Mutation-positive Subjects Research Articles

Novel mutations and electrophysiological findings in RGS9 and R9AP associated retinal dysfunction (Bradyopsia)

Abstract Purpose To describe the phenotypic characteristics of patients with mutation in RGS9 or R9AP, which give rise to the disorder "bradyopsia" Methods Patients were ascertained according to history, examination and electrophysiological recordings. ERGs were performed in excess of the ISCEV standard better to characterise retinal function, and included S‐cone ERGs, ON‐OFF ERGs, and dark adapted ERGs to a bright flash with extended inter‐stimulus intervals. Blood samples were taken for DNA extraction from affected subjects and where possible unaffected relatives. Mutation screening of RGS9 and R9AP was performed. Results Four subjects were found to harbour mutations in RGS9 or R9AP, with three being novel. Three subjects, two Pakistani sisters and an Afghani female, had mutations in R9AP. The remaining patient, a British male, had a compound heterozygous mutation in RGS9. All mutation positive subjects had characteristic electrophysiological findings with, in particular, severely abnormal photopic cone ERGs (with relative S‐cone sparing) but normal dark adapted cone ERGs, and the need to use a prolonged inter‐stimulus interval to allow recovery from a dark‐adapted bright flash. Clinically, the patients had a stationary cone dysfunction syndrome, including mild photophobia; normal colour vision; normal fundi; and no nystagmus. Conclusion Novel electrophysiological features are described in association with RGS9 and R9AP mutations. The distinctive electrophysiological features associated with RGS9 and R9AP mutations enable directed genetic screening, and allow the distinction from patients with Oligocone Trichromacy, who present with a similar clinical phenotype.

High Prevalence of Impaired Glucose Homeostasis and Myopathy in Asymptomatic and Oligosymptomatic 3243A>G Mitochondrial DNA Mutation-Positive Subjects

The point mutation of 3243A>G mtDNA is the most frequent cause of mitochondrial diabetes, often presenting as the syndrome maternally inherited diabetes and deafness (MIDD). The mutation may also cause myopathy, ataxia, strokes, ophthalmoplegia, epilepsy, and cardiomyopathy in various combinations. Consequently, it is difficult to predict the "phenotypic risk profile" of 3243A>G mutation-positive subjects. The 3243A>G mutation coexists in cells with wild-type mtDNA, a phenomenon called heteroplasmy. The marked variability in mutation loads in different tissues is the main explanation for the different phenotypes associated with this mutation. The aim of the study was to screen asymptomatic and oligosymptomatic 3243A>G mtDNA carriers for diabetes and myopathy. The study is a case-control study. Nineteen adult 3243A>G carriers presumed to be normoglycemic and matched healthy controls were subjected to an oral glucose tolerance test. Twenty-six adult 3243A>G carriers with unknown myopathy status and 17 healthy controls had a maximal cycle test and a muscle biopsy performed. The mutation loads were quantified in blood and muscle biopsies and correlated to the clinical manifestations of the mutation. In the presumed normoglycemic 3243A>G-positive subjects, one subject had overt diabetes, and 10 subjects had impaired glucose tolerance. Sixteen of the 26 subjects with unknown oxidative capacity fulfilled criteria for myopathy. The mutation load in blood and muscle correlated with the age for diagnosis of impaired glucose homeostasis and hearing impairment (rho = -0.71 to -0.78; P < 0.0001). The findings suggest that 3243A>G mutation carriers should be screened for diabetes and myopathy.

Relationship between CAG repeat length and brain volume in premanifest and early Huntington’s disease

Huntington's disease (HD) is caused by an expanded CAG repeat on the gene encoding for the protein huntingtin. There are conflicting findings about the extent to which repeat length predicts signs of the disease or severity of disease progression in adults. This study examined the relationship between CAG repeat length and brain volume in a large cohort of pre- and post-motor onset HD gene carriers, using voxel-based morphometry (VBM), an approach which allowed us to investigate the whole brain without defining a priori regions of interest. We also used VBM to examine group differences between 20 controls, 21 premanifest, and 40 early HD subjects. In the 61 mutation-positive subjects higher CAG repeat length was significantly associated with reduced volume of the body of the caudate nucleus bilaterally, left putamen, right insula, right parahippocampal gyrus, right anterior cingulate, and right occipital lobe, after correcting for age. The group contrasts showed significant reduction in grey matter volume in the early HD group relative to controls in widespread cortical as well as subcortical areas but there was no evidence of difference between controls and premanifest subjects. Overall we have demonstrated that increased CAG repeat length is associated with atrophy in extra-striatal as well as striatal regions, which has implications for the monitoring of disease-modifying therapies in the condition.

