Strategies and outcomes of PGD of familial adenomatous polyposis

Owing to adult onset of hereditary cancer, prenatal diagnosis (PND) raises numerous ethical issues on the acceptability to terminate an affected pregnancy (TOP). PND for these disorders is often considered as unacceptable by couples as well as geneticists and legal or ethical authorities, but preimplantation genetic diagnosis (PGD), even if subject to controversy, seems to be a more acceptable option. Therefore, many couples, who do not want to transmit their cancer to their children, consider PGD as their only reproductive option. This article describes our experience of PGD for familial adenomatous polyposis (FAP). Twelve couples were referred between 2000 and 2005. We developed PGD tests to detect the mutation alone, but we rapidly set up multiplex PCR combining mutation detection and indirect diagnosis. Finally, we set up duplex and triplex indirect diagnoses to be able to offer a PGD, whatever mutation was involved in familial cases. PGD strategies were based on (i) a new double allele-specific PCR approach (D-ARMS) allowing the detection of the wild-type and mutated allele; (ii) PCR fragments sizing and (iii) restriction length polymorphisms. For the 12 referrals, we developed eight tests, and 11 cycles have been performed for four couples, resulting in eight embryo transfers and five pregnancies, with the birth of one healthy boy and two ongoing pregnancies. We are now able to propose PGD to most couples at risk of transmitting FAP to their offspring, whether the mutation is familial or occurred de novo.     

Germline mutations in APC and MUTYH are responsible for the majority of families with attenuated familial adenomatous polyposis

A small fraction of families with familial adenomatous polyposis (FAP) display an attenuated form of FAP (AFAP). We aimed to assess the presence of germline mutations in the MUTYH and adenomatous polyposis coli (APC) genes in AFAP families and to compare the clinical features between the two causative genes. Families with clinical AFAP were selected from the Dutch Polyposis Registry according to the following criteria: (a) at least two patients with 10-99 adenomas diagnosed at age >30 years or (b) one patient with 10-99 adenomas at age >30 years and a first-degree relative with colorectal cancer (CRC) with a few adenomas, and, applying for both criteria, no family members with more than 100 polyps before the age of 30 years. All probands were screened for germline mutations in the APC and MUTYH genes. Twenty-five of 315 Dutch families with FAP (8%) met our criteria for AFAP. These families included 146 patients with adenomas and/or CRC. Germline APC mutations were identified in nine families and biallelic MUTYH mutations in another nine families. CRC was identified at a mean age of 54 years (range 24-83 years) in families with APC and at 50 years (range 39-70 years) in families with MUTYH (p = 0.29). APC and biallelic MUTYH mutations are responsible for the majority of AFAP families. Based on our results and those reported in the literature, we recommend colonoscopy once every 2 years in AFAP families, starting surveillance from the late teens in APC mutation carriers and from age 20-25 years in biallelic MUTYH mutation carriers.     

Mutation analysis of the adenomatous polyposis coli (APC) gene in northwest Spanish patients with familial adenomatous polyposis (FAP) and sporadic colorectal cancer

Germline mutations in the tumor‐suppresor APC gene are associated with hereditary familial adenomatous polyposis (FAP) and somatic mutations are common in sporadic colorectal cancer. In this study, we report the identification of three novel germline mutations: 1682‐1683insA, 3252‐3253insAT, 3544A>T and a new somatic mutation 4130‐4131delTT, all giving rise to truncated APC proteins. The majority of the mutations we found originate a truncated APC protein and cause the FAP phenotype. However, special attention must be given to the missense mutations Asp1822Val and Ser2621Cys since their segregation with the FAP phenotype is questionable. In our FAP families we did not find any genetical alterations at codon 1309, being this mutation the most frequent reported in APC. Differences in the recurrence of pathological mutations in APC could exist among populations. However, epidemiological studies must be performed to confirm this hypothesis. Hum Mutat 18:355, 2001. © 2001 Wiley‐Liss, Inc.     

