The 1993-94 Généthon human genetic linkage map

Practice guidelines for the molecular analysis of Prader-Willi and Angelman syndromes

Background

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are clinically distinct neurodevelopmental genetic disorders that map to 15q11-q13. The primary phenotypes are attributable to loss of expression of imprinted genes within this region which can arise by means of a number of mechanisms. The most sensitive single approach to diagnosing both PWS and AS is to study methylation patterns within 15q11-q13; however many techniques exist for this purpose. Given the diversity of techniques available, there is a need for consensus testing and reporting guidelines.

Methods

Testing and reporting guidelines have been drawn up and agreed in accordance with the procedures of the UK Clinical Molecular Genetics Society and the European Molecular Genetics Quality Network.

Results

A practical set of molecular genetic testing and reporting guidelines has been developed for these two disorders. In addition, advice is given on appropriate reporting policies, including advice on test sensitivity and recurrence risks. In considering test sensitivity, the possibility of differential diagnoses is discussed.

Conclusion

An agreed set of practice guidelines has been developed for the diagnostic molecular genetic testing of PWS and AS.

Identification of a second kindred with familial hypocalciuric hypercalcemia type 3 (FHH3) narrows localization to a <3.5 megabase pair region on chromosome 19q13.3

CONTEXT

Familial hypocalciuric hypercalcemia (FHH) is a genetically heterogenous disorder that consists of three defined types, FHH1, FHH2, and FHH3 whose chromosomal locations are 3q21.1, 19p, and 19q13, respectively. FHH1, caused by mutations of the calcium-sensing receptor (CASR), occurs in more than 65% of patients, whereas the abnormalities underlying FHH2 and FHH3, which have each been described in single North American kindreds, are unknown.

OBJECTIVE

The aim of this study was to determine the basis of FHH in a proband, who did not have CASR mutations, and her kindred.

PATIENTS AND METHODS

The proband was a 43-yr-old woman who presented with a corrected serum calcium of 2.74 mmol/liter (normal = 2.15-2.55 mmol/liter), a serum PTH of 47 pg/ml (normal = 10-65 pg/ml), and a urinary calcium clearance:creatinine clearance of 0.006. She did not have a CASR mutation within the coding region and splice sites, and 24 members from three generations of her kindred were ascertained and investigated for serum abnormalities and cosegregation with polymorphic loci from chromosomes 3q21.1 and 19q13 using leukocyte DNA.

RESULTS

Sixteen members were hypercalcemic with normal or elevated serum PTH concentrations and mild hypophosphatemia, features consistent with FHH3. Use of microsatellite and single nucleotide polymorphic loci from chromosome 19q13.3 demonstrated cosegregation with FHH in the kindred, with a peak LOD score = 5.98 at 0% recombination with D19S412. Analysis of recombinants mapped FHH to a 3.46-Mbp interval flanked centromerically by single nucleotide polymorphism rs1990932 and telomerically by D19S604.

CONCLUSIONS

FHH3 may explain the calcium homeostasis disorder in those FHH patients who do not have CASR mutations.

Single-sperm analysis for recurrence risk assessment of spinal muscular atrophy

With the detection of a homozygous deletion of the survival motor neuron 1 gene (SMN1), prenatal and preimplantation genetic diagnosis (PGD) for spinal muscular atrophy has become feasible and widely applied. The finding of a de novo rearrangement, resulting in the loss of the SMN1 gene, reduces the recurrence risk from 25% to a lower percentage, the residual risk arising from recurrent de novo mutation or germline mosaicism. In a couple referred to our PGD center because their first child was affected with SMA, the male partner was shown to carry two SMN1 copies. An analysis of the SMN1 gene and two flanking markers was performed on 12 single spermatozoa, to determine whether the father carried a CIS duplication of the SMN1 gene on one chromosome and was a carrier, or if the deletion has occurred de novo. We showed that all spermatozoa that were carriers of the 'at-risk haplotype' were deleted for the SMN1 gene, confirming the carrier status of the father. We provide an original application of single germ cell studies to recessive disorders using coamplification of the gene and its linked markers. This efficient and easy procedure might be useful to elucidate complex genetic situations when samples from other family members are not available.

Trinucleotide repeats in human genome and exome

Trinucleotide repeats (TNRs) are of interest in genetics because they are used as markers for tracing genotype–phenotype relations and because they are directly involved in numerous human genetic diseases. In this study, we searched the human genome reference sequence and annotated exons (exome) for the presence of uninterrupted triplet repeat tracts composed of six or more repeated units. A list of 32 448 TNRs and 878 TNR-containing genes was generated and is provided herein. We found that some triplet repeats, specifically CNG, are overrepresented, while CTT, ATC, AAC and AAT are underrepresented in exons. This observation suggests that the occurrence of TNRs in exons is not random, but undergoes positive or negative selective pressure. Additionally, TNR types strongly determine their localization in mRNA sections (ORF, UTRs). Most genes containing exon-overrepresented TNRs are associated with gene ontology-defined functions. Surprisingly, many groups of genes that contain TNR types coding for different homo-amino acid tracts associate with the same transcription-related GO categories. We propose that TNRs have potential to be functional genetic elements and that their variation may be involved in the regulation of many common phenotypes; as such, TNR polymorphisms should be considered a priority in association studies.

Reducing polycystic liver volume in ADPKD: effects of somatostatin analogue octreotide

BACKGROUND AND OBJECTIVES

No medical treatment is available for polycystic liver disease, a frequent manifestation of autosomal-dominant polycystic kidney disease (ADPKD). In a prospective, randomized, double-blind, crossover study, 6 months of octreotide (40 mg every 28 days) therapy limited kidney volume growth more effectively than placebo in 12 patients with ADPKD.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

In this secondary, post hoc analysis of the above study, octreotide-induced changes in liver volumes compared with placebo and the relationship between concomitant changes in liver and kidney volumes were evaluated. Those analyzing liver and kidney volumes were blinded to treatment.

RESULTS

Liver volumes significantly decreased from 1595 +/- 478 ml to 1524 +/- 453 ml with octreotide whereas they did not appreciably change with placebo. Changes in liver volumes were significantly different between the two treatment periods (-71 +/- 57 ml versus +14 +/- 85 ml). Octreotide-induced liver volume reduction was fully explained by a reduction in parenchyma volume from 1506 +/- 431 ml to 1432 +/- 403 ml. Changes in liver volumes were significantly correlated with concomitant changes in kidney volumes (r = 0.67) during octreotide but not during placebo treatment. Liver and kidney volume changes significantly differed with both treatments (octreotide: -71 +/- 57 ml versus +71 +/- 107; placebo: +14 +/- 85 ml versus +162 +/- 114), but net reductions in liver (-85 +/- 103 ml) and kidney (-91 +/- 125 ml) volume growth on octreotide versus placebo were similar.

