KRAS genetic variant as a prognostic factor for recurrence in resectable non-small cell lung cancer

Purpose. Several angiogenic prognostic markers are under investigation because of their potential clinical utility, aiming to improve patient outcomes. We hypothesized that genetic variant in the VEGF pathway could be used as prognostic markers of survival in non-small cell lung cancer (NSCLC) patients undergoing pulmonary resection. Methods. We evaluated the relationship between genetic variants in the VEGF pathway and relapse-free survival (RFS, main endpoint) and overall survival (OS, secondary endpoint) among 131 patients with stage I-III NSCLC treated with surgical resection from 2009 to 2013. Clinical, pathological and surgical data were prospectively collected. Twenty-five variants in sixteen relevant genes were selected and genotyped in tumor samples by real time PCR. The Kaplan-Meier method with the log-rank test and Cox’s regression models were used for RFS and OS analyses. Results. With a median follow-up of 36 (min = 2.8; max = 67.4) months, there were 31 (24%) relapses and 31 (24%) deaths. Overall, median RFS was not reached and median OS was 65 [95% confidence interval (CI) 56-75] months. The KRAS rs1137282 and PIK3C2A rs4356203 variants were significantly associated with RFS. For KRAS rs1137282, the 3-year RFS was 76% [95% CI 64-84%] in patients harboring an A/A genotype compared to 53% [95% CI 37-69%] in patients harboring an A/G or G/G genotype (p = 0.02). For PIK3C2A rs4356203, patients with an A/A or an A/G genotype had a 3-year RFS of 72% [95% CI 58-76%], whereas in patients with a G/G genotype was 49% [95% CI 28-70%] (p = 0.02). These associations remained statistically significant after adjusting for all the relevant clinical parameters in the multivariable analysis. Conclusion. Genetic variants in VEGF pathway may be associated with recurrence in stage I-III NSCLC. Specifically, the KRAS rs1137282 could be considered as a prognostic factor for recurrence in resectable NSCLC patients. Although PIK3C2A rs4356203 was associated with RFS, further analyses are necessary to confirm these data (AU)

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KRAS genetic variant as a prognostic factor for recurrence in resectable non-small cell lung cancer.

PURPOSE: Several angiogenic prognostic markers are under investigation because of their potential clinical utility, aiming to improve patient outcomes. We hypothesized that genetic variant in the VEGF pathway could be used as prognostic markers of survival in non-small cell lung cancer (NSCLC) patients undergoing pulmonary resection. METHODS: We evaluated the relationship between genetic variants in the VEGF pathway and relapse-free survival (RFS, main endpoint) and overall survival (OS, secondary endpoint) among 131 patients with stage I-III NSCLC treated with surgical resection from 2009 to 2013. Clinical, pathological and surgical data were prospectively collected. Twenty-five variants in sixteen relevant genes were selected and genotyped in tumor samples by real time PCR. The Kaplan-Meier method with the log-rank test and Cox's regression models were used for RFS and OS analyses. RESULTS: With a median follow-up of 36 (min = 2.8; max = 67.4) months, there were 31 (24%) relapses and 31 (24%) deaths. Overall, median RFS was not reached and median OS was 65 [95% confidence interval (CI) 56-75] months. The KRAS rs1137282 and PIK3C2A rs4356203 variants were significantly associated with RFS. For KRAS rs1137282, the 3-year RFS was 76% [95% CI 64-84%] in patients harboring an A/A genotype compared to 53% [95% CI 37-69%] in patients harboring an A/G or G/G genotype (p = 0.02). For PIK3C2A rs4356203, patients with an A/A or an A/G genotype had a 3-year RFS of 72% [95% CI 58-76%], whereas in patients with a G/G genotype was 49% [95% CI 28-70%] (p = 0.02). These associations remained statistically significant after adjusting for all the relevant clinical parameters in the multivariable analysis. CONCLUSION: Genetic variants in VEGF pathway may be associated with recurrence in stage I-III NSCLC. Specifically, the KRAS rs1137282 could be considered as a prognostic factor for recurrence in resectable NSCLC patients. Although PIK3C2A rs4356203 was associated with RFS, further analyses are necessary to confirm these data.

Genetic variations in the VEGF pathway as prognostic factors in metastatic colorectal cancer patients treated with oxaliplatin-based chemotherapy.

