Mutations of PVRL1, encoding a cell-cell adhesion molecule/herpesvirus receptor, in cleft lip/palate-ectodermal dysplasia.

Cleft lip, with or without cleft palate (CL/P), is one of the most common birth defects, occurring in 0.4 to 2.0 per 1,000 infants born alive. Approximately 70% of CL/P cases are non-syndromic (MIM 119530), but CL/P also occurs in many single-gene syndromes, each affecting a protein critical for orofacial development. Here we describe positional cloning of the gene responsible for an autosomal recessive CL/P-ectodermal dysplasia (ED) syndrome (CLPED1; previously ED4; ref. 2), which we identify as PVRL1, encoding nectin-1, an immunoglobulin (Ig)-related transmembrane cell-cell adhesion molecule that is part of the NAP cell adhesion system. Nectin-1 is also the principal cell surface receptor for alpha-herpesviruses (HveC; ref. 7), and the high frequency of CLPED1 on Margarita Island in the Caribbean Sea might result from resistance of heterozygotes to infection by these viruses.

Identification of a prevalent founder mutation in an Israeli Muslim Arab village confirms the role of PRCD in the aetiology of retinitis pigmentosa in humans.

BACKGROUND: Retinitis pigmentosa (RP) is the most common form of hereditary retinal degeneration. At least 32 genes and loci have been implicated in non-syndromic autosomal recessive RP. Progressive rod-cone degeneration is a canine form of autosomal recessive retinal degeneration, which serves as an animal model for human RP, and is caused by a missense mutation of the PRCD gene. The same homozygous PRCD mutation has been previously identified in a single human RP patient from Bangladesh. To date, this is the only RP-causing mutation of PRCD reported in humans. METHODS: The cause of the high incidence rate of autosomal recessive RP in an isolated Muslim Arab village in Northern Israel was investigated by haplotype analysis in affected families. The underlying mutation was detected by direct sequencing of the causative gene, and its prevalence in affected and unaffected individuals from the village was determined. Patients who were homozygotes for this mutation underwent ophthalmic evaluation, including funduscopy and electroretinography. RESULTS AND CONCLUSIONS: The identification of a novel pathogenic nonsense mutation of PRCD is reported. This founder mutation was found in a homozygous state in 18 patients from nine families, and its carrier frequency in the investigated village is 10%. The mutation is associated with a typical RP phenotype, including bone spicule-type pigment deposits and non-recordable electroretinograms. Additional findings include signs of macular degeneration and cataract. The identification of a second pathogenic mutation of PRCD in multiple RP patients confirms the role of PRCD in the aetiology of RP in humans.

Medical genetics in Israel.

The state of Israel was founded in 1948 and includes approximately 4.5 million Jews and 1 million of non-Jews, mainly Muslim Arabs. Subgroups can be distinguished within each of these two groups: among the Jews according to their country of origin and among the non-Jews according to their religion or even their village of origin. The precise origin of each patient is particularly important for the medical geneticists since in each subgroup some hereditary disorders have been reported with an increased frequency. This knowledge also allows in some cases for preventive genetic screening. The reasons for the relatively high frequency of mendelian disorders in the different communities in Israel are numerous including mainly a founder effect with genetic drift and selection. In Israel, medical genetics is a recognized medical speciality and there are eleven clinical genetic centers in the country. These centers are in close contact with the individuals active in the different fields of human genetics in Israel both for service and research.

Machado-Joseph disease: correlation between the clinical features, the CAG repeat length and homozygosity for the mutation.

Machado-Joseph disease (MJD) is an autosomal dominant neurodegenerative disorder associated with the expansion of a CAG trinucleotide repeat in the MJD1 gene located on 14q32.1. We confirmed that the CAG expansion caused MJD in a Yemenite Jewish family and demonstrated that most of the clinical variation among members of this family was due to the genotype of the affected individuals. Six patients who presented with an early onset (25 years) and severe disorder were found to be homozygous for the CAG expansion. Among 5 heterozygotes for the CAG expansion older than 40 years, one had neurological symptoms from the age of 45, while the others were asymptomatic. In one of the heterozygotes, no neurological symptoms were present when last examined at the age of 66. Homozygosity for the MJD1 mutation was the main cause of variability in this large family, however, other factors clearly played a role in the expression of the gene. We could demonstrate that homozygote sibs with similar expansion in both alleles had significant differences in disease severity. Gender did not affect the clinical expression in this family.

Transient neonatal diabetes mellitus in a child with invdup(6)(q22q23) of paternal origin.

An association between the rare condition of transient neonatal diabetes mellitus and either uniparental disomy for chromosome 6 or dup(6)(q22q23) raised the assumption that in this location on chromosome 6 there is an imprinted gene. We diagnosed diabetes that developed in a baby girl immediately after birth and resolved after 7 weeks of insulin treatment. Due to some minor dysmorphic features, we investigated her karyotype and identified invdup(6)(q22q23). The duplication spans at least 10 cM including the DNA sites DS270,S314,S1684 and S310. This case further supports the assumption that an imprinted gene exists on chromosome 6q22-23.

Mucolipidosis type IV: clinical spectrum and natural history.

The clinical spectrum and developmental features of mucolipidosis type IV, a recessive lysosomal storage disorder, are presented. The evaluation was based on information from the clinical charts and information obtained from the families of 20 patients between the ages of 2 to 17 years. The clinical manifestations of the disease, profound psychomotor retardation and visual impairment, appear during the first year of life. Definitive diagnosis is made by electron microscopy which reveals storage organelles typical of the mucolipidoses. This study details, for the first time, the heterogeneity of the ophthalmologic features, specifically as pertains to the age of onset, degree and clinical course of the corneal opacities, and the retinal involvement. Although the top developmental level was found to be 12 to 15 months in language and motor function, the course of the disease is protracted for some children, who show only a slight improvement, and others, little if any deterioration despite the early infantile onset of the disease. This presentation provides guidelines for the clinical diagnosis of mucolipidosis type IV.

Mental retardation, spasticity, and transverse limb defects.

We report on a mentally retarded boy with spastic paraplegia and a transverse defect of the left foot who was born to consanguineous parents. This association of ectrodactyly, mental retardation, and spastic paraplegia was first reported by Jancar [1967].

Genetic disorders among Palestinian Arabs. 2. Hydrocephalus and neural tube defects.

Congenital hydrocephalus and/or open neural tube defect was present in at least one individual of 98 families out of the 2,000 Palestinian Arabic families who have visited the Genetic clinic at the Hadassah Medical Center. In 22 families the brain malformation was part of a syndrome: Meckel syndrome in 10, Warburg syndrome in another 5, Carpenter in one, and undiagnosed in 6 families. In 76 of the families the neural tube defect and/or the hydrocephalus were non-syndromal. It seems that most of the cases of isolated non-syndromal hydrocephalus represented autosomal recessive traits and that an abnormal allele is common among Palestinian Muslim Arabs.

Hereditary disorders among Iranian Jews.

Iranian Jews represent an ancient community with a very high degree of inbreeding. Although the community remained relatively isolated, it had strong ties with Babylonian Jewry in Iraq. Several genetic disorders have been reported to be frequent among Iranian Jews, in particular, corticosterone methyloxydase deficiency type II, polyglandular syndrome, and rimmed vacuole myopathy. Based on the data collected in our clinic, recessive and dominant deafness also appear to be frequent. Other diseases, such as beta-thalassemia, achromatopsia, colobomatous microphthalmia, Dubin-Johnson syndrome, and congenital myasthenia gravis, were frequent in both the Iranian and Iraqi Jewish communities. The place of origin of the families within Iran and the results of molecular studies suggest some reason(s) for the high frequency of these disorders among Iranian Jews. While the high frequency of some of the disorders, such as corticosterone methyloxydase deficiency type II, represents a founder effect, in other diseases (such as beta-thalassemia) it was secondary to heterozygote advantage.

Major gene is responsible for anencephaly among Iranian Jews.

Anencephaly is relatively frequent in Jews originating from Iran, in particular when its incidence is compared to that of open spina bifida in the same population (12 cases of anencephaly out of 14 cases of neural tube defects). The high incidence of this disorder in Iranian Jews, a relatively isolated community with a very high rate of consanguinity, suggests that anencephaly is caused by a major recessive gene. This possibility is supported by the sex ratio among these patients, which was significantly different from that observed for patients with anencephaly in other populations.

Dominance and homozygosity.

Because of the high consanguinity rates in many communities in Israel we had the opportunity to study homozygosity for some dominant disorders. This experience and a review confirmed that in most cases homozygotes of dominant disorders are more severely affected than heterozygotes. In some cases molecular analysis allowed an understanding of the mechanisms involved. While heterozygosity for point mutations or deletions of PAX3 lead to similar manifestations (Waardenburg syndrome), in homozygotes the phenotype is much more severe, probably in direct relation to the loss of function. Charcot-Marie-Tooth 1A is caused by a duplication of PMP22 and further over-expression lead to a more severe disorder. In diseases in which the mutation leads to an abnormal structural protein, the homozygote may be as severely affected as the heterozygote (epidermolysis bullosa simplex) or more severely (achondroplasia, Marfan syndrome). The polyglutamine tract is translated in disorders caused by CAG triplet expansions. In homozygotes for Machado-Joseph disease the onset is earlier and the symptoms are more severe than in heterozygotes, while in Huntington disease homozygotes are affected like heterozygotes.

Genetic disorders among Palestinian Arabs: 1. Effects of consanguinity.

Among Palestinian Arabs the rate of consanguinity is very high and some 44.3% of the marriages are between relatives (22.6% of them between first cousins). In almost 2,000 files from Palestinian Arab families who attended the genetics clinic in the Hadassah Medical Center; we were able to study the effects of consanguinity on different disorders. The consanguinity rate in families with dominant or X-linked disorders and chromosome aberrations was similar to the one observed in the general population. We did not find any significant differences in the rate of consanguineous marriages between the parents and grandparents of children affected with trisomy 21 and the general population. Thus, we were not able to confirm the suggestion that there is an increase risk for trisomies in children/grandchildren of consanguineous parents. Among the parents of patients with rare autosomal recessive disorders the consanguinity rate was much higher than the one of the general population (92.5%). Among the autosomal recessive disorders, which were relatively frequent in the population, there were fewer marriages between relatives; but in most cases the difference from rare disorders is relatively small. The importance of genetic factors in various congenital malformations, such as neural tube defects and cleft lip/palate or in various forms of infertility, was confirmed by the observation of a significantly higher consanguinity rate in the parents of these patients than is observed in the general population.

High frequencies of human genetic diseases: founder effect with genetic drift or selection?

Rare genetic diseases have been reported with high frequency in some populations. The mechanisms which were proposed to explain most of these observations include founder effect, genetic drift or selective advantage. In recent years, many genes have been sequenced and mutations causing some of these disorders were characterized. According to the analysis of haplotypes and/or mutations, it may be possible to distinguish 3 groups of disorders frequent in isolated populations. In the first group, all the affected patients have only one frequent mutation, suggesting a founder effect with genetic drift. In the second group, more than one mutation is found among the patients; however, most of the patients are homozygotes for one frequent mutation which most probably originated from a common founder; the other patients are compound heterozygotes for the common mutation and a rare mutation. In the third group, more than one frequent mutation is found responsible for each disease. This may be due to a selective advantage which allows the expansion of each new mutation in the particular population or to multiple founder effect with genetic drift in smaller communities which thereafter mixed to form the larger population.

Problems in diagnosis and delineation of inherited disorders in highly inbred populations.

Two families were chosen as examples of the problems that may arise in the identification of inherited disorders in populations in which the rate of consanguineous marriages is high. In the first family, mentally retarded children of both sexes were born to 4 sisters married to close relative, and the possibility of an autosomal recessive disease was raised. The diagnosis of an X-linked disease, Martin Bell Syndrome, was made after the results of the chromosome analysis were at hand. In inbred communities, individuals affected with X-linked diseases are often born to parents who are related. In the second family two different autosomal recessive disorders were diagnosed among the children of a couple originating from a very inbred community. One of the children was affected with both disorders. The finding of two sibs with different symptoms may suggest that they have the same syndrome, and the differences in manifestations represent variability. The possibility that there may be more than one common abnormal gene in very inbred communities must be kept in mind, in particular when one is dealing with syndromes in which the diagnosis is based on clinical symptoms only. Another problem is that when the child has a complex unknown syndrome, the possibility that the child is affected simultaneously with 2 different genetic disorders should be raised.

Intrafamilial variability in lysosomal storage diseases.

In most lysosomal storage diseases clinical variability is found and affects age-of-onset, severity, and the degree of neurological involvement. Very often the variability is due to the existence of different mutations leading to the same enzyme deficiency. In most of the families with more than one person affected, the clinical picture is very similar. Intrafamilial variation has been reported in the lysosomal storage diseases related or not related to genetic heterogeneity. In families in which different affected persons have the same genotype, as in X-linked disorders, or autosomal recessive diseases in which both parents of the affected sibs are carriers and healthy, the variability must be due to factors not related to the genotype. On the other hand, when different mutations are present in the same family, the variability may be related to the differences in the genotype of the affected persons. Knowledge of the possibility of intrafamilial variability and its genetic basis is essential in clinical and prenatal diagnosis of the lysosomal storage diseases.

Selection for carriers of recessive diseases: a common phenomenon?

Several autosomal recessive disorders are present in high frequency in isolated populations because of either multiple allelic mutations or mutations in different genes. These observations are best explained by selection, which may be an important mechanism in the determination of the distribution of genetic disorders.

On the inheritance of the split hand/split foot malformation.

Analysis of families with non-syndromal split hand/split foot (SHSF) confirms the existence of 2 distinct entities, most probably caused by at least 2 different autosomal dominant genes. In the families in which the SHSF malformation is non-syndromal and limited to the hands and feet (type I), the pattern of inheritance is of a regular autosomal dominant gene with a high penetrance (96%). In families in which at least one individual has other limb malformations and SHSF (type II), the transmission is often unusual. In most families, the gene is non-penetrant, sometimes for generations, before the birth of the first affected individual. Thereafter, among the descendants of affected individuals, the penetrance is reduced (66%), suggesting the possible existence of another gene which controls the appearance of the clinical manifestations. The possibility that SHSF associated with other limb malformations is a disorder caused by trinucleotide repeat instability is raised.

Mutations in von Recklinghausen neurofibromatosis: an hypothesis.

Von Recklinghausen neurofibromatosis or neurofibromatosis type I (NF1) is a relatively frequent (1/3,000 livebirths) autosomal dominant condition. Some unusual aspects are noted in this disorder: new mutations are frequent and almost all are of paternal origin without parental age effect. The recurrence of NF1 among children of healthy parents is rare as opposed to other dominant disorders. I propose that in NF1 (1) new mutations occur often in somatic cells or in late germinal cells, however, they occur very rarely in early germinal cells leading to germinal mosaicism and (2) the individual with somatic mosaicism presents symptoms of the disease. Therefore, an NF1 patient with an apparent new mutation is often a somatic mosaic for the mutation and if the mosaic is also present in germinal cells some of his children will be affected. This hypothesis may explain the unusual aspects of mutation in NF1.

Are the wolffian anomalies in males the phenotype corresponding to the müllerian anomalies in females?

The Wolffian and Müllerian ducts are embryologically closely related. This survey was aimed to determine whether, as has been suggested, Wolffian duct anomalies in males are the phenotype corresponding to Müllerian anomalies in females. This was done by analysis of the phenotype of the males in associations and syndromes in which Müllerian duct anomalies are frequent, and of the phenotype of the females in families in which anomalies of the Wolffian ducts are found. It appears that the only relation between the 2 types of defects seems to be a developmental one. In cases of early insult, anomalies are seen in males and females (e.g., in the association of renal agenesis with Müllerian or Wolffian defects). On the other hand, in all the other associations or syndromes, no relation was observed. This suggests that Wolffian anomalies in males are not the phenotype corresponding to Müllerian anomalies in females.

Heterozygote detection in Hunter syndrome.

Iduronate sulfate sulfatase activity was determined in 36 women, relatives of Hunter syndrome patients. The use of serum and lymphocyte extracts for the determination of enzyme levels enabled the detection of 13 out of 15 (86%) obligate heterozygotes and identification of 10 of 21 other relatives as carriers. These methods are relatively simple and can easily be applied for routine examinations of all women at risk of being a Hunter heterozygote. These results permit for the first time meaningful genetic counseling for the families of Hunter patients.