CONCLUSIONS

Octreotide therapy reduces liver volumes in patients with ADPKD and is safe.

Appearance of epithelial and stromal genomic instability in background colorectal mucosa of sporadic colorectal cancer patients: relation to age and gender

BACKGROUND

We have previously demonstrated that not only epithelial but also stromal genetic instability possibly contributes to colorectal tumorigenesis. To assess the increasing risk of carcinogenesis in the colorectum with aging, we examined genomic instability in both epithelia and stroma in the background noncancerous mucosa of patients with colorectal carcinomas.

METHODS

In 213 noncancerous colorectal mucosa samples from colorectal cancer cases and 51 normal mucosa specimens of diverticulosis cases, epithelial and stromal genomic instability was analyzed with National Cancer Institute standard microsatellite markers, chromosome 17 (Chr.17) markers and tumor suppressor gene-related markers, using a combination of laser-capture microdissection and GeneScan approaches. Results were compared with immunohistochemically demonstrated expression of FHIT, Rb, WT1, hMLH1 and hMSH2.

RESULTS

Genomic instability (MSI and LOH) in both epithelia and stroma appeared after around 40 years of age and remained relatively constant thereafter at relatively low frequencies (4.8-30.4%). The Epithelial LOH tended to show a stepwise increase in people in their 40s and 50s along with aging, especially in males. Overall frequencies of both epithelial MSI and LOH in left-side colon and LOH in right-side colon were significantly higher in males than in females. Epithelial hMLH1 expression in MSI (-) cases tended to be reduced with aging.

CONCLUSIONS

Genomic instability of both MSI and LOH in noncancerous colonic mucosa, and more particularly epithelial and stromal LOH, appears relatively early in adults, suggesting age-related changes which increase the risk of cancer development, particularly in males.

Comparison of human chromosome 19q13 and syntenic region on mouse chromosome 7 reveals absence, in man, of 11.6 Mb containing four mouse calcium-sensing receptor-related sequences: relevance to familial benign hypocalciuric hypercalcaemia type 3

Familial benign hypocalciuric hypercalcaemia (FBHH) is a genetically heterogeneous disorder that consists of three designated types, FBHH1, FBHH2 and FBHH3, whose chromosomal locations are 3q21.1, 19p and 19q13, respectively. FBHH1 is caused by mutations of a calcium-sensing receptor (CaSR), but the abnormalities underlying FBHH2 and FBHH3 are unknown. FBHH3, also referred to as the Oklahoma variant (FBHH(Ok)), has been mapped to a 12cM interval, flanked by D19S908 and D19S866. To refine the location of FBHH3, we pursued linkage studies using 24 polymorphic loci. Our results establish a linkage between FBHH3 and 17 of these loci, and indicate that FBHH3 is located in a 4.1 Mb region flanked centromerically by D19S112 and telomerically by rs245111, which in the syntenic region on mouse chromosome 7 contains four Casr-related sequences (Gprc2a-rss). However, human homologues of these Gprc2a-rss were not found and a comparative analysis of the 22.0 Mb human and 39.3 Mb mouse syntenic regions showed evolutionary conservation of two segments that were inverted with loss from the human genome of 11.6 Mb that contained the four Gprc2a-rss. Thus, FBHH3 cannot be attributed to Gprc2a-rss abnormalities. DNA sequence analysis of 12 other genes from the interval that were expressed in the parathyroids and/or kidneys did not detect any abnormalities, thereby indicating that these genes are unlikely to be the cause of FBHH3. The results of this study have refined the map location of FBHH3, which will facilitate the identification of another CaSR or a mediator of calcium homeostasis.

Multiplex Manager 1.0: a cross-platform computer program that plans and optimizes multiplex PCR

Multiplex Manager 1.0 is a user-friendly cross-platform program that designs efficient combinations of existing genetic marker loci into multiplex polymerase chain reactions and optimizes using prior marker information. The program has the flexibility to solve two design problems: combining all markers into the smallest number of reactions, or alternatively, selecting a subset from many available markers to design an efficient and robust multiplex. Our program minimizes the number of reactions, the genetic linkage, and the difference in annealing temperature. At the same time it maximizes the spacing between markers, the heterozygosity, and the number of alleles. The final output provides easily interpreted and informative graphical representations of reactions, as well as the option of manually editing final reactions. Multiplex Manager 1.0 is freely available at www.multiplexmanager.com.

Development of an integrative database with 499 novel microsatellite markers for Macaca fascicularis

BACKGROUND

Cynomolgus macaques (Macaca fascicularis) are a valuable resource for linkage studies of genetic disorders, but their microsatellite markers are not sufficient. In genetic studies, a prerequisite for mapping genes is development of a genome-wide set of microsatellite markers in target organisms. A whole genome sequence and its annotation also facilitate identification of markers for causative mutations. The aim of this study is to establish hundreds of microsatellite markers and to develop an integrative cynomolgus macaque genome database with a variety of datasets including marker and gene information that will be useful for further genetic analyses in this species.

RESULTS

We investigated the level of polymorphisms in cynomolgus monkeys for 671 microsatellite markers that are covered by our established Bacterial Artificial Chromosome (BAC) clones. Four hundred and ninety-nine (74.4%) of the markers were found to be polymorphic using standard PCR analysis. The average number of alleles and average expected heterozygosity at these polymorphic loci in ten cynomolgus macaques were 8.20 and 0.75, respectively.

CONCLUSION

BAC clones and novel microsatellite markers were assigned to the rhesus genome sequence and linked with our cynomolgus macaque cDNA database (QFbase). Our novel microsatellite marker set and genomic database will be valuable integrative resources in analyzing genetic disorders in cynomolgus macaques.