APC mutations in familial adenomatous polyposis families in the Northwest of England

We have investigated a series of FAP patients in the Northwest of England in order to identify and characterise the specific APCmutations. Using SSCP, we found 27 mutations in a total of 50 families investigated. The mutations were predominantly frameshift or nonsense mutations and there were two splice site changes. We have described two patients with severe Gardner's phenotype from different ethnic backgrounds who share the same mutation at codon 1537. Although the frequency of the most common mutation appears low, it is not dissimilar to that reported by other groups. Hum Mutat 10:376–380, 1997. © 1997 Wiley-Liss, Inc.     

家族性腺瘤样息肉病中APC基因的胚系突变分析

目的 探索有效的突变检测技术，系统分析家族性腺瘤样息肉病(familial adenomatous polyposis，FAP)相关基因结肠腺瘤病(adenomatous polyposis coli，APC)基因的胚系突变，及其与疾病表型的关系。方法 从22例临床确诊的FAP患者，外周静脉血中提取基因组DNA。变性高效液相色谱、蛋白截短检测、测序技术结合应用进行全基因分析。根据患者临床资料，进行基因型-表型分析。结果 22例FAP患者中13例检出APC基因胚系突变，均为无义或移码突变。基因型-表型关系的初步分析表明，在基因5′端或3′端发生突变的患者临床症状较轻，在基因中段发生突变的患者临床症状典型或严重。结论 本研究中所采用的技术体系可敏感、高效地检出APC基因突变，APC基因的突变型与FAP患者的临床表型存在关联，所采用的技术体系适用于FAP症状出现前的基因诊断。     

Predictive testing for familial adenomatous polyposis in a rural South Indian community

During the course of genome studies in a rural community in the South Indian state of Karnataka, DNA-based investigations and counselling for familial adenomatous polyposis (FAP) were requested via the community physician. The proposita died in 1940 and FAP had been clinically diagnosed in 2 of her 5 children, both deceased. DNA samples from 2 affected individuals in the third generation were screened for mutations in the APC gene, and a frame-shift mutation was identified in exon 15 with a common deletion at codon 1061. Predictive testing for the mutation was then organized on a voluntary basis. There were 11 positive tests, including confirmatory positives on 2 persons diagnosed by colonoscopy, and to date surgery has been successfully undertaken on 3 previously undiagnosed adults. The ongoing success of the study indicates that, with appropriate access to the facilities offered by collaborating centres, predictive testing is feasible for diseases such as FAP and could be of significant benefit to communities in economically less developed countries.     

Mutations of the APC adenomatous polyposis coli) gene

Several investigators have reported germline mutations of the APC gene in patients with familial adenomatous polyposis (FAP) as well as somatic mutations in tumors developed in digestive organs (stomach, pancreas, colon, and rectum). Those results provide evidence that inactivation of the APC gene plays a significant role in FAP and in sporadic tumors of these tissues. APC mutations have led to some interesting observations. First, the great majority of the mutations found to date would result in truncation of the APC product. Second, almost all the mutations have occurred within the first half of the coding sequence, and somatic mutations in colorectal tumors are further clustered in a particular region called MCR (mutation cluster region). Third, most identified point mutations in the APC gene are transitions from cytosine to other nucleotides. Fourth, the location of germ-line mutations tends to correlate with the number of colorectal polyps in FAP patients. Furthermore, inactivation of both alleles of the APC gene seems to be required as an early event to develop most of adenomas and carcinomas in the colon and rectum as well as some of those in the stomach. © 1993 Wiley-Liss, Inc.     

APC promoter 1B deletion in seven American families with familial adenomatous polyposis

Familial adenomatous polyposis (FAP) is a colorectal cancer predisposition syndrome caused by mutations in the adenomatous polyposis coli (APC) gene. Clinical genetic testing fails to identify disease causing mutations in up to 20% of clinically apparent FAP cases. Following the inclusion of multiplex ligation‐dependent probe amplification (MLPA) probes specific for APC promoter 1B, seven probands were identified with a deletion of promoter 1B. Using haplotype analysis spanning the APC locus, the seven families appear to be identical by descent from a common founder. The clinical phenotype of 19 mutation carriers is classical FAP with colectomy at an average age of 24. The majority of cases had a large number of duodenal and gastric polyps. Measurements of allele‐specific expression of APC mRNA using TaqMan assay confirmed that relative expression in the allele containing the promoter 1B deletion was reduced 42–98%, depending on tissue type. This study confirms the importance of APC promoter deletions as a cause of FAP and identifies a founder mutation in FAP patients from the United States.     

