Characteristics of genetic diseases in consanguineous populations in the genomic era: Lessons from Arab communities in North Israel.

The health outcome of consanguineous/endogamous unions is an increased risk of autosomal recessive disorders in their progeny. This manuscript is focused on consanguineous/endogamous populations living in North Israel. Molecular tools show that spouses' relatedness and hence their risks for congenital diseases among offspring are often greater than the risk calculated on the basis of reported pedigrees. Revealing founder mutations allow for effective genetic counseling, but also induce genetic screening of the whole community in case the mutations are found to be frequent. More complex genetic mechanisms, such as co-inheritance of more than one condition, allelic and even locus heterogeneity, have been identified. These mechanisms make genetic counseling more challenging but with the advancement of molecular techniques, diseases can be better deciphered. Yet, the presence of multiple mutations responsible for genetic diseases in isolated populations, and occasionally locus heterogeneity of diseases, is an unexpected phenomenon that still needs mechanistic clarification. It seems probably that addressing genetic counseling challenges and estimations of risks for genetic morbidity in consanguineous/endogamous couples will be achieved by introducing high-throughput genetic technologies into daily practice. The genomic era has expanded dramatically the translation of research products to genetic counseling tools, and this tendency is expected to yield a stronger impact in a near future.

Expanding the genotypic spectrum of Perrault syndrome.

Perrault syndrome is a rare autosomal recessive disorder characterized by sensorineural hearing loss (SNHL) in both sexes and primary ovarian insufficiency in 46, XX karyotype females. Biallelic variants in five genes are reported to be causative: HSD17B4, HARS2, LARS2, CLPP and C10orf2. Here we present eight families affected by Perrault syndrome. In five families we identified novel or previously reported variants in HSD17B4, LARS2, CLPP and C10orf2. The proband from each family was whole exome sequenced and variants confirmed by Sanger sequencing. A female was compound heterozygous for a known, p.(Gly16Ser) and novel, p.(Val82Phe) variant in D-bifunctional protein (HSD17B4). A family was homozygous for mitochondrial leucyl aminocyl tRNA synthetase (mtLeuRS) (LARS2) p.(Thr522Asn), previously associated with Perrault syndrome. A further family was compound heterozygous for mtLeuRS, p.(Thr522Asn) and a novel variant, p.(Met117Ile). Affected individuals with LARS2 variants had low frequency SNHL, a feature previously described in Perrault syndrome. A female with significant neurological disability was compound heterozygous for p.(Arg323Gln) and p.(Asn399Ser) variants in Twinkle (C10orf2). A male was homozygous for a novel variant in CLPP, p.(Cys144Arg). In three families there were no putative pathogenic variants in these genes confirming additional disease-causing genes remain unidentified. We have expanded the spectrum of disease-causing variants associated with Perrault syndrome.

DMRTA2 (DMRT5) is mutated in a novel cortical brain malformation.

Lissencephaly is a phenotypically and genetically heterogeneous group of cortical brain malformations due to abnormal neuronal migration. The identification of many causative genes has increased the understanding of normal brain development. A consanguineous family was ascertained with three siblings affected by a severe prenatal neurodevelopmental disorder characterised by fronto-parietal pachygyria, agenesis of the corpus callosum and progressive severe microcephaly. Autozygosity mapping and exome sequencing identified a homozygous novel single base pair deletion, c.1197delT in DMRTA2, predicted to result in a frameshift variant p.(Pro400Leufs*33). DMRTA2 encodes doublesex and mab-3-related transcription factor a2, a transcription factor key to the development of the dorsal telencephalon. Data from murine and zebrafish knockout models are consistent with the variant of DMTRA2 (DMRT5) as responsible for the cortical brain phenotype. Our study suggests that loss of function of DMRTA2 leads to a novel disorder of cortical development.

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.

Severe infantile type of carnitine palmitoyltransferase II (CPT II) deficiency due to homozygous R503C mutation.

We report a patient with severe infantile carnitine palmitoyltransferase II (CPT II) deficiency who died at the age of 3 months. Genetic analysis of the CPT2 gene revealed that the patient was homozygous, and her parents were heterozygous, for a R503C missense mutation. Heterozygosity for R503C, without a second mutation, has previously been reported in symptomatic patients from two families, one with the mild adult myopathic form and one with malignant hyperthermia. In contrast, the R503C heterozygous parents of the patient were entirely asymptomatic, suggesting that additional genetic and/or environmental factors must have contributed to the occurrence of symptoms in previously reported carriers. Our findings indicate that the mutation R503C should be added to the handful of mutations associated with the severe phenotype when present in the homozygous state or combined with another severe mutation.

Mutations in the steroidogenic acute regulatory protein (StAR) in six patients with congenital lipoid adrenal hyperplasia.

Congenital lipoid adrenal hyperplasia (lipoid CAH), the most severe form of CAH, is caused by mutations in the steroidogenic acute regulatory protein (StAR). Lipoid CAH is common among the Japanese, Korean, and Palestinian Arab populations, but is rare elsewhere. We describe six patients with lipoid CAH: four Japanese, one Palestinian, and one Guatemalan Native American. All had classical clinical presentations of normal female external genitalia in both genetic sexes, with severe glucocorticoid and mineralocorticoid deficiency presenting in the first month of life. Quite atypically, one patient had small adrenal glands shown by computed tomographic scanning. The StAR genes were characterized in all six patients. Three of the Japanese patients were compound heterozygotes for the common Japanese mutation Q258X in association with three different novel frameshift mutations; the fourth Japanese patient was homozygous for the mutation R182L, which is common among Palestinian patients but has not been described previously in a Japanese patient. Our Palestinian and Native American patients were each homozygous for novel frameshift mutations. Thus we have found five new frameshift mutations, but no new amino acid replacement (missense) mutations. This would be consistent with the view that only a small number of residues in the StAR protein are crucial for biological activity. The tomographic finding of small adrenals in a patient with genetically proven lipoid CAH due to a StAR mutation suggests a substantially broader spectrum of clinical findings in this disease than has been appreciated previously.

Osteosclerosis, hypoplastic nose, and proptosis (Raine syndrome): further delineation.

We describe a newborn girl with a lethal sclerosing bone dysplasia leading to prenatal skeletal alterations and microcephaly, proptosis, hypoplastic nose and midface, small jaw, cleft palate, hypertrophied gums, intracranial calcifications, and generalized osteosclerosis. There is a remarkable similarity between our patient and six previously reported infants subsequently categorized as having a distinct entity: Raine syndrome. Autosomal recessive inheritance is postulated based on parental consanguinity in several of the previous cases and in our patient.

The fate of 12 recessive mutations in a single village.

In a Muslim Arab village, relatively isolated because of the preference of consanguineous marriages, we studied the fate of 12 mutations in 5 different genes. The study was based on carriers detected among relatives of affected patients and of carriers discovered in a random sample of 424 adults. Most of the mutations have been introduced by a carrier(s) originating from another village, but a few have been de novo events. Mutations that are very frequent in the entire village were introduced soon after the foundation of the village. Examples of such mutations are [GBJ2, 35Gdel] and [MEFV, M680I], with a carrier frequency of 7.8% and 6.2%, respectively. Many of the other mutations that are rare were introduced recently into the village and are frequent only among the descendants of the first couple carrying the mutation. For instance all the carriers of [ARSA, Q190H], responsible for metachromatic leukodystrophy, were found among the 218 descendants of a couple who were living in the village 4 generations ago. Since the village is typical for the region this study allows for some general conclusions to be drawn. In a population with a high degree of inbreeding the diagnosis of a single family with a patient(s) affected with a recessive disorder points to a recent event, while the finding of a rare disease in several families from an inbred population points to an older mutation. Mutations are often "exported" from one population to another by marriage. In the new inbred population this novel mutation will either be lost or will become frequent as the result of a founder effect. These observations are important for genetic counselling in the case of a recent mutation, since only the descendants of the founder couple are at risk, while in the case of older mutations the risk may be for the entire village. In the case of those frequent ancient mutations, the risk for a relative of an affected individual will be similar whether he marries a close relative or any random individual in the village.

Evidence for blood chimerism in dizygotic spontaneous twin pregnancy discordant for Down syndrome.

BACKGROUND: A monochorionic-diamniotic placenta (MCDAP) is rare in dizygotic (DZ) twinning. All reported cases have been documented in pregnancies achieved by the induction of ovulation alone or during the IVF cycle. METHODS AND RESULTS: We report a spontaneous pregnancy in a 39-year-old patient with evidence of MCDAP in DZ twins, discordant for trisomy 21. The first and second-trimester sonographic scans indicated male twins with MCDAP. Amniocentesis, performed because of advanced maternal age, revealed a normal karyotype in one fetus, and trisomy 21 in the other. Molecular studies, performed in order to confirm the zygosity and chorionicity, demonstrated that the fetuses were DZ. In order to identity the affected twin, a detailed sonographic examination was repeated, but no abnormal findings associated with Down syndrome were demonstrated in any of the fetuses. Therefore, umbilical cord blood samples were obtained from both fetuses. Chromosomal analysis revealed in both fetuses two cell lines: a normal cell line of 46,XY and a 47,XY,+ 21 cell line, in 65 and 80% of the cells, respectively. This result was independently confirmed by both FISH and G-banding. DNA extracted from both cord blood samples demonstrated an admixture of two distinct genotypes in each sample. CONCLUSIONS: We propose that this case represents a monochorionic-dizygotic twin pregnancy with blood chimerism. The most plausible mechanism underlying this phenomenon is placental fusion early in pregnancy, resulting in an architecturally single placenta originating from two distinct zygotes. The newly formed blood vessels created anastomoses between the DZ twins and allowed reciprocal blood chimerism between the normal and the trisomic twin.