Prevalence and clinical characteristics of maternally inherited diabetes and deafness caused by the mt3243A &gt; G mutation in young adult diabetic subjects in Sri Lanka

The maternally inherited mt3243A > G mutation is associated with a variable clinical phenotype including diabetes and deafness (MIDD). We aimed to determine the prevalence and clinical characteristics of MIDD in a large South Asian cohort of young adult-onset diabetic patients from Sri Lanka. DNA was available from 994 subjects (age of diagnosis 16-40 years, age at recruitment < or = 45 years). Mutation screening was performed using a QRT-PCR method on an ABI 7900HT system using sequence-specific probes. Samples with heteroplasm > or = 5.0% were considered positive. Nine (four males) mutation-positive subjects were identified (prevalence 0.9%). They were diagnosed at a younger age (25.9 +/- 4.8 years vs. 31.9 +/- 5.6 years, P = 0.002) and were lean (body mass index [BMI] 18.7 +/- 2.7 kg/m(2) vs. 24.7 +/- 4.0 kg/m(2), P < 0.001) compared to NMCs. One mutation-positive subject (11.1%) had metabolic syndrome, compared to 633 (64.3%) of NMCs. Insulin therapy within 6 months of diagnosis was used in four (44.0%) carriers compared to 6.9% of NMCs (P = 0.002). Combined screening criteria of any two of maternal history of diabetes, personal history of hearing impairment and family history of hearing impairment only identified five (55%) of the carriers, with a positive predictive value of 7.4%. The prevalence of mt3243A > G mutation among young adult-onset diabetic subjects from Sri Lanka was 0.9%. Our study demonstrates that a maternal family history of diabetes and either a personal and/or family history of deafness only distinguish half of patients with MIDD from Sri Lankan subjects with young-onset diabetes.

Spinocerebellar ataxia type 17 (SCA17): Oculomotor phenotype and clinical characterization of 15 Italian patients

SCA17 is a rare type of autosomal dominant spinocerebellar ataxia caused by a CAG/CAA expansion in the gene encoding the TATA-binding protein (TBP). We screened for triplet expansion in the TBP gene 110 subjects with progressive cerebellar ataxia and 94 subjects with Huntington-like phenotype negative at specific molecular tests. SCA17 mutation-positive subjects were found in both groups of patients. Expanded alleles with > or = 44 CAG/CAA repeats were identified in 11 individuals and in 4 non-symptomatic relatives. Eleven de novo diagnosed patients and four patients previously reported underwent extensive clinical, neuroradiological and oculographic examination. Cerebellar signs and symptoms were present in all cases; 80% of the patients had mild to severe cognitive deficits; 66% of patients showed choreic movements; pyramidal signs, bradykinesia and dystonia were observed in approx 50% of the cases. MRI demonstrated cortical and cerebellar atrophy in all patients, whereas neurophysiological examination excluded signs of peripheral nervous system involvement. Oculographic examinations were performed in 9 out of 15 patients and showed a distinct pattern of oculomotor abnormalities, characterized by impairment of smooth pursuit, defects in the saccade accuracy, normal saccade velocity, hyperreflexia of vestibuloocular reflexes, and absence of nystagmus. In summary, this study presents one of the largest series of SCA17 patients in Europe. In our group of patients, SCA17 represents the third most frequent SCA genotype. Our clinical data confirm the large variability in SCA17 phenotypic presentation, and indicate that a peculiar combination of neuroradiological, electrophysiological and oculomotor findings is recognizable in SCA17.