Molecular and clinical characteristics in 32 families affected with familial adenomatous polyposis

Germ‐line mutations in the 5′ half of the Adenomatous Polyposis Coli (APC) gene are found in about 80% of the patients affected with familial adenomatous polyposis (FAP). The vast majority of these are nonsense or frameshift mutations which result in the loss of the carboxyl terminus of the APC protein. Using an in vivo assay in yeast, we have identified pathogenic germ‐line mutations in 26 of 32 (81%) unrelated Swiss families affected with FAP. Nine mutations were novel and eight families were shown to harbor two recurrent mutations. Correlations were attempted between the location of APC germ‐line mutations and clinical manifestations of the disease. © 2001 Wiley‐Liss, Inc.     

APC germline mutations identified in Czech patients with familial adenomatous polyposis

Familial adenomatous polyposis (FAP) is an autosomal dominantly inherited predisposition to colorectal cancer, which is caused by germline mutations in the adenomatous polyposis coli (APC) gene. The APC mutations have been investigated in 46 Czech unrelated FAP families and 9 suspected FAP families using DGGE analysis and direct DNA sequencing. We found 25 germline APC mutations and identified 11 which were not previously reported. Of the identified mutations, 10 were 1 to 5 bp deletions, four were 1 bp insertions and six were nonsense, all leading to the formation of premature stop codon. In addition, we detected two mutations in the splice-donor region of APC intron 11, one missense and two samesense mutations. Phenotypes of patients with known and novel types of mutations are presented and discussed.     

中国人家族性腺瘤性患肉病患者结肠腺瘤性患肉病基因的胚系突变

目的 探讨中国人家族性腺瘤性息肉病(familial adenomatous polyposis,FAr)患者的结肠腺瘤性息肉病(adenomatous polyposis coli,APC)基因的胚系突变类型.方法 对9个FAP家系18名成员进行多重连接依赖性探针扩增(multiplex ligation-dependent probe amplification,MLPA)检测APC基因有无大片段缺失.再应用PCR扩增APC基因的15个外显子区域,经变性高效液相色谱(denaturing high performance liquid chromatography,DHPLC)对每个扩增片段进行筛查,流出峰异常的片段,经DNA测序验证小片段的改变.结果 9个家系中有3个家系发现有APC基因的胚系突变:家系2为c.3184-3187 del CAhA,家系4为c.5432C＞T,家系9为c.3925-3929 del AAAAG.3种突变中c.5432C＞T在数据库中未见报道.结论 中国人不同的APC基因的胚系突变可引起FAP;无APC胚系突变的FAP患者的发病可能存在其他的机制.     

Somatic APC mosaicism: a frequent cause of familial adenomatous polyposis (FAP)

Somatic mutational mosaicism presents a challenge for both molecular and clinical diagnostics and may contribute to deviations from predicted genotype-phenotype correlations. During APC mutation screening in 1,248 unrelated patients with familial adenomatous polyposis (FAP), we identified 75 cases with an assumed or confirmed de novo mutation. Prescreening methods (protein truncation test [PTT], DHPLC) indicated the presence of somatic mosaicism in eight cases (11%). Sequencing of the corresponding fragments revealed very weak mutation signals, pointing to the presence of either nonsense or frameshift mutations at low level. All mutations were confirmed and quantified by SNaPshot analysis: in leukocyte DNA from the eight patients, the percentage of mosaicism varied between 5.5% and 77%, while the proportion of the mutation in DNA extracted from adenomas of the respective patient was consistently higher. The eight mutations identified as mosaic are localized within codons 216-1464 of the APC gene. According to the known genotype-phenotype correlation, patients with mutations in this region exhibit typical or severe FAP. However, six of the eight patients presented with an attenuated or atypical polyposis phenotype. Our data demonstrate that in a fraction of FAP patients the causative APC mutation may not be detected due to weak signals or somatic mosaicism that is restricted to tissues other than blood. SNaPshot analysis was proven to be an easy, rapid, and reliable method of confirming low-level mutations and evaluating the degree of mosaicism. Some of the deviations from the expected phenotype in FAP can be explained by the presence of somatic mosaicism.     