Strong linkage disequilibrium for the frequent GJB2 35delG mutation in the Greek population

Approximately one in 1,000 children is affected by severe or profound hearing loss at birth or during early childhood (prelingual deafness). Up to 40% of congenital, autosomal recessive, severe to profound hearing impairment cases result from mutations in a single gene, GJB2, that encodes the connexin 26 protein. One specific mutation in this gene, 35delG, accounts for the majority of GJB2 mutations detected in Caucasian populations. Some previous studies have assumed that the high frequency of the 35delG mutation reflects the presence of a mutational hot spot, while other studies support the theory of a common founder. Greece is among the countries with the highest carrier frequency of the 35delG mutation (3.5%), and a recent study raised the hypothesis of the origin of this mutation in ancient Greece. We genotyped 60 Greek deafness patients homozygous for the 35delG mutation for six single nucleotide polymorphisms (SNPs) and two microsatellite markers inside or flanking the GJB2 gene. The allele distribution in the patients was compared to 60 Greek normal hearing controls. A strong linkage disequilibrium was found between the 35delG mutation and markers inside or flanking the GJB2 gene. Furthermore, we found a common haplotype with a previous study, suggesting a common founder for the 35delG mutation.

Linkage mapping of CVD risk traits in the isolated Norfolk Island population

To understand the underlying genetic architecture of cardiovascular disease (CVD) risk traits, we undertook a genome-wide linkage scan to identify CVD quantitative trait loci (QTLs) in 377 individuals from the Norfolk Island population. The central aim of this research focused on the utilization of a genetically and geographically isolated population of individuals from Norfolk Island for the purposes of variance component linkage analysis to identify QTLs involved in CVD risk traits. Substantial evidence supports the involvement of traits such as systolic and diastolic blood pressures, high-density lipoprotein-cholesterol, low-density lipoprotein-cholesterol, body mass index and triglycerides as important risk factors for CVD pathogenesis. In addition to the environmental influences of poor diet, reduced physical activity, increasing age, cigarette smoking and alcohol consumption, many studies have illustrated a strong involvement of genetic components in the CVD phenotype through family and twin studies. We undertook a genome scan using 400 markers spaced approximately 10 cM in 600 individuals from Norfolk Island. Genotype data was analyzed using the variance components methods of SOLAR. Our results gave a peak LOD score of 2.01 localizing to chromosome 1p36 for systolic blood pressure and replicated previously implicated loci for other CVD relevant QTLs.

Overview of linkage analysis in complex traits

The goal of this unit is to instruct the reader on the general approaches now available for mapping complex disease. The term complex genetic disease refers to a trait phenotype with an unknown mode of inheritance. The goal of this unit is to instruct the reader on the general approaches now available for mapping complex disease.

Plant simple sequence repeats: distribution, variation, and effects on gene expression

Genome-wide simple sequence repeat (SSR) information was analyzed together with functional annotations of Arabidopsis genes and public gene expression data for Arabidopsis and rice. Analysis of more than 15,000 Arabidopsis and more than 16,000 rice SSRs indicated that SSRs may affect the expression of hundreds of genes. Data from experiments on DNA methylation, histone acetylation, and transcript turnover suggest that SSRs may affect gene expression at transcriptional and posttranscriptional levels. Members of some functional groups were shown to be enriched with SSRs and often contained similar but non-homologous repeats within the same gene regions. In addition, the distribution of perfect and imperfect SSRs in some Arabidopsis, maize, and rice genes was used to demonstrate how two-level control of SSR variation may contribute to protein evolution.

[Human genome project: a federator program of genomic medicine]

The Human Genome Project improves our understanding of the molecular genetics basis of the inherited and complex diseases such as diabetes, schizophrenia, and cancer. Information from the human genome sequence is essential for several antenatal and neonatal screening programmes. The new genomic tools emerging from this project have revolutionized biology and medicine and have transformed our understanding of health and the provision of healthcare. Its implications pervade all areas of medicine, from disease prediction and prevention to the diagnosis and treatment of all forms of disease. Increasingly, it will be possible to drive predisposition testing into clinical practice, to develop new treatments or to adapt available treatments more specifically to an individual's genetic make-up. This genomic information should transform the traditional medications that are effective for every members of the population to personalized medicine and personalized therapy. The pharmacogenomics could give rise to a new generation of highly effective drugs that treat causes, not just symptoms.

Conservation of synteny between guppy and Xiphophorus genomes

The guppy and fish in the genus Xiphophorus have both been important model systems for the study of natural and sexual selection for over 50 years. Whereas the guppy is unique in the degree to which the environmental variables shaping phenotypic variation are known, Xiphophorus has the advantage that genomic resources have been developed due to the utility of this taxon for the study of melanoma. If linkage maps for the guppy and Xiphophorus are similar, genomic resources developed in Xiphophorus will be useful in the guppy. The authors used an F2 mapping cross of divergent populations of the guppy to construct partial female and male genetic linkage maps incorporating microsatellite markers derived from Xiphophorus mapping efforts. Flanking regions for a sample of microsatellites occurring in maps for both taxa were sequenced in the guppy and compared to published sequences from Xiphophorus. This confirmed that these loci were homologous and estimated the divergence in neutral nuclear DNA to be 0.21 substitutions per site. The female map comprises 16 linked markers on six linkage groups, and the male map comprises 24 markers on nine linkage groups. Linkage relationships among loci homologous in the guppy and Xiphophorus primarily show conservation of genetic architecture between species, but several major changes were detected.

Somatic APC inactivation mechanisms in sporadic colorectal cancer cases in Hungary

The role of germline inactivation of the adenomatosis polyposis coli (APC) gene in hereditary colorectal cancer is well known, being the most important cause of familial adenomatosus polyposis (FAP) syndrome. Hereditary cases with germline mutations, however, account only for 5-10% of colorectal cancers. The somatic inactivation of this gene has also been observed in sporadic cases. In order to examine the inactivation mechanisms of the APC gene we screened 70 sporadic colorectal cancer cases (27 rectal, 43 intestinal) of different stages for promoter hypermethylation, allelic imbalance (AI) and somatic mutations. The presence of promoter hypermethylation was observed in 21 cases (30%). Fifteen of the examined tumors (21%) showed AI, and also 15 tumors (21%) carried at least one somatic mutation. Thirteen of the detected alterations were novel variations: seven frameshifts, four missense mutations and two polymorphisms. Biallelic inactivation was found in 15 patients (21%). These results suggest that the inactivation of the APC gene is very common in sporadic colorectal cancer, and the main inactivation mechanism of the APC gene is promoter hypermethylation. Allelic imbalance has the same frequency as mutations, and mutations in the APC gene are more common in the early stages and in tumors located in the rectum.