Variable pathological and clinical features of a large Brazilian family harboring a mutation in the aryl hydrocarbon receptor-interacting protein gene

Germline aryl hydrocarbon receptor-interacting protein (AIP) mutations occur in 15% of familial isolated pituitary adenoma (FIPA) cases. To date, studies have focused on the identification of such mutations in large international cohorts. Detailed genetic and clinical studies within AIP mutation-positive families have been limited. To undertake a comprehensive study of a large Brazilian FIPA kindred with an E174 frameshift (E174fs) AIP mutation to assess clinical, hormonal, and radiological features in mutation carriers. The kindred included 122 subjects across six generations; all underwent clinical examination. Genetic studies were performed to identify E174fs mutation carriers. E174fs-positive subjects underwent magnetic resonance imaging (MRI) and hormonal assessments. Of the ten germline AIP mutation carriers, three had pituitary tumors, while seven were asymptomatic carriers. Three patients with pituitary tumors showed variability in terms of tumor phenotype (two with acromegaly, one with prolactinoma, or mixed prolactin/GH-secreting tumor) and age at diagnosis; both patients with acromegaly had poor responses to octreotide. Tumor AIP immunohistochemistry from the operated patient showed decreased expression when compared with normal tissue. Two adult subjects with normal MRI had elevated IGF-I in the absence of other causes. A 2-year-old child with the E174fs mutation and a normal MRI had premature thelarche, ovarian development, and advanced bone age in the absence of other underlying causes. The penetrance of pituitary tumors in AIP mutation-positive adult subjects was 33.3%, while clinical/hormonal features were variable. The features noted in AIP-mutation carriers in this kindred suggest that clinical characteristics of such carriers may extend beyond pituitary tumors.

Aryl Hydrocarbon Receptor-Interacting Protein Gene Mutations in Familial Isolated Pituitary Adenomas: Analysis in 73 Families

An association between germline aryl hydrocarbon receptor-interacting protein (AIP) gene mutations and pituitary adenomas was recently shown. The objective of the study was to assess the frequency of AIP gene mutations in a large cohort of patients with familial isolated pituitary adenoma (FIPA). This was a multicenter, international, collaborative study. The study was conducted in 34 university endocrinology and genetics departments in nine countries. Affected members from each FIPA family were studied. Relatives of patients with AIP mutations underwent AIP sequence analysis. Presence/absence and description of AIP gene mutations were the main outcome measures. There was no intervention. Seventy-three FIPA families were identified, with 156 patients with pituitary adenomas; the FIPA cohort was evenly divided between families with homogeneous and heterogeneous tumor expression. Eleven FIPA families had 10 germline AIP mutations. Nine mutations, R16H, G47_R54del, Q142X, E174frameshift, Q217X, Q239X, K241E, R271W, and Q285frameshift, have not been described previously. Tumors were significantly larger (P = 0.0005) and diagnosed at a younger age (P = 0.0006) in AIP mutation-positive vs. mutation-negative subjects. Somatotropinomas predominated among FIPA families with AIP mutations, but mixed GH/prolactin-secreting tumors, prolactinomas, and nonsecreting adenomas were also noted. Approximately 85% of the FIPA cohort and 50% of those with familial somatotropinomas were negative for AIP mutations. AIP mutations, of which nine new mutations have been described here, occur in approximately 15% of FIPA families. Although pituitary tumors occurring in association with AIP mutations are predominantly somatotropinomas, other tumor types are also seen. Further study of the impact of AIP mutations on protein expression and activity is necessary to elucidate their role in pituitary tumorigenesis in FIPA.

Wolframin mutations and hospitalization for psychiatric illness

Genetic predisposition plays an important role in most common psychiatric disorders. The identification of a specific gene associated with a psychiatric illness can lead to improved management of the gene-associated disorder. Mutations in the wolframin gene are associated with mental illness. Many patients with the Wolfram syndrome (WS), who are homozygous or compound heterozygous for wolframin mutations, have severe psychiatric symptoms. In WS families, close blood relatives, who have a high probability of carrying a single wolframin mutation, had a statistically significant excess, over spouse controls, of psychiatric hospitalizations, attempted and completed suicides, and self-reports of mental illness. Since heterozygous carriers of wolframin mutations are relatively frequent in the population according to the general Hardy-Weinberg principle, such mutations might be responsible for the illnesses of many psychiatric patients. The hypothesis that heterozygous carriers of a wolframin mutation are predisposed to psychiatric illness was tested in subjects from 25 WS families. In all, 11 relatives who had psychiatric hospitalizations could be genotyped through mutation analysis. Eight of these carried the wolframin mutation transmitted in their family, significantly (one-sided P=0.0022) more than the 3.0 expected if there were no association between psychiatric hospitalizations and mutations at this locus. All eight mutation-positive subjects had been hospitalized for a major depression. This confirmation of the association is not influenced by confounders, undetected stratification, or genetic heterogeneity. The relative risk of psychiatric hospitalization for depression was estimated to be 7.1 (95% CI 1.9-26.6) for carriers of a single wolframin mutation compared to noncarriers.