Angiogenesis is a significant biological mechanism in the progression and metastasis of solid tumors. Vascular endothelial growth factor (VEGF), its receptors and signaling effectors have a central role in tumor-induced angiogenesis. Genetic variation in the VEGF pathway may impact on tumor angiogenesis and, hence, on clinical cancer outcomes. This study evaluates the influence of common genetic variations within the VEGF pathway in the clinical outcomes of 172 metastatic colorectal cancer (mCRC) patients treated with first-line oxaliplatin/5-fluorouracil chemotherapy. A total of 27 single-nucleotide polymorphisms (SNPs) in 16 genes in the VEGF-dependent angionenesis process were genotyped using a dynamic array on the BioMark™ system. After assessing the KRAS mutational status, we found that four SNPs located in three genes (KISS1, KRAS and VEGFR2) were associated with progression-free survival. Five SNPs in three genes (ITGAV, KRAS and VEGFR2) correlated with overall survival. The gene-gene interactions identified in the survival tree analysis support the importance of VEGFR2 rs2071559 and KISS1 rs71745629 in modulating these outcomes. This study provides evidence that functional germline polymorphisms in the VEGF pathway may help to predict outcome in mCRC patients who undergo oxaliplatin/5-fluorouracil chemotherapy.

EGFR ligands and DNA repair genes: genomic predictors of complete response after capecitabine-based chemoradiotherapy in locally advanced rectal cancer.

Epidermal growth factor receptor (EGFR) activation by radiation leads to increased cell proliferation and acts as a radioresistance mechanism. Neoadjuvant chemoradiation is the standard of care for locally advanced rectal cancer, and to date, no biomarkers of response have been found. We analyzed polymorphisms in the EGFR and its ligands, DNA repair genes and the thymidylate synthase in 84 stages II and III rectal cancer patients treated with neoadjuvant capecitabine plus radiotherapy. The rs11942466 polymorphism in the amphiregulin (AREG) gene region was associated with a pathological complete response (ypCR) (odds ratio: 0.26; 95% confidence interval: 0.06-0.79; P=0.014). The rs11615 C>T polymorphism in the ERCC1 gene also correlated with the ypCR as no patients with a C/C genotype achieved ypCR; P=0.023. This is the first work to propose variants within the AREG and the ERCC1 genes as promising predictive biomarkers of ypCR in rectal cancer.

Improving detection and genetic counseling in carriers of spinal muscular atrophy with two copies of the SMN1 gene.

Spinal muscular atrophy (SMA) is an autosomal recessive disease caused by mutations in the survival motor neuron1 gene (SMN1). Global carrier frequency is around 1 in 50 and carrier detection is crucial to define couples at risk to have SMA offspring. Most SMA carriers have one SMN1 copy and are currently detected using quantitative methods. A few, however, have two SMN1 genes in cis (2/0 carriers), complicating carrier diagnosis in SMA. We analyzed our experience in detecting 2/0 carriers from a cohort of 1562 individuals, including SMA parents, SMA relatives, and unrelated individuals of the general population. Interestingly, in three couples who had an SMA child, both the parents had two SMN1 copies. Families of this type have not been previously reported. Our results emphasize the importance of performing a detailed carrier study in SMA parents with two SMN1 copies. Expanding the analysis to other key family members might confirm potential 2/0 carriers. Finally, when a partner of a known carrier presents two SMN1 copies, the study of both parents will provide a more accurate diagnosis, thus optimizing genetic counseling.

Intergenic polymorphisms in the amphiregulin gene region as biomarkers in metastatic colorectal cancer patients treated with anti-EGFR plus irinotecan.

In the epidermal growth factor receptor (EGFR) pathway, polymorphisms in EGFR and its ligand EGF have been studied as biomarkers for anti-EGFR treatment. However, the potential pharmacogenetic role of other EGFR ligands such as amphiregulin (AREG) and epiregulin (EREG) has not been elucidated. We studied 74 KRAS and BRAF wild-type metastatic colorectal cancer patients treated with anti-EGFR plus irinotecan. Twenty-two genetic variants in EGFR, EGF, AREG and EREG genes were selected using HapMap database and literature resources. Three tagging single-nucleotide polymorphisms in the AREG gene region (rs11942466 C>A, rs13104811 A>G, and rs9996584 C>T) predicted disease control in the multivariate analyses. AREG rs11942466 C>A and rs9996584 C>T were also associated with overall survival (OS). The functional polymorphism, EGFR rs712829 G>T, was associated with progression-free and OS. Our findings support that intergenic polymorphisms in the AREG gene region might help to identify colorectal cancer patients that will benefit from irinotecan plus anti-EGFR therapy.