Preimplantation genetic diagnosis for autosomal recessive polycystic kidney disease

Autosomal recessive polycystic kidney disease (ARPKD) is one of the most common hereditary renal cystic diseases, and is caused by mutations in the PKHD1 gene. Due to the poor prognosis, there is a strong demand for prenatal diagnosis. Preimplantation genetic diagnosis (PGD) represents an alternative because it avoids the physical and emotional trauma of a pregnancy termination in the case of an affected fetus. A standardized single-cell diagnostic procedure was developed, based on haplotype analysis, enabling PGD to be offered to couples at risk of transmitting ARPKD. Six linked markers within (D6S1714 and D6S243), or in close proximity to (D6S272, D6S436, KIAA0057, D6S1662) the PKHD1 gene were tested by multiplex nested-polymerase chain reaction (PCR), using a Qiagen multiplex PCR kit. PCR analyses were carried out on 50 single lymphocytes. The amplification rate was excellent (100%), with an allele drop-out (ADO) rate ranging from 0 to 8%. Five PGD cycles were performed and 23 embryos were biopsied and analysed using this test. Transferable embryos were obtained in 4 cycles, resulting in two pregnancies and the birth of a healthy boy. This standardized diagnostic procedure allowed the detection of recombination, contamination, and ADO events, providing high assay accuracy with wide applicability.

Copy number variants and genetic traits: closer to the resolution of phenotypic to genotypic variability

A considerable and unanticipated plasticity of the human genome, manifested as inter-individual copy number variation, has been discovered. These structural changes constitute a major source of inter-individual genetic variation that could explain variable penetrance of inherited (Mendelian and polygenic) diseases and variation in the phenotypic expression of aneuploidies and sporadic traits, and might represent a major factor in the aetiology of complex, multifactorial traits. For these reasons, an effort should be made to discover all common and rare copy number variants (CNVs) in the human population. This will also enable systematic exploration of both SNPs and CNVs in association studies to identify the genomic contributors to the common disorders and complex traits.

A novel obesity locus on chromosome 4q: the Strong Heart Family Study

OBJECTIVE

Obesity is a growing and important public health problem in Western countries and worldwide. There is ample evidence that both environmental and genetic factors influence the risk of developing obesity. Although a number of genes influencing obesity and obesity-related measures have been localized, it is clear that others remain to be identified. The rate of obesity is particularly high in American Indian populations. This study reports the results of a genome-wide scan for loci influencing BMI and weight in 963 individuals in 58 families from three American Indian populations in Arizona, Oklahoma, and North and South Dakota participating in the Strong Heart Family Study.

RESEARCH METHODS AND PROCEDURES

Short tandem repeat markers were genotyped, resulting in a marker map with an average spacing of 10 centimorgans. Standard multipoint variance component linkage methods were used.

RESULTS

Significant evidence of linkage was observed in the overall sample, including all three study sites, for a locus on chromosome 4q35 [logarithm of the odds (LOD)=5.17 for weight, 5.08 for BMI]. Analyses of the three study sites individually showed that the greatest linkage support for the chromosome 4 locus came from Arizona (LOD=2.6 for BMI), but that LOD scores for weight were >1 in all three samples. Suggestive linkage signals (LOD>2) were also observed on chromosomes 5, 7, 8, and 10.

DISCUSSION

The chromosome 4 locus detected in this scan is in a region lacking any obvious positional candidate genes with known functions related to obesity. This locus may represent a novel obesity gene.

CAG and CTG repeat polymorphism in exons of human genes shows distinct features at the expandable loci

Although the trinucleotide repeats are present in the exons of numerous human genes, the allele distribution is not well known, and the factors responsible for their intergenic and intragenic variability are not well understood. We have analyzed the length and sequence variation within the most commonly occurring CAG and CTG repeats in a large number of human genes selected to contain the longest reported repeat tracts. Our study revealed that in genes other than those implicated in the Triplet Repeat Expansion Diseases (TREDs), the very long and highly polymorphic repeats are rather infrequent. The length of pure repeat tract in the most frequent allele was found to correlate well with the rate of the repeat length polymorphism, and CAA triplets were shown to be the most frequent CAG repeat interruptions. As both the CAG and CAA triplets code for glutamine, our results may suggest that the selective pressure disfavors the long uninterrupted CAG repeats in genes and transcripts but not the long normal polyglutamine tracts in proteins. This may indicate that hairpin structures formed in ssDNA and RNA by long pure CAG repeats would be selected against as they may impede normal cellular processes.

Microsatellite megatracts in the maize (Zea mays L.) genome

Long tracts (megatracts) of (CAG)n, (TAG)n, and (GAA)n microsatellite sequences capable of forming composite DNA segments were found in the maize (Zea mays L.) genome. Some of the (CAG)n and (TAG)n megatracts were organized in clusters of up to 1 Mb on several chromosomes, as detected by fluorescence in situ hybridization (FISH), as well as on extended DNA fibers. Extensive polymorphism was found among different maize inbred lines with respect to the number and size of microsatellite megatract clusters on the A chromosomes. Polymorphism was also common among B chromosomes of different nuclei in the inbred line Zapalote Chico. Different retrotransposable elements were often inserted into the microsatellite tracts. Size variation in some (TAG)n and (GAA)n megatracts was observed in consecutive generations among siblings of the inbred lines, indicating that these loci are highly unstable and predisposed to dynamic mutations similar to those described in mammalian systems.

Microsatellite analysis of recurrent vestibular schwannoma (acoustic neuroma) following stereotactic radiosurgery

HYPOTHESIS

Genetic and immunohistochemical studies may provide insight into the mechanisms of vestibular schwannoma (VS) recurrence following radiation therapy.

BACKGROUND

Stereotactic radiation therapy is an increasingly common alternative to microsurgical resection for the primary management of sporadic VS. The molecular mechanisms associated with recurrent vestibular schwannoma (VS) following radiation therapy are not known.

METHODS

Primary or irradiated VS tumors were fresh-frozen at the time of surgical resection and microdissected to undergo DNA extraction. Lymphocytic control DNA was isolated from blood obtained by venipuncture. Paired normal and tumor DNA specimens were analyzed for allelic loss by PCR amplification of polymorphic dinucleotide repeat sequences. Immunohistochemical studies were performed on paraffin-embedded, irradiated surgical specimens.

