Locus and allelic heterogeneity and phenotypic variability in Waardenburg syndrome

Waardenburg syndrome (WS) is a disorder of neural crest cell migration characterized by auditory and pigmentary abnormalities. We investigated a cohort of 14 families (16 subjects) either by targeted sequencing or whole-exome sequencing. Thirteen of these families were clinically diagnosed with WS and one family with isolated non-syndromic hearing loss (NSHL). Intra-familial phenotypic variability and non-penetrance were observed in families diagnosed with WS1, WS2 and WS4 with pathogenic variants in PAX3, MITF and EDNRB, respectively. We observed gonosomal mosaicism for a variant in PAX3 in an asymptomatic father of two affected siblings. For the first time, we report a biallelic pathogenic variant in MITF in a subject with WS2 and a biallelic variant in EDNRB was noted in a subject with WS2. An individual with isolated NSHL carried a pathogenic variant in MITF. Blended phenotype of NSHL and albinism was observed in a subject clinically diagnosed to have WS2. A phenocopy of WS1 was observed in a subject with a reported pathogenic variant in GJB2, known to cause isolated NSHL. These novel and infrequently reported observations exemplify the allelic and genetic heterogeneity and show phenotypic diversity of WS.     

The second report of Knobloch syndrome.

Two sibs affected with Knobloch syndrome comprising severe visual handicap (high myopia and retinal detachment with other defects), atypical occipital cephalocele, and unusual plantar creases are described.     

The second report of Knobloch syndrome

Two sibs affected with Knobloch syndrome comprising severe visual handicap (high myopia and retinal detachment with other defects), atypical occipital cephalocele, and unusual plantar creases are described.     

Congenital scalp defects and vitreoretinal degeneration: Redefining the Knobloch syndrome

An apparently autosomal recessive syndrome of hereditary vitreoretinal degeneration (VRD) with retinal detachment, high myopia, and congenital encephalocele was described in 1971 by Knobloch and Layer [J Pediatr Ophthalmol 8:181–184]. Clinical confirmation of the presence of encephaloceles was lacking, and no neuropathologic studies were reported. We have evaluated a similarly affected family with 2 sibs with high myopia, VRD, and occipital scalp defects. Histologic examination of the scalp defects showed heterotopic neuronal tissue in both instances. The older girl has had a unilateral retinal detachment. Her other eye and both eyes of the younger sib have so far been treated successfully with prophylactic retinal cryotherapy. Both children have normal to above normal intelligence. The family reported by Knoblock and Lyer [1971] and the sibship herein described appear to represent a distinct autosomal recessive trait. Analysis of the associated defects suggests an underlying defect in early cephalic neuroectodermal morphogeneis. Data from these families imply that congenital occipital scalp defects rather than true encephaloceles may, as is true in some cases of Meckel syndrome, accompany Knobloch syndrome. The presence of a congenital midline scalp defect should alert the clinician to possible underlying central nervous system and/or ocular pathology and should lead to consideration of further diagnostic evaluations And prophylactic measures. © 1993 Wiley-Liss, Inc.     

Knobloch syndrome involving midline scalp defect of the frontal region

We report on a 4-year-old boy with Knobloch syndrome. He has vitreoretinal degeneration, high myopia, cataract, telecanthus, hypertelorism, and a high-arched palate. He also has a defect of the anterior midline scalp with involvement of the frontal bone as documented by a computed tomography (CT) scan. The brain was normal on CT scan and magnetic resonance imaging. We present a review of the 23 published cases with this syndrome. Our patient illustrates the importance of investigating for underlying ocular and central nervous system pathology whenever midline scalp defects are present.     

Knobloch syndrome: novel mutations in COL18A1, evidence for genetic heterogeneity, and a functionally impaired polymorphism in endostatin

Knobloch syndrome (KNO) is an autosomal recessive disorder characterized by high myopia, vitreoretinal degeneration with retinal detachment, and congenital encephalocele. Pathogenic mutations in the COL18A1 gene on 21q22.3 were recently identified in KNO families. Analysis of two unrelated KNO families from Hungary and New Zealand allowed us to confirm the involvement of COL18A1 in the pathogenesis of KNO and to demonstrate the existence of genetic heterogeneity. Two COL18A1 mutations were identified in the Hungarian family: a 1-bp insertion causing a frameshift and a premature in-frame stop codon and an amino acid substitution. This missense variant is located in a conserved amino acid of endostatin, a cleavage product of the carboxy-terminal domain of collagen alpha 1 XVIII. D1437N (D104N in endostatin) likely represents a pathogenic mutation, as we show that the endostatin N104 mutant is impaired in its affinity towards laminin. Linkage to the COL18A1 locus was excluded in the New Zealand family, providing evidence for the existence of a second KNO locus. We named the second unmapped locus for Knobloch syndrome KNO2. Mutation analysis excluded COL15A1, a member of the multiplexin collagen subfamily similar to COL18A1, as being responsible for KNO2.     

Locus heterogeneity in progressive familial intrahepatic cholestasis.

Progressive familial intrahepatic cholestasis (PFIC or Byler disease) is a rare autosomal recessive form of severe and fatal cholestatic liver disease. A locus for PFIC has recently been mapped to chromosome 18q21-q22 in the original Byler pedigree. This region harbours the locus for a related phenotype, benign recurrent intrahepatic cholestasis (BRIC), suggesting that these traits are allelic. Linkage analysis was undertaken in five consanguineous PFIC pedigrees from Saudi Arabia using marker loci (D18S69, D18S41, D18S64, D18S38, D18S42, D18S55, D18S68, and D18S61) which span the Byler disease/BRIC region on 18q21-q22. In this family set the disease locus was excluded from this region, showing that locus heterogeneity exists for the PFIC phenotype.     

Report of two sibs with Knobloch syndrome (encephalocoele and viteroretinal degeneration) and other anomalies

We report on two sibs with high myopia, vitreoretinal degeneration (VRD), and occipital encephalocoele or scalp lesion. We review the literature on Knobloch syndrome, discuss possible causes, and suggest a possible involvement of mesoderm in the morphogenesis. One case presents with very early onset of severe eye disease, whereas the other is notable for the very mild scalp defect. In addition, both appear to have an unusual pulmonary lymphatic condition. Am. J. Med. Genet. 78:286–290, 1998. © 1998 Wiley-Liss, Inc.     

Locus heterogeneity in autosomal dominant congenital external ophthalmoplegia (CFEOM).

Congenital external ophthalmoplegia (CFEOM) is an uncommon autosomal dominant condition that has previously been mapped to the pericentromeric region of chromosome 12 in seven families with no evidence of locus heterogeneity. We report three families with typical CFEOM. One family does not map to this region of chromosome 12 or to other chromosomal locations implicated in disorders of lid or ocular movement. Recombinants in two CFEOM families potentially help to reduce the size of the candidate region on chromosome 12.     

Genetic heterogeneity in Usher syndrome

Mutations in seven different genes have been associated with Usher syndrome, and an additional four loci have been mapped. The identified genes encode myosin VIIa, harmonin (a PDZ-domain protein), cadherin 23, protocadherin 15, sans (a scaffold-like protein), usherin and clarin. Three clinical types of Usher syndrome have been described: USH1 patients have severe to profound congenital hearing loss, vestibular dysfunction, and retinal degeneration beginning in childhood, those with USH2 have moderate to severe congenital hearing loss, normal vestibular function, and later onset of retinitis pigmentosa, and USH3 patients have progressive hearing loss, which distinguishes them from the other two types. The shaker-1, waltzer, Ames waltzer, and Jackson shaker mice provide murine models for four of the genetic forms of Usher syndrome. Ongoing studies are enabling early diagnosis of Usher syndrome in children who present with hearing loss, thus providing time to prepare for the onset of visual loss.     

Cognitive heterogeneity in Williams syndrome

This study used the Woodcock-Johnson Tests of Cognitive Ability-Revised to investigate a wide range of cognitive abilities in people with Williams syndrome (WS). It involved a comparatively large sample of 31 people with WS, but took a case-series approach. The study addressed the widespread claims of a characteristic "WS cognitive profile" by looking for heterogeneity rather than homogeneity. People with WS showed a variety of preserved (significantly above mental age [MA]), expected (at MA), and significantly impaired (significantly below MA) levels of functioning. Such results provide clear evidence for heterogeneity in cognitive functions within WS. We found the most homogeneity on a test of phonological processing and a test of phonological short-term memory, with half of the WS sample performing at MA levels on these tests. Interestingly, no WS individual showed a weakness on a test of nonverbal reasoning, and only one WS individual showed a weakness on a test of verbal comprehension. In addition, we found that strengths on analysis-synthesis and verbal analogies occurred only for WS individuals with an MA less than 5.5 years (our sample median MA); people with an MA greater than 5.5 years performed at MA level on these 2 tests. Results also provided preliminary evidence for distinct subgroups of WS people based on their cognitive strengths and weaknesses on a broad range of cognitive functions. On the basis of the findings, caution should be made in declaring a single cognitive profile that is characteristic of all individuals with WS. Just as there is heterogeneity in genetic and physical anomalies within WS, not all WS individuals share the same cognitive strengths and weaknesses. Also, not all WS individuals show the profile of a strength in verbal abilities and a weakness in spatial functions.     

Genetic heterogeneity in Peutz-Jeghers syndrome

LKB1, the human gene encoding a serine threonine kinase, was recently identified as a susceptibility gene for Peutz-Jeghers syndrome (PJS), a disease characterized by the constellation of intestinal hamartomata, oral mucocutaneous hyperpigmentation, and an increased risk for gastrointestinal as well as extraintestinal malignancies. To date, the majority of individuals with PJS have been found to have genetic alterations in LKB1, most of which result in protein truncation. Additionally, linkage analyses have suggested a modicum of genetic heterogeneity, with the majority of PJS families showing linkage to the LKB1 locus. In this study, we evaluated five kindreds with greater than two affected family members, five PJS probands with only one other affected family member, as well as 23 individuals with sporadic PJS for mutations within the LKB1 gene. Conformation sensitive gel electrophoresis was utilized for the initial screen, followed by direct sequence analysis for characterization. Long-range PCR was used for the detection of larger genetic insertions or deletions. Mutation analysis revealed genetic alterations in LKB1 in two probands who had a family history of PJS. LKB1 mutations were detected in only four of the remaining 23 cases of sporadic PJS. These data suggest the presence of significant genetic heterogeneity for PJS and the involvement of other loci in this syndrome.     

Clinical and genetic heterogeneity of Seckel syndrome

Seckel syndrome is a rare autosomal recessive condition belonging to the group of osteodysplastic primordial "dwarfism" and characterized by the association of 1) severe pre- and postnatal growth retardation, 2) microcephