mutagenesis screens suggests that the piebald region is characterized by a low gene density and high essential gene density with a distinct genomic content and organization that supports complex regulatory interactions and promotes evolutionary stability. genesis 47:392–403, 2009. © 2009 Wiley‐Liss, Inc.     

Germline mutations in the 3′ part of APC exon 15 do not result in truncated proteins and are associated with attenuated adenomatous polyposis coli

Familial adenomatous polyposis (FAP) is an inherited predisposition to colorectal cancer characterized by the development of numerous adenomatous polyps predominantly in the colorectal region. Germline mutations in the adenomatous polyposis coli (APC) gene are responsible for most cases of FAP. Mutations at the 5′ end of APC are known to be associated with a relatively mild form of the disease, called attenuated adenomatous polyposis coli (AAPC). We identified a frameshift mutation in the 3′ part of exon 15, resulting in a stop codon at 1862, in a large Dutch kindred with AAPC. Western blot analysis of lymphoblastoid cell lines derived from affected family members from this kindred, as well as from a previously reported Swiss family carrying a frameshift mutation at codon 1987 and displaying a similar attenuated phenotype, showed only the wild-type APC protein. Our study indicates that chain-terminating mutations located in the 3′ part of APC do not result in detectable truncated polypeptides and we hypothesize that this is likely to be the basis for the observed AAPC phenotype. Received: 18 June 1996 / Revised: 8 July 1996     

Fine Structure Mapping and Deletion Analysis of the Murine Piebald Locus

piebald (s) is a recessive mutation that affects the development of two cell types of neural crest origin: the melanocytes, responsible for pigment synthesis in the skin, and enteric ganglia, which innervate the lower bowel. As a result, mice carrying piebald mutations exhibit white spotting in the coat and aganglionic megacolon. Previously the gene had been localized to the distal half of mouse chromosome 14. To determine its precise location relative to molecular markers, an intersubspecific backcross was generated. Two anchor loci of chromosome 14, slaty and hypogonadal, in addition to simple sequence length repeat markers, were used to localize s to a 2-cM interval defined by the markers D14Mit38 and D14Mit42. The molecular markers were also used to characterize nine induced s alleles. Three of these mutations exhibited no deletions or rearrangements of the flanking markers, whereas the other six had two or more of these markers deleted. The extent of the deletions was found to be consistent with the severity of the homozygous phenotype. The location of deletion breakpoints in the induced alleles, coupled with the recombination breakpoints in the backcross progeny, provide useful molecular landmarks to define the location of the piebald gene.     

KIT (c-kit oncogene product) pathway is constitutively activated in human testicular germ cell tumors

We investigated the expression of KIT (product of c-kit oncogene), gain-of-function mutations, and activation of its downstream signal transduction in human testicular cancers. KIT was expressed in 88% (22/25) of seminomas and in 44.4% (4/9) of non-seminomas compared to adjacent normal testicular tissue. Nine of the KIT-expressing seminomas had mutations (40.9%; 9/22) in the c-kit gene; two cases in exon 11 and 7 cases in exon 17. Two of these mutations in exon 17 were novel, and the other seven mutations were identical to the already known gain-of-function mutations which cause activation of KIT without ligand stem cell factor. All of the mutant KIT and 53.8% (7/13) of wild-type KIT were phosphorylated (activated) and associated with phosphorylated phosphatidylinositol 3-kinase (PI3K). Akt was also phosphorylated in these seminomas, suggesting that the KIT-PI3K-Akt pathway is activated in seminoma. These findings suggest that the KIT-PI3K-Akt pathway is constitutively activated in testicular germ cell tumors, due to overexpression of KIT protein and/or gain-of-function mutations in the c-kit gene.     

Association of Hereditary Nonpolyposis Colorectal Cancer–Related Tumors Displaying Low Microsatellite Instability with MSH6 Germline Mutations

Hereditary nonpolyposis colorectal cancer (HNPCC) (Amsterdam criteria) is often caused by mutations in mismatch repair (MMR) genes, and tumors of patients with HNPCC show microsatellite instability (MSI-high phenotype). Germline mutations of MMR genes have rarely been found in families that have HNPCC or suspected HNPCC and that do not show microsatellite instability (MSI-low phenotype). Therefore, an MSI-high phenotype is often used as an inclusion criterion for mutation testing of MMR genes. Correction of base-base mismatches is the major function of MSH6. Since mismatches present with an MSI-low phenotype, we assumed that the phenotype in patients with HNPCC-related tumors might be associated with MSH6 germline mutations. We divided 36 patients with suspected HNPCC into an MSI-low group (n=18) and an MSI-high group (n=18), on the basis of the results of MSI testing. Additionally, three unrelated patients from Amsterdam families with MSI-low tumors were investigated. All patients were screened for MSH2, MLH1, and MSH6 mutations. Four presumably causative MSH6 mutations were detected in the patients (22%) who had suspected HNPCC and MSI-low tumors. Furthermore, we detected one frameshift mutation in one of the three patients with HNPCC and MSI-low tumors. In the MSI-high group, one MSH6 missense mutation was found, but the same patient also had an MLH1 mutation, which may explain the MSI-high phenotype. These results suggest that MSH6 may be involved in a substantial proportion of patients with HNPCC or suspected HNPCC and MSI-low tumors. Our data emphasize that an MSI-low phenotype cannot be considered an exclusion criterion for mutation testing of MMR genes in general.     

Germline and somatic mutations in hMSH6 and hMSH3 in gastrointestinal cancers of the microsatellite mutator phenotype

Hereditary and sporadic gastrointestinal cancer of the microsatellite mutator phenotype (MMP) is characterized by a remarkable genomic instability at simple repeated sequences. The genomic instability is often caused by germline and somatic mutations in DNA mismatch repair (MMR) genes hMSH2 and hMLH1. The MMP can be also caused by epigenetic inactivation of hMLH1. The MMP generates many somatic frameshift mutations in genes containing mononucleotide repeats. We previously reported that in MMP tumors the hMSH6 and hMSH3 MMR genes often carry frameshift mutations in their (C)(8) and (A)(8) tracks, respectively. We proposed that these 'secondary mutator mutations' contribute to a gradual manifestation of the MMP. Here we report the detection of other frameshift, nonsense, and missense mutations in these genes in colon and gastric cancers of the MMP. A germline frameshift mutation was found in hMSH6 in a colon tumor harboring another somatic frameshift mutation. Several germline sequence variants and somatic missense mutations at conserved residues were detected in hMSH6 and only one was detected in hMSH3. Of the three hMSH6 germline variants in conserved residues, one coexisted with a somatic mutation at the (C)(8) track and another had a somatic missense mutation. We suggest that some of these germline and somatic missense variants are pathogenic. While biallelic hMSH6 and hMSH3 frameshift mutations were found in some tumors, many tumors seemed to contain only monoallelic mutations. In some tumors, these somatic monoallelic frameshift mutations at the (C)(8) and (A)(8) tracks were found to coexist with other somatic mutations in the other allele, supporting their functionality during tumorigenesis. However, the low incidence of these additional somatic mutations in hMSH6 and hMSH3 leaves many tumors with only monoallelic mutations. The impact of the frameshift mutations in gene expression was studied by comparative analysis of RNA and protein expression in different tumor cell clones with different genotypes. The results show that the hMSH6 (C)(8) frameshift mutation abolishes protein expression, ruling out a dominant negative effect by a truncated protein. We suggest the functionality of these secondary monoallelic mutator mutations in the context of an accumulative haploinsufficiency model.