A genome-wide association study on copy-number variation identifies a 11q11 loss as a candidate susceptibility variant for colorectal cancer.

Colorectal cancer (CRC) is a complex disease, and therefore its development is determined by the combination of both environmental factors and genetic variants. Although genome-wide association studies (GWAS) of SNP variation have conveniently identified 20 genetic variants so far, a significant proportion of the observed heritability is yet to be explained. Common copy-number variants (CNVs) are one of the most important genomic sources of variability, and hence a potential source to explain part of this missing genetic fraction. Therefore, we have performed a GWAS on CNVs to explore the relationship between common structural variation and CRC development. Phase 1 of the GWAS consisted of 881 cases and 667 controls from a Spanish cohort. Copy-number status was validated by quantitative PCR for each of those common CNVs potentially associated with CRC in phase I. Subsequently, SNPs were chosen as proxies for the validated CNVs for phase II replication (1,342 Spanish cases and 1,874 Spanish controls). Four common CNVs were found to be associated with CRC and were further replicated in Phase II. Finally, we found that SNP rs1944682, tagging a 11q11 CNV, was nominally associated with CRC susceptibility (p value = 0.039; OR = 1.122). This locus has been previously related to extreme obesity phenotypes, which could suggest a relationship between body weight and CRC susceptibility.

Pharmacogenomics in colorectal cancer: a genome-wide association study to predict toxicity after 5-fluorouracil or FOLFOX administration.

The development of genotyping technologies has allowed for wider screening for inherited causes of variable outcomes following drug administration. We have performed a genome-wide association study (GWAS) on 221 colorectal cancer (CRC) patients that had been treated with 5-fluorouracil (5-FU), either alone or in combination with oxaliplatin (FOLFOX). A validation set of 791 patients was also studied. Seven SNPs (rs16857540, rs2465403, rs10876844, rs10784749, rs17626122, rs7325568 and rs4243761) showed evidence of association (pooled P-values 0.020, 9.426E-03, 0.010, 0.017, 0.042, 2.302E-04, 2.803E-03) with adverse drug reactions (ADRs). This is the first study to explore the genetic basis of inter-individual variation in toxicity responses to the administration of 5-FU or FOLFOX in CRC patients on a genome-wide scale.

F8 gene dosage defects in atypical patients with severe haemophilia A.

We performed molecular analysis of the factor 8 gene (F8) in 272 unrelated Spanish patients with haemophilia A (HA) and detected a mutation by routine analysis in 267 of them (98.1%). No mutation was detected in the remaining five patients despite clinical and laboratory confirmation of HA. The aim is to describe the molecular alterations in F8 discovered by gene dosage methodologies in three of these patients. For methodology, F8 sequencing, intragenic marker analysis, multiplex ligation-dependent probe amplification and quantitative real time-PCR were followed. One patient had Klinefelter syndrome (47,XXY) and a large deletion spanning exons 1-12 masked by the other F8 allele; the second patient showed a large duplication spanning exons 2-10 and the third patient revealed a non-contiguous double duplication of exons 14 and 23-25. The remaining two patients had mild HA and dosage results were normal. The application of gene dosage methods is useful to define haemophilic patients in whom mutations are not detected using other routine methods. Nevertheless, in a small percentage of patients (<1%), no molecular pathology can be identified after testing several genetic methodologies.

Methotrexate consolidation treatment according to pharmacogenetics of MTHFR ameliorates event-free survival in childhood acute lymphoblastic leukaemia.