Gonadotrophin therapy in Kallmann syndrome caused by heterozygous mutations of the gene for fibroblast growth factor receptor 1: report of three families:Case report

Gonadotrophin therapy (GT) is frequently used to induce fertility in Kallmann syndrome (KS). We studied the effects and the consequences of GT in autosomal dominant KS caused by heterozygous FGFR1 mutations. Three Japanese families were examined. In family A, an adult male received GT and had two sons. In family B, an adult female received GT and gave birth to dizygotic male and female twins. In family C, an adult female received GT and produced a son and a daughter. Direct sequencing was performed for FGFR1, and clinical assessment was carried out for KS features. The father and the elder son of family A had P745S mutation, the mother and the female twin of family B had G687R mutation, and the mother and the two children of family C had S107X mutation. KS phenotype was detected for the mutation-positive subjects, except for the elder son of family A who had apparently normal phenotype. GT in FGFR1 mutations is effective in acquiring fertility but has a risk of transmitting the mutation and the disease phenotype to the next generation.

Genotypic and phenotypic characterization of Noonan syndrome: New data and review of the literature

Noonan syndrome (NS) is an autosomal dominant disorder, characterized by short stature, minor facial anomalies, and congenital heart defects. In approximately 50% of cases the condition is caused by missense mutations in the PTPN11 gene on chromosome 12, resulting in a gain of function of the protein SHP-2. In this study, PTPN11 mutation analysis was performed in 170 NS patients. In 76 (45%) of them a mutation was identified. We report on the distribution of these mutations, as well as on genotype-phenotype relationships. The benefit of the NS scoring system developed by van der Burgt et al. [(1994); Am J Med Genet 53:187-191] is shown, among physicians who consequently based their diagnosis on the NS scoring system the percentage mutation positive subjects was 54%, whereas this percentage was only 39% among physicians who made less use of the scoring system. In two patients with some uncommon manifestations mutations were found in the C-SH2 domain, a region in which defects are not often identified in NS. A trend was observed in patients carrying the 922A --> G change (Asn308Asp) receiving normal education. In one patient with NS and mild juvenile myelomonocytic leukemia (JMML) the mutation 218C --> T (Thr73Ile) was found. This confirms previous findings indicating that individuals with NS with specific mutations in PTPN11 are at risk of developing JMML.

Individuals with an increased risk of colorectal cancer: perceived benefits and psychological aspects of surveillance by means of regular colonoscopies.

To evaluate the psychological consequences of genetic counseling followed by a surveillance program using colonoscopy among individuals with increased risk of colorectal cancer. Two hundred sixty-five individuals, participating in a surveillance program with colonoscopy, were mailed a survey questionnaire that assessed their experience of the surveillance program and their perception of the risk of colorectal cancer. The Hospital Anxiety and Depression scale and the Swedish Short Form-36 Health Survey was also included. Two hundred forty individuals completed the questionnaire and were divided into the following risk groups: risk group 1, an individual with a mutation in hMLH1 or hMSH2 and a lifetime colorectal cancer risk of 80% (n = 28); risk group 2, a lifetime colorectal cancer risk of 40% (n = 129); and risk group 3, a lifetime colorectal cancer risk of 20% (n = 83). Among all individuals, the mean for perceived benefit was 8.0, and the perception of discomfort was 3.3 on the visual analog scale (1-10). In risk group 1, 61% underestimated personal risks as being 40% or less. Approximately 50% of the subjects in risk groups 2 and 3 either under- or overestimated their lifetime risk. According to the Swedish Short Form-36 Health Survey and the Hospital Anxiety and Depression scale, the study sample resembled the reference population. A majority of the study sample understood why they were under surveillance, and regular colonoscopies were well-tolerated. The wide range of risk perception as well as low-risk perception in mutation positive subjects is acceptable, as long as these individuals adhere to surveillance programs and do not demonstrate increased levels of anxiety or depression.