High resolution genetic map of the adenomatous polyposis coli gene (APC) region

Familial adenomatous polyposis coli (APC) is a dominantly inherited colorectal cancer susceptibility disease caused by mutation in a gene called APC and located on chromosome 5q21. Presymptomatic diagnosis of this condition is recommended because it enables restriction of the efficient but demanding prevention program to those relatives that are genetically affected. The large size of the APC gene makes the direct search for the causal alteration difficult to implement in routine diagnostic laboratories. Because APC appears to be genetically homogeneous with alteration in a single locus causing the disease, cosegregation analysis may represent an alternative efficient method for presymptomatic diagnosis. However, the reliability of the risk estimation by linkage analysis in APC families is hampered by the lack of a short range genetic map of the APC locus. A combined approach including genotyping of 65 APC families, analysis of the CEPH database, and complementary typing of both APC and CEPH families has made it possible to derive the following genetic map: Centromere-[D5S82-D5S49]-0.02-D5S122-0.01-D5S136-0.01-D5S135-0.02-[APC-D5S346-MCC]-0.04-[D5S81-D5S64]-Telomere. This order, which differs from previously proposed genetic maps, is fully compatible with recent physical mapping data. These data should contribute to increase the reliability of the presymptomatic test for APC. © 1995 Wiley-Liss, Inc.     

应用变性高效液相色谱检测31例家族性腺瘤性息肉病家系结肠腺瘤性息肉病基因突变

目的 应用变性高效液相色谱(denaturing high performance liquid chromatography,DHPLC)技术检测我国家族性腺瘤性息肉病(familial adenomatons polyposis,FAP)家系的结肠腺瘤性息肉病(adenoinatous pelyposis coli,APC)基因变异特征,研究其病因机制.方法 采集31个家系的先证者、患者和家系成员的外周血淋巴细胞,抽提DNA并以降落式PCR扩增APC基因各外显子和启动子.基因突变检测先由DHPLC进行筛选,发现异常峰者进行测序鉴定并TA克隆鉴定,结果与网络数据进行比对.结果 31个家系中共有15个家系检出了12种不同的突变类型,FAP家系APC基因的突变检出率为48.39%.发现了4种新的突变及3例不同的内含子突变.4个新的突变分别位于255、677、1192、1403密码子,均为移码突变.证明了DHPLC能检出APC基因的突变.在APC基因的突变中,移码突变占86.67%,无义突变占13.33%,说明移码突变是中国人APC基因突变的主要方式.在突变位点上,第15外显子突变最常见,约占86.67%.结论 FAP家系APC基因的突变检出率为48.39%,发现了4种新的导致蛋白编码改变的突变.证实中国人FAP家系中APC基因突变位点以第15外显子最常见,类型以移码突变为主.     

家族性腺瘤性息肉病家系中一种新的APC基因的胚系突变

目的研究1个家族性腺瘤性息肉病家系的腺瘤样息肉病基因（adenomatous polyposis coli，APC）的胚系突变。方法经结肠镜、组织病理学检查和家族史的调查，确定了1例家族性腺瘤性息肉病（familial adenomatous polyposis，FAP）患者。应用多重连接依赖性探针扩增（multiplex ligation-dependent probe amplification，MLPA）、变性高效液相色谱（denaturing high-performance liquid chromatography，DHPLC）测序等技术对这一家系的成员进行系统的APC全基因筛查。结果在此家系中发现一个新的APC基因的胚系突变c。1999 C〉T（Q667X），这一突变造成了APC基因终止密码子的形成，从而形成有功能障碍的截短蛋白。临床上，此突变可引起严重的FAP症状，早发结直肠腺瘤和腺癌。结论Q667X胚系突变是引起该家系临床表型的原因，受累成员可考虑大肠预防性切除手术。