Recent advances in treatment for childhood acute lymphoblastic leukaemia (ALL) have significantly increased outcome. High-dose methotrexate (MTX) is the most commonly used regimen during the consolidation period, but the optimal dose remains to be defined. We investigated the usefulness of the MTHFR genotype to increase the MTX dosage in the consolidation phase in 141 childhood ALL patients enrolled in the ALL/SHOP-2005 protocol. We also investigated the pharmacogenetic role of polymorphisms in genes involved in MTX metabolism on therapy-related toxicity and survival. Patients with a favourable MTHFR genotype (normal enzymatic activity) treated with MTX doses of 5 g m⁻² had a significantly lower risk of suffering an event than patients with an unfavourable MTHFR genotype (reduced enzymatic activity) that were treated with the classical MTX dose of 3 g m⁻² (P=0.012). Our results indicate that analysis of the MTHFR genotype is a useful tool to optimise MTX therapy in childhood patients with ALL.

Isolated cardiomyopathy caused by a DMD nonsense mutation in somatic mosaicism: genetic normalization in skeletal muscle.

X-linked dilated cardiomyopathy is a pure cardiac dystrophinopathy phenotype mainly caused by DMD mutations that present a specific transcription effect in cardiac tissue. We report a 26-year-old male who presented with severe dilated cardiomyopathy and high creatine kinase. The patient did not complain of skeletal muscle weakness. A muscle biopsy showed mild dystrophic changes and a low proportion of dystrophin-negative fibres. A molecular study identified a nonsense DMD mutation (p.Arg2098X) in somatic mosaicism. The ratio of mutant versus normal allele in blood and skeletal muscle suggests selective pressure against mutant muscle cells, a process known as genetic normalization. We hypothesize that this process may have mitigated skeletal muscle symptoms in this patient. This is the second report of a DMD somatic mosaic with evidence of genetic normalization in muscle. Somatic DMD mutations should be considered in patients presenting with idiopathic dilated cardiomyopathy.

Diagnóstico de la enfermedad de CADASIL en pacientes normotensos y no diabéticos con infarto lacunar / Diagnosis of CADASIL disease in normotensive and non-diabetics with lacunar infarct

Introducción: la enfermedad de CADASIL (Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukoencephalopathy) se caracteriza por isquemias cerebrales recurrentes de tipo lacunar, habitualmente en pacientes sin factores de riesgo vascular. Analizamos la frecuencia de enfermedad de CADASIL en pacientes con infarto lacunar sin factores de riesgo vascular clásicos. Métodos: estudiamos pacientes con un primer infarto lacunar menores de 65 años sin hipertensión, diabetes mellitus u otra causa que justificara la isquemia cerebral. Realizamos estudio inmunohistoquímico de 5μm de espesor sobre biopsia cutánea usando el anticuerpo monoclonal anti-Notch 3 (1E4). Además del estudio inmunohistoquímico se realizó en todos los casos el estudio genético del gen Notch 3 de los exones 3, 4, 5, 6, 11 y 19. Resultados: de 1.519 pacientes con infarto lacunar, sólo 57 (3,7%) cumplieron los criterios de selección, y 30 de ellos aceptaron participar en el estudio. Analizamos 30 pacientes con edad media de 53 años; el 50% fueron hombres y todos presentaron un primer infarto cerebral tipo lacunar. El estudio inmunohistoquímico y genético confirmó la enfermedad de CADASIL en dos pacientes (6,6%) en el exón 4 nt 622C/T (Arg 182 Cys) y 694 T/C (Cys206Arg) respectivamente (AU)

Background: CADASIL (Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukoencephalopathy) is characterized by recurrent cerebral ischemic episodes of the lacunar subtype usually without traditional vascular risk factors. We investigated the frequency of CADASIL among selected patients with cerebral ischemia of the lacunar subtype. Methods: we studied patients under 65 years old who presented cerebral ischemia of the lacunar subtype without hypertension, diabetes mellitus or other causes that explained the cerebral ischemia. On the skin biopsies, we performed immunostaining analysis on 5μm frozen sections with monoclonal antibody anti-Notch 3 (1E4). We also performed a genetic analysis of the Notch 3 gene (exons 3,4,5,6,11 and 19). Results: of 1.519 patients analyzed, only 57 (3.7%) fulfilled the selection criteria, and 30 of them accepted to participated in the study. We studied 30 patients, mean age was 53 years (range 34 to 65), 50% were men and all patients suffered a lacunar stroke. Immunostaining analysis was positive in two patients (6.6%) and the genetic analysis confirmed a mutation characteristic of CADASIL in exon 4 nt 622C/T (Arg 182 Cys) and 694 T/C (Cys206Arg) respectively. Conclusions: CADASIL disease was present in 6.6% of patients younger than 65 years with a lacunar stroke and without hypertension or diabetes mellitus. Screening for CADASIL should be considered in these patients (AU)

LMNA mutation in progeroid syndrome in association with strokes.