RESULTS

Using 16 polymorphic microsatellite markers, 20 of 26 non-irradiated VS demonstrated loss of heterozygosity (LOH) in at least one locus of chromosome 22q. In contrast, none of the four irradiated recurrent VS demonstrated LOH on chromosome 22q (p = 0.008). No allelic loss was seen in either the primary or irradiated VS utilizing markers mapping to chromosome 10. Deletions on chromosome 10 are seen in both benign and higher-grade meningiomas and intracranial malignancies associated with radiotherapy. Immunohistochemical studies were performed to detect the protein product of the NF2 gene, merlin, in the four irradiated VS. NF2 staining was not observed.

CONCLUSION

This study represents the first microsatellite and immunohistochemical analysis of recurrent VS following radiation therapy. Our preliminary observations suggest an alternative mechanism of NF2 inactivation that may correlate with radioresistance in VS.

A novel locus for autosomal recessive nonsyndromic hearing impairment, DFNB63, maps to chromosome 11q13.2-q13.4

Hereditary hearing impairment is a genetically heterogeneous disorder. To date, 49 autosomal recessive nonsyndromic hearing impairment (ARNSHI) loci have been described, and there are more than 16 additional loci announced. In 25 of the known loci, causative genes have been identified. A genome scan and fine mapping revealed a novel locus for ARNSHI (DFNB63) on chromosome 11q13.2-q13.4 in a five-generation Turkish family (TR57). The homozygous linkage interval is flanked by the markers D11S1337 and D11S2371 and spans a 5.3-Mb interval. A maximum two-point log of odds score of 6.27 at a recombination fraction of theta = 0.0 was calculated for the marker D11S4139. DFNB63 represents the eighth ARNSHI locus mapped to chromosome 11, and about 3.33 Mb separate the DFNB63 region from MYO7A (DFNB2/DFNB11). Sequencing of coding regions and exon-intron boundaries of 13 candidate genes, namely SHANK2, CTTN, TPCN2, FGF3, FGF4, FGF19, FCHSD2, PHR1, TMEM16A, RAB6A, MYEOV, P2RY2 and KIAA0280, in genomic DNA from an affected individual of family TR57 revealed no disease-causing mutations.

Clinical-etiologic correlation in children with Prader-Willi syndrome (PWS): An interdisciplinary study

Prader-Willi syndrome (PWS) is a multisystemic disorder caused by the loss of expression of paternally transcribed genes within chromosome 15q11-q13. Most cases are due to paternal deletion of this region; the remaining cases result from maternal uniparental disomy (UPD) and imprinting defects. To better understand the phenotypic variability of PWS, a genotype-phenotype correlation study was performed in 91 children with PWS. Patients were diagnosed by Southern Blot Methylation assay and genetic subtypes were established using FISH and microsatellite analyses. Fifty-nine subjects with deletion (31/28 males/females; mean age 3.86 years), 30 with UPD (14/16 males/females; mean age 3.89 years) and 2 girls with a presumed imprinting defect (mean age 0.43 yrs) were identified. For correlation purposes patients were grouped as "deleted" and "non-deleted." An increased maternal age was found in the UPD group. Four of Holm's criteria were more frequently present in the deleted group: need for special feeding techniques, sleep disturbance, hypopigmentation, and speech articulation defects. Concerning cognitive assessments, only 9.52% of subjects with deletion had Full-Scale IQ (FSIQ) > or =70, while 61.53% of subjects without deletion had FSIQ > or =70. Similar results were found in behavioral measures. Sleep disorders and carbohydrate metabolism were systematically assessed. Polysomnoghaphic studies revealed a higher frequency of central events with desaturations > or =10% in the deleted group (P = 0.020). In summary, the phenotype was significantly different between both groups in certain parameters related to the CNS. These results might be related to the differences in brain gene expression of the genetic subtypes.

Multiple displacement amplification improves PGD for fragile X syndrome

We report an improvement in the PGD test for fragile X syndrome (FXS). Recently, multiple displacement amplification (MDA) has been reported to yield large amounts of DNA from single cells. Taking into account this technique, we developed a new PGD test for FXS, enabling combined analysis of linked polymorphic markers with the study of the non-expanded CGG repeat. Single cell amplification efficiency was first assessed on single lymphocytes. Amplification rate of the different markers ranged from 85 to 95% with an allele drop-out (ADO) rate comprised between 7 and 34%. Using this test, eight PGD cycles were carried out for six couples, and 37 embryos were analysed after preliminary MDA. Amplification rate was increased by this technique from 41 to 66% so that embryos with no results were rarer (14 versus 45% without MDA). Reliability of the test was considerably improved by combining direct with indirect genetic analysis. Furthermore, in cases of fully expanded alleles too large to be amplified by PCR, this test gives an internal amplification control. Embryonic transfers were carried out in all but one PGD cycles. One biochemical and one clinical pregnancy resulted, and a healthy child was born. This single diagnosis procedure could be suitable to most patients carrying FXS.

Microsatellite analysis of the APC gene and immunoexpression of E-cadherin, catenin, and tubulin in esophageal squamous cell carcinoma

The histological features that accompany the development and progression of solid tumors are known to be controlled by a distinct cascade of molecular events. One such event is the inactivation of tumor suppressor genes, such as the adenomatous polyposis coli (APC) gene. Disruption of the cadherin-catenin cell adhesion complex also plays a role in the initial steps of cancer invasion and metastasis whereas alterations in cell structural molecules, such as tubulin, may contribute to the cancer phenotype. The understanding of the status of these molecules in ESSC may provide novel markers that could impact on management of the disease. The present study examined alterations in the microsatellite sequence of the APC gene via fluorescent-based polymerase chain reaction in 100 cases of primary esophageal squamous cell carcinoma. In addition, the expression of E-cadherin, alpha- and beta-catenin, and alpha- and beta-tubulin was analyzed using immunohistochemistry. These data were then statistically compared with each other as well as the relevant clinicopathologic data. Although the APC markers (D5S210, D5S346, D5S299, and D5S82) tested did show an overall high frequency of allelic imbalance/loss of heterozygosity (62.48%) and microsatellite instability (41.27%), they did not show prognostic significance in the study cohort and were not correlated with the immunohistochemical data. The tubulin proteins showed no significant change in expression in the tumor tissue The decreased immunoreactivity of E-cadherin was statistically correlated with the presence of lymph node metastases (P = .0180). Although alpha- and beta-catenin as well as E-cadherin showed no direct prognostic value, E-cadherin may warrant further investigation as an indirect prognostic indicator by allowing more accurate prediction of lymph node metastases.