Genetic testing for polyposis: practical and ethical aspects

The three autosomal dominant inherited polyposis syndromes, familial adenomatous polyposis, juvenile polyposis, and Peutz-Jeghers polyposis predispose to colorectal cancer as does hereditary non-polyposis colorectal cancer syndrome. Uncovering the genetic background...

Neuropsychological manifestations of the genetic mutation for Huntington's disease in presymptomatic individuals.

A triplet repeat (CAG) expansion mutation in the huntingtin gene on chromosome 4 is responsible for Huntington's disease (HD). Presymptomatic genetic testing for this mutation has identified clinically normal persons who are virtually certain to develop this dementing illness if they live a normal lifespan. The present study sought to determine whether these "mutation-positive" persons have impairments in cognitive functioning. Seventy-five mutation-positive persons did not differ from 128 mutation-negative persons on tests selected for their sensitivity to early-stage HD. Interestingly, however, those with the mutation viewed themselves as more likely to develop HD than did those without the mutation. Among mutation-positive subjects, having a longer CAG repeat mutation was likewise not associated with cognitive impairment. However, being closer to estimated disease onset (a product of repeat length and parent's age at onset) was associated with selected cognitive impairments. When viewed in light of previous studies showing atrophy of the caudate nucleus and putamen in mutation-carriers who are close to onset but not those far from onset, these results suggest that subtle changes in brain and behavior may be detected shortly before subjects with the HD mutation develop sufficient signs and symptoms for diagnosis. Conceptual and methodological problems associated with the search for presymptomatic cognitive and behavioral indicators of dementing illness are discussed.

No Support for Endoscopic Surveillance for Gastric Cancer in Hereditary Non-Polyposis Colorectal Cancer

Background: The DNA mismatch repair gene mutations underlying hereditary non-polyposis colorectal cancer syndrome (HNPCC) also predispose, besides colorectal and endometrial cancer, to gastric cancer, usually of the intestinal type. The carcinogenetic pathway behind the elevated gastric cancer risk is largely unknown. Methods: The aim of this study was to determine whether there are any premalignant lesions to search for in gastric surveillance in HNPCC by comparing gastric histopathology between mutation-positive and mutation-negative family members. We searched for differences in occurrence of Helicobacter pylori , inflammation, atrophy, intestinal metaplasia and dysplastic changes. Upper gastrointestinal endoscopy was performed for 73 mutation-positive and 32 mutation-negative family members. Results: One case of duodenal cancer was detected in the mutation-positive group, but no gastric neoplastic lesions were seen in either group. There were no differences in the occurrence of polyps, H. pylori , inflammation, activity, atrophy nor intestinal metaplasia tested with binaric, logistic, regression analysis. Conclusions: We conclude that surveillance gastroscopy may not be beneficial in HNPCC, since neither cases of early cancer nor premalignant lesions could be detected in our series of 73 mutation-positive subjects.

VHL c.505 T>C mutation confers a high age related penetrance but no increased overall mortality

BACKGROUNDGermline mutations of the VHL gene cause von Hippel-Lindau syndrome (VHL). In southern Germany, a specific mutation in this gene, c.505 T>C, is one of the most frequent alterations owing...

Psychological consequences of predictive genetic testing for hereditary non-polyposis colorectal cancer (HNPCC): a prospective follow-up study.

Predictive genetic testing for cancer allows identification of those with the mutation (mutation positive) who should undergo cancer surveillance aiming at early detection of cancer and those without the mutation (mutation negative), whose unnecessary worry can be alleviated and who need not undergo frequent surveillance. However, there is a risk that predictive testing might have a harmful emotional impact on an individual. In the course of a predictive genetic testing protocol, we assessed general anxiety (by the State-Trait Anxiety Inventory [STAI]), fear of cancer and death, satisfaction with life and attitude to the future using a questionnaire survey in 271 individuals tested for hereditary non-polyposis colorectal cancer (HNPCC). Measurements were made before the first counseling (baseline), at the test disclosure session (STAI only) and 1 and 12 months after disclosure. Although at every measurement, the mutation-positive individuals were more afraid of cancer than those who were mutation negative, in both groups fear of cancer decreased significantly from baseline after disclosure. The mutation-positive subjects were more anxious than their counterparts immediately after the test disclosure, but the differences had disappeared at the follow-ups. In other variables, neither differences between the groups defined by mutation status nor changes with time were detected. Our findings suggest that counseling and testing relieve fear of cancer; no harmful emotional impact was detectable at the 1-year follow-up. To confirm these findings, however, the impact of testing should be studied after a longer interval. Furthermore, to evaluate the ultimate interpretation of these results, studies are needed to investigate the impact of fear of cancer on surveillance behavior among the mutation-positive subjects.

Life and Health Insurance Behaviour of Individuals Having Undergone a Predictive Genetic Testing Programme for Hereditary Non-Polyposis Colorectal Cancer

Objective and Methods: We describe the insurance behaviour of subjects (n = 271) who had previously taken a predictive genetic test for hereditary non-polyposis colorectal cancer (HNPCC); 31% of them were mutation positive, indicating a high risk of cancer. One year after testing, the subjects were sent a questionnaire including questions about their present life and health insurance before participation in the study, and their actual and planned purchase of the insurance policies during the testing programme which comprised a pre-test counselling session, a period for reflection, the testing, and a test disclosure session. Results: Thirty percent reported that they already had a life insurance and 14% a health insurance before participating in the study. The mutation-positive subjects possessed a health insurance significantly more often than the mutation-negative individuals (21 vs. 11%, p = 0.02) and a similar trend was observed for life insurance (36 vs. 28%, p = 0.12). Life and health insurance policies purchased just before testing was reported by 3 and 2% of the subjects, respectively. Life and health insurance policies purchased after testing were reported by 3 and <1%, respectively, and planned purchase by 3 and 2%, respectively. No statistically significant differences were found between the groups defined by mutation status in reports of life or health insurance behaviour during or after the programme. Conclusion: According to self-reported data, the mutation-positive subjects did not differ from the others in the purchase of life or health insurance policies. However, the mutation-positive individuals reported that they possessed health insurance policies before entering the study more often than their counterparts.

Controlled 15-year trial on screening for colorectal cancer in families with hereditary nonpolyposis colorectal cancer

Background & Aims: Identification of the hereditary nonpolyposis colorectal cancer (HNPCC) syndrome enables prevention of colorectal cancer (CRC) by means of colonoscopy and polypectomies. We evaluated the efficacy of screening in a controlled trial over 15 years. Methods: Incidence of CRC and survival were compared in 2 cohorts of at-risk members of 22 families with HNPCC. Colonic screening at 3-year intervals was arranged for 133 subjects; 119 control subjects had no screening. Genetic testing was offered to subjects in whose families the causative mutation was known. Results: CRC developed in 8 screened subjects (6%) compared with 19 control subjects (16 %; P = 0.014). The CRC rate was reduced by 62%. In mutation-positive subjects alone, the CRC rates were 18% in screened subjects and 41% in controls ( P = 0.02). The decrease resulted from the removal of adenomas in 13 mutation-positive individuals (30%) and in 6 subjects with unknown mutation status (40%). All CRCs in the study group were local, causing no deaths, compared with 9 deaths caused by CRC in the controls. The overall death rates were 10 vs. 26 subjects in the study and control groups ( P = 0.003), 4 vs. 12 in mutation-positive subjects ( P = 0.05). Conclusions: Colonoscopic screening at 3-year intervals more than halves the risk of CRC, prevents CRC deaths, and decreases overall mortality by about 65% in HNPCC families. GASTROENTEROLOGY 2000;118:829-834

A novel deletion at codon 441 of the APC gene associated with ophthalmic lesions (CHRPE) in a South African family.

A novel mutation at codon 441 in exon 10 of the adenomatous polyposis coli (APC) gene was identified in a South African family of mixed ancestry, using a convenient, non-radioactive, heteroduplex-SSCP screening assay. This single thymidine deletion after nucleotide position 1322 creates a frameshift resulting in a downstream stop codon at amino acid residue 453 of the APC gene. Genotypes of nine family members were subsequently correlated with the presence or absence of congenital hypertrophy of the retinal pigment epithelium (CHRPE), since expression of this common extracolonic manifestation of FAP is largely determined by the length of the truncated protein. CHRPE was absent in the five unaffected family members analysed, while four mutation positive subjects showed these ophthalmic lesions. Correlation between the molecular analysis and ophthalmic examinations, performed without knowledge of clinical and genetic status respectively, provided additional evidence in favour of the view that the range of phenotypic expression in FAP may result from different allelic manifestations of APC mutations.