Hutchinson-Gilford progeria syndrome is a very rare but well-characterized genetic disorder that causes premature ageing. Clinical features affect growth, skeleton, body fat, skin, hair and the cardiovascular system. It is caused by mutations in LMNA gene, the most frequent being p.Gly608Gly (c.1824C > T) in exon 11. Here we present a four-year-old HGPS patient who presented several severe strokes and carried a heterozygous LMNA missense mutation in exon 2: p.Glu138Lys. This mutation is located far from the C-terminal region implicated in the posttranslational processing of prelamin A, but it lies within the rod domain of lamin A/C that represents a highly conserved domain specific to nuclear lamins. We hypothesize that this region could be involved in early and severe strokes in HGPS, such as those presented by our patient.

A genotype-directed phase I-IV dose-finding study of irinotecan in combination with fluorouracil/leucovorin as first-line treatment in advanced colorectal cancer.

BACKGROUND: Infusional fluorouracil/leucovorin (FU/LV) plus irinotecan (FOLFIRI) is one of the standard first-line options for patients with metastatic colorectal cancer (mCRC). Irinotecan is converted into 7-ethyl-10-hydroxycamptothecin (SN-38) by a carboxylsterase and metabolised through uridine diphosphate glucuronosyl transferase (UGT1A1). The UGT1A1*28 allele has been associated with the risk of developing severe toxicities. The present trial was designed to define the maximum tolerated dose according to UGT1A1 genotype. This report focuses on the results of tolerance to different escalated doses of FOLFIRI first-line of chemotherapy. PATIENTS AND METHODS: Patients undergoing first-line treatment for mCRC and eligible for treatment with FOLFIRI were classified according to UGT1A1 genotype. A total of 94 patients were eligible for dose escalation of irinotecan. The starting dose of biweekly irinotecan was 180 mg m(-2) for the *1/*1, 110 mg m(-2) for the *1/*28 and 90 mg m(-2) for the *28/*28 genotypes. RESULTS: The dose of irinotecan was escalated to 450 mg m(-2) in patients with the *1/*1 genotype, to 390 mg m(-2) in those with the *1/*28 genotype and to 150 mg m(-2) in those with the *28/*28 genotype. Neutropenia and diarrhoea were the most common grade 3 or 4 toxicities. CONCLUSIONS: Our results demonstrated that the recommended dose of 180 mg m(-2) for irinotecan in FOLFIRI is considerably lower than the dose that can be tolerated for patients with the UGT1A1 *1/*1 and *1/*28 genotypes. The maximum tolerable dose (MTD) in patients with a high-risk UGT1A1 *28/*28 genotype is 30% lower than the standard dose of 180 mg m(-2).

Diagnosis of CADASIL disease in normotensive and non-diabetics with lacunar infarct.

BACKGROUND: CADASIL (Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukoencephalopathy) is characterized by recurrent cerebral ischemic episodes of the lacunar subtype usually without traditional vascular risk factors. We investigated the frequency of CADASIL among selected patients with cerebral ischemia of the lacunar subtype. METHODS: we studied patients under 65 years old who presented cerebral ischemia of the lacunar subtype without hypertension, diabetes mellitus or other causes that explained the cerebral ischemia. On the skin biopsies, we performed immunostaining analysis on 5µm frozen sections with monoclonal antibody anti-Notch 3 (1E4). We also performed a genetic analysis of the Notch 3 gene (exons 3,4,5,6,11 and 19). RESULTS: of 1.519 patients analyzed, only 57 (3.7%) fulfilled the selection criteria, and 30 of them accepted to participated in the study. We studied 30 patients, mean age was 53 years (range 34 to 65), 50% were men and all patients suffered a lacunar stroke. Immunostaining analysis was positive in two patients (6.6%) and the genetic analysis confirmed a mutation characteristic of CADASIL in exon 4 nt 622C/T (Arg 182 Cys) and 694 T/C (Cys206Arg) respectively. CONCLUSIONS: CADASIL disease was present in 6.6% of patients younger than 65 years with a lacunar stroke and without hypertension or diabetes mellitus. Screening for CADASIL should be considered in these patients.

Pharmacogenetic studies on the antipsychotic treatment. Current status and perspectives.

Based on present knowledge, in this work we review the importance of the pharmacogenetic tests in the treatment with antipsychotic drugs. Many associations have been reported between different genetic markers and response to treatment as well as to the appearance of adverse reactions. However, up to now, no "prime" biomarker capable of unequivocally predicting the clinical benefits of a specific treatment or its toxicity has been identified. The use of individual pharmacogenetic markers has been demonstrated to have little clinical utility, and therefore the combination of information obtained from the analysis of different genes seems to be a more promising strategy. Inclusion of pharmacogenetic tests in clinical trials conducted prospectively and that include a large number of cases could, undoubtedly, significantly contribute to the development of individualized medicine protocols.

Estudios Farmacogenéticos del tratamiento con Antipsicóticos: Estado actual y perspectivas / Pharmacogenetic studies on the antipsychotic treatment. Current status and perspectives

En este trabajo se revisa, a la luz de los conocimientos actuales, la relevancia de los estudios farmacogenéticos en el tratamiento con fármacos antipsicóticos. Se han descrito un gran número de asociaciones entre distintos marcadores genéticos y la respuesta al tratamiento, así como a la aparición de efectos adversos. Sin embargo, no se ha identificado aún ningún biomarcador “estrella” capaz de predecir de forma inequívoca el beneficio clínico de un determinado tratamiento ni su toxicidad. La utilización de marcadores farmacogenéticos individuales se ha demostrado de poca utilidad clínica, por lo que la combinación de la información obtenida del estudio de diversos genes parece una estrategia más prometedora. La inclusión de estudios farmacogenéticos en ensayos clínicos realizados de forma prospectiva incluyendo un elevado número de pacientes podría, sin duda, contribuir de forma significativa al desarrollo de protocolos de medicina personalizada (AU)

Based on present knowledge, in this work we review the importance of the pharmacogenetic tests in the treatment with antipsychotic drugs. Many association shave been reported between different genetic markers and response to treatment as well as to the appearance of adverse reactions. However, up to now, no “prime” biomarker capable of unequivocally predicting the clinical benefits of a specific treatment or its toxicity has been identified. The use of individual pharmacogenetic markers has been demonstrated to have little clinical utility, and therefore the combination of information obtained from the analysis of different genes seems to be a more promising strategy. Inclusion of pharmacogenetic tests in clinical trials conducted prospectively and that include a large number of cases could, undoubtedly, significantly contribute to the development of individualized medicine protocols (AU)

The c.859G>C variant in the SMN2 gene is associated with types II and III SMA and originates from a common ancestor.

Homozygous mutations of the telomeric SMN1 gene lead to degeneration of motor neurons causing spinal muscular atrophy (SMA). A highly similar centromeric gene (SMN2) can only partially compensate for SMN1 deficiency. The c.859G>C variant in SMN2 has been recently reported as a positive disease modifier. We identified the variant in 10 unrelated chronic SMA patients with a wide spectrum of phenotypes ranging from type II patients who can only sit to adult walkers. Haplotype analysis strongly suggests that the variant originated from a common ancestor. Our results confirm that the c.859G>C variant is a milder SMN2 allele and predict a direct correlation between SMN activity and phenotypic severity.

Copy number variations are not modifiers of phenotypic expression in a pair of identical twins carrying a BRCA1 mutation.

Mutations in BRCA1 and BRCA2 genes confer a high risk of breast and ovarian cancer but the incomplete penetrance of these mutations suggests that other genetic and/or environmental factors may modify this risk. We present a family where all affected members carried a mutation in the BRCA1 gene and the index case had suffered from cancer twice in the last 27 years, whereas her monozygotic twin sister, also a carrier of the mutation, remained healthy. As copy number variants (CNVs) contribute to phenotypic diversity, a comparative genomic hybridization array (CGH) was performed to see whether the differences in the CNV profile were a modifier factor of the phenotype in our monozygotic twins. Our results show that differences in the CNVs profile were not the cause of the extremely variable penetrance observed in our MZ twin. The search for an explanation should not therefore be limited to genetic changes at the level of the DNA sequence.