Evaluation of linkage disequilibrium and its effect on non-parametric multipoint linkage analysis using two high density single-nucleotide polymorphism mapping panels

Genotype data from the Illumina Linkage III SNP panel (n = 4,720 SNPs) and the Affymetrix 10 k mapping array (n = 11,120 SNPs) were used to test the effects of linkage disequilibrium (LD) between SNPs in a linkage analysis in the Collaborative Study on the Genetics of Alcoholism pedigree collection (143 pedigrees; 1,614 individuals). The average r² between adjacent markers across the genetic map was 0.099 ± 0.003 in the Illumina III panel and 0.17 ± 0.003 in the Affymetrix 10 k array. In order to determine the effect of LD between marker loci in a nonparametric multipoint linkage analysis, markers in strong LD with another marker (r² > 0.40) were removed (n = 471 loci in the Illumina panel; n = 1,804 loci in the Affymetrix panel) and the linkage analysis results were compared to the results using the entire marker sets. In all analyses using the ALDX1 phenotype, 8 linkage regions on 5 chromosomes (2, 7, 10, 11, X) were detected (peak markers p < 0.01), and the Illumina panel detected an additional region on chromosome 6. Analysis of the same pedigree set and ALDX1 phenotype using short tandem repeat markers (STRs) resulted in 3 linkage regions on 3 chromosomes (peak markers p < 0.01). These results suggest that in this pedigree set, LD between loci with spacing similar to the SNP panels tested may not significantly affect the overall detection of linkage regions in a genome scan. Moreover, since the data quality and information content are greatly improved in the SNP panels over STR genotyping methods, new linkage regions may be identified due to higher information content and data quality in a dense SNP linkage panel.

Persistent familial hematuria in children and the locus for thin basement membrane nephropathy

This study examined how often children with persistent familial hematuria were from families where hematuria segregated with the known genetic locus for the condition known as benign familial hematuria or thin basement membrane nephropathy (TBMN) at COL4A3/COL4A4. Twenty-one unrelated children with persistent familial hematuria as well as their families were studied for segregation of hematuria with haplotypes at the COL4A3/COL4A4 locus for benign familial hematuria and at the COL4A5 locus for X-linked Alport syndrome. Eight families (38%) had hematuria that segregated with COL4A3/COL4A4, and four (19%) had hematuria that segregated with COL4A5. At most, eight of the other nine families could be explained by disease at the COL4A3/COL4A4 locus if de novo mutations, non-penetrant hematuria or coincidental hematuria in unaffected family members was present individually or in combination. This study confirms that persistent familial hematuria is not always linked to COL4A3/COL4A4 (or COL4A5) and suggests the possibility of a further genetic locus for benign familial hematuria. This study also highlights the risk of excluding X-linked Alport syndrome on the basis of the absence of a family history or of kidney failure.

Discriminating power of localized three-dimensional facial morphology

Many genetic syndromes involve a facial gestalt that suggests a preliminary diagnosis to an experienced clinical geneticist even before a clinical examination and genotyping are undertaken. Previously, using visualization and pattern recognition, we showed that dense surface models (DSMs) of full face shape characterize facial dysmorphology in Noonan and in 22q11 deletion syndromes. In this much larger study of 696 individuals, we extend the use of DSMs of the full face to establish accurate discrimination between controls and individuals with Williams, Smith-Magenis, 22q11 deletion, or Noonan syndromes and between individuals with different syndromes in these groups. However, the full power of the DSM approach is demonstrated by the comparable discriminating abilities of localized facial features, such as periorbital, perinasal, and perioral patches, and the correlation of DSM-based predictions and molecular findings. This study demonstrates the potential of face shape models to assist clinical training through visualization, to support clinical diagnosis of affected individuals through pattern recognition, and to enable the objective comparison of individuals sharing other phenotypic or genotypic properties.

Strategies and clinical outcome of 250 cycles of Preimplantation Genetic Diagnosis for single gene disorders

BACKGROUND

We report on our experience with preimplantation genetic diagnosis (PGD) for single gene disorders (SGDs), from 1999 to 2004, describing strategies and overall clinical outcome of 250 cycles in 174 couples for 23 different genetic conditions.

METHODS

PGD cycles included 15 for autosomal dominant, 148 for autosomal recessive and 19 for X-linked SGDs. In addition, 68 cycles of PGD for SGDs were performed in combination with HLA matching. The strategy in each case used an initial multiplex PCR, followed by minisequencing to identify the mutation(s) combined with multiplex PCR for closely linked informative markers to increase accuracy. Linkage analysis, using intragenic and/or extragenic polymorphic microsatellite markers, was performed in cases where the disease-causing mutation(s) was unknown or undetectable.

RESULTS

In 250 PGD cycles, a total of 1961 cleavage stage embryos were biopsied. PCR was successful in 3409 out of 3149 (92.4%) biopsied blastomeres and a diagnosis was possible in 1849 (94.3%) embryos. Four hundred and twenty-seven embryos were transferred in 211 cycles, resulting in 71 pregnancies (33.6% per embryo transfer), including 15 biochemical pregnancies, six spontaneous miscarriages, two ectopic pregnancies, which were terminated, and nine pregnancies which are still ongoing. The remaining pregnancies were confirmed to be unaffected and went to term without complications, resulting in the birth of 35 healthy babies.

CONCLUSIONS

Minisequencing for mutation detection combined with multiplex fluorescence PCR for linkage analysis is an efficient, accurate and widely applicable strategy for PGD of SGDs. Our experience provides a further demonstration that PGD is an effective clinical tool and a useful option for many couples with a high risk of transmitting a genetic disease.

A novel D458V mutation in the SANS PDZ binding motif causes atypical Usher syndrome

Homozygosity mapping and linkage analysis in a Turkish family with autosomal recessive prelingual sensorineural hearing loss revealed a 15-cM critical region at 17q25.1-25.3 flanked by the polymorphic markers D17S1807 and D17S1806. The maximum two-point lod score was 4.07 at theta=0.0 for the marker D17S801. The linkage interval contains the Usher syndrome 1G gene (USH1G) that is mutated in patients with Usher syndrome (USH) type 1g and encodes the SANS protein. Mutation analysis of USH1G led to the identification of a homozygous missense mutation D458V at the -3 position of the PDZ binding motif of SANS. This mutation was also present homozygously in one out of 64 additional families from Turkey with autosomal recessive nonsyndromic hearing loss and heterozygously in one out of 498 control chromosomes. By molecular modeling, we provide evidence that this mutation impairs the interaction of SANS with harmonin. Ophthalmologic examination and vestibular evaluation of patients from both families revealed mild retinitis pigmentosa and normal vestibular function. These results suggest that these patients suffer from atypical USH.

Multiple correcting COL17A1 mutations in patients with revertant mosaicism of epidermolysis bullosa

Revertant mosaicism by somatic reversion of inherited mutations has been described for a number of genetic diseases. Several mechanisms can underlie this reversion process, such as gene conversion, crossing-over, true back mutation, and second-site mutation. Here, we report the occurrence of multiple corrections in two unrelated probands with revertant mosaicism of non-Herlitz junctional epidermolysis bullosa, an autosomal recessive genodermatosis due to mutations in the COL17A1 gene. Immunofluorescence microscopy and laser dissection microscopy, followed by DNA and RNA analysis, were performed on skin biopsy specimens. In patient 1, a true back mutation, 3781T-->C, was identified in the specimen from the arm, and a second-site mutation, 4463-1G-->A, which compensated for the frameshift caused by the inherited 4424-5insC mutation, was identified in the 3' splice site of exon 55 in a specimen from the middle finger. Patient 2 showed--besides two distinct gene conversion events in specimens from the arm and hand sites, both of which corrected the 1706delA mutation--a second-site mutation (3782G-->C) in an ankle specimen, which prevented the premature ending of the protein by the 3781C-->T nonsense mutation (R1226X). Thus, both inherited mutations, paternal as well as maternal, reverted at least once by different reversion events in distinct cell clusters in the described patients. The occurrence of multiple correcting mutations within the same patient indicates that in vivo reversion is less unusual than was generally thought. Furthermore, in the male patient, mosaic patterns of type XVII collagen-positive keratinocytes were present in clinically unaffected and affected skin. This latter observation makes it likely that reversion may be overlooked and may happen more often than expected.

MMP13 mutation causes spondyloepimetaphyseal dysplasia, Missouri type (SEMD(MO)

MMPs, which degrade components of the ECM, have roles in embryonic development, tissue repair, cancer, arthritis, and cardiovascular disease. We show that a missense mutation of MMP13 causes the Missouri type of human spondyloepimetaphyseal dysplasia (SEMD(MO)), an autosomal dominant disorder characterized by defective growth and modeling of vertebrae and long bones. Genome-wide linkage analysis mapped SEMD(MO) to a 17-cM region on chromosome 11q14.3-23.2 that contains a cluster of 9 MMP genes. Among these, MMP13 represented the best candidate for SEMD(MO), since it preferentially degrades collagen type II, abnormalities of which cause skeletal dysplasias that include Strudwick type SEMD. DNA sequence analysis revealed a missense mutation, F56S, that substituted an evolutionarily conserved phenylalanine residue for a serine in the proregion domain of MMP13. We predicted, by modeling MMP13 structure, that this F56S mutation would result in a hydrophobic cavity with misfolding, autoactivation, and degradation of mutant protein intracellularly. Expression of wild-type and mutant MMP13s in human embryonic kidney cells confirmed abnormal intracellular autoactivation and autodegradation of F56S MMP13 such that only enzymatically inactive, small fragments were secreted. Thus, the F56S mutation results in deficiency of MMP13, which leads to the human skeletal developmental anomaly of SEMD(MO).

A genetic and epidemiologic study of cardiovascular disease in Alaska natives (GOCADAN): design and methods

This article is a report of the design and methods of the Genetics of Coronary Artery Disease in Alaska Natives (GOCADAN) Study. This longitudinal, population-based study was initiated to investigate the genetic determinants of cardiovascular disease and its risk factors. Between October 2000 and April 2004, this family study enrolled 1,214 Eskimos from several coastal villages in the Norton Sound region of Western Alaska. Examinations included a physical, laboratory determinations, and measures of subclinical disease. This study will generate a genome-wide scan for loci influencing cardiovascular disease-related traits. Relations between subclinical atherosclerosis and markers of inflammation will be examined using historic and newly drawn samples. The study will provide data on CVD prevalence, risk factors and the relative contribution of genetic and environmental determinants in Alaska Native peoples. Data from this study will contribute to the delivery of health-care and prevention of CVD in Alaska Eskimos and other populations.

Autosomal dominant congenital non-progressive ataxia overlaps with the SCA15 locus

BACKGROUND

Most patients with pure nonprogressive congenital cerebellar ataxia have a sporadic form of unknown heredity and etiology. Several small families have been reported with a dominantly inherited nonprogressive congenital ataxia (NPCA).

METHODS

The authors ascertained and clinically characterized a four-generation pedigree segregating an autosomal dominant type of congenital nonprogressive cerebellar ataxia associated with cognitive impairment. Following the exclusion of several SCA localizations (SCA-1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 17, IOSCA, and DRPLA), a genome-wide linkage study was performed.

RESULTS

Examination of the family showed that all affected members had gait ataxia and cognitive disability with variable features of dysarthria, dysmetria, dysdiadochokinesia, nystagmus, dystonic movements, and cerebellar hypoplasia on imaging. Clinical signs of pyramidal tract dysfunction and sensory changes were absent. A genome-wide search in this family detected linkage to chromosome 3p with a maximum two-point lod score of 4.26 at D3S3630. This localization to the pter is distal to D3S1304, as defined by a recombination event. This overlaps with the SCA15 locus, with the critical overlapping region between the microsatellite markers, D3S1304 and D3S1620 (approximately 8 cM).

CONCLUSION

Autosomal dominant congenital nonprogressive cerebellar ataxia with or without cerebellar hypoplasia overlaps with the SCA15 locus on chromosome 3pter.

Genetic mosaicism in basal cell carcinoma

Human basal cell cancer (BCC) shows unique growth characteristics, including a virtual inability to metastasize, absence of a precursor stage and lack of tumour progression. The clonal nature of BCC has long been a subject for debate because of the tumour growth pattern. Despite a morphologically multifocal appearance, genetic analysis and three-dimensional reconstructions of tumours have favoured a unicellular origin. We have utilized the X-chromosome inactivation assay in order to examine clonality in 13 cases of BCC. Four parts of each individual tumour plus isolated samples of stroma were analysed following laser-assisted microdissection. In 12/13 tumours, the epithelial component of the tumour showed a monoclonal pattern suggesting a unicellular origin. Surprisingly, one tumour showed evidence of being composed of at least two non-related monoclonal clones. This finding was supported by the analysis of the ptch and p53 gene. Clonality analysis of tumour stroma showed both mono- and polyclonal patterns. A prerequisite for this assay is that the extent of skewing is determined and compensated for in each case. Owing to the mosaic pattern of normal human epidermis, accurate coefficients are difficult to obtain; we, therefore, performed all analyses both with and without considering skewing. This study concludes that BCC are monoclonal neoplastic growths of epithelial cells, embedded in a connective tissue stroma at least in part of polyclonal origin. The study results show that what appears to be one tumour may occasionally constitute two or more independent tumours intermingled or adjacent to each other, possibly reflecting a local predisposition to malignant transformation.

Th1 Cytokine Polymorphisms in Spanish Patients With Type 1 Diabetes

Several polymorphisms in regions where Th1 cytokines (IL12B and IFNG genes) are located were analyzed in 303 Spanish subjects with type 1 diabetes and compared with a control cohort (n = 548). Both groups comprised residents of the Madrid area. The haplotype frequencies were estimated by the expectation-maximization algorithm, and p values were corrected by the number of haplotypes taken into account in the study. Two haplotypes were significantly associated with the disease, one in the IL12B region (D5S2038*8/D5S1352*2/SNP1188C; OR = 3.01, p(c) = 0.0255) and another involved the IFNGgene (D12S313*9/IFNG*1; OR = 1.58, p(c) = 0.0217). Furthermore, a protective IL12B haplotype was found (D5S2038*4/D5S1352*1/SNP1188A; OR = 0.40, p(c) = 0.0405). No association was found for any of IL12B and IFNG markers individually.

Genotypes at chromosome 22q12-13 are associated with HIV-1-exposed but uninfected status in Italians

OBJECTIVE

Despite multiple and repeated exposures to HIV-1, some individuals possess no detectable HIV genome and show T-cell memory responses to the viral antigens. HIV-1-reactive mucosal IgA detected in such uninfected individuals suggests their possible immune resistance against HIV. We tested if the above HIV-1-exposed but uninfected status was associated with genetic markers other than a homozygous deletion of the CCR5 gene.

METHODS

Based on our mapping in chromosome 15 of a gene controlling the production of neutralizing antibodies in a mouse retrovirus infection, we genotyped 42 HIV-1-exposed but uninfected Italians at polymorphic loci in the syntenic segment of human chromosome 22, and compared them with 49 HIV-1-infected and 47 uninfected healthy control individuals by a closed testing procedure.

RESULTS

A significant association was found between chromosome 22q12-13 genotypes and a putative dominant locus conferring anti-HIV-1 immune responses in the exposed but uninfected individuals. Distributions of linkage disequilibrium across chromosome 22 also differed between the exposed but uninfected and two other phenotypic groups.

CONCLUSIONS

The data indicated the presence of a new genetic factor associated with the HIV-1-exposed but uninfected status.

Value of microsatellite instability typing in detecting hereditary non-polyposis colorectal cancer

AIM OF THE STUDY

To detect hereditary non-polyposis colorectal cancer (HNPCC) patients with a strategy combining clinical selection (patient age at onset of cancer less than 50 years or family history of HNPCC tumors) and microsatellite instability typing plus immunohistochemistry, leading to mismatch repair (MMR) germline mutation analysis.

METHODS

Tumors were screened for microsatellite instability (MSI) and for hmlh1 and hmsh2 immunohistochemical expression. Germline mutation analysis was performed to search for MLH1 and MSH2 mutations in patients with MSI-High and MSI-Low tumors.

RESULTS

197 adenocarcinomas were studied: 164 patients were< or =50 years old, 33 were older than 50 years but had a family history of HNPCC tumors. Fifty tumors (25.4%) were MSI-High, 10 were MSI-Low (5.1%), and 130 were MS-Stable (66%). MSI typing was inconclusive in 7 (3.5%). Immunohistochemistry screening was performed on 165 tumors: sensitivity was 63.6%, specificity was 99%. Germline mutation analysis was performed in 33/60 MSI-High or Low tumors: 23 mutations were noted (70% of the tested patients).

CONCLUSION

This proposed strategy of determining microsatellite instability in young colorectal cancer patients or in patients with a family history of HNPCC tumors led to an increased frequency in the detection of MMR germline mutations.

Anatomy of a founder effect: myotonic dystrophy in Northeastern Quebec

Founder effects are largely responsible for changes in frequency profiles of genetic variants in local populations or isolates. They are often recognized by elevated incidence of certain hereditary disorders as observed in regions of Charlevoix and Saguenay-Lac-Saint-Jean (SLSJ) in Northeastern Quebec. Dominantly transmitted myotonic dystrophy (DM1) is highly prevalent in SLSJ where its carrier rate reaches 1/550, compared with 1/5,000 to 1/50,000 elsewhere. To shed light on the origin of DM1 in this region, we have screened 50 nuclear DM1 families from SLSJ and studied the genetic variation in a 2.05 Mb (2.9 cM) segment spanning the site of the expansion mutation. The markers analyzed included 22 biallelic SNPs and two microsatellites. Among 50 independent DM1 chromosomes, we distinguished ten DM1-associated haplotypes and grouped them into three haplotype families, A, B and C, based on the relevant extent of allele sharing between them. To test whether the data were consistent with a single entry of the mutation into SLSJ, we evaluated the age of the founder effect from the proportion of recombinant haplotypes. Taking the prevalent haplotype A1_21 (58%) as ancestral to all the disease-associated haplotypes in this study, the estimated age of the founder effect was 19 generations, long predating the colonization of Nouvelle-France. In contrast, considering A1_21 as ancestral to the haplotype family A only, yielded the estimated founder age of nine generations, consistent with the settlement of Charlevoix at the turn of 17th century and subsequent colonization of SLSJ. We conclude that it was the carrier of haplotype A (present day carrier rate of 1/730) that was a "driver" of the founder effect, while minor haplotypes B and C, with corresponding carrier rates of 1/3,000 and 1/10,000, respectively, contribute DM1 to the incidence level known in other populations. Other studies confirm that this might be a general scenario in which a major "driver" mutation/haplotype issued from a founder effect is found accompanied by distinct minor mutations/haplotypes occurring at background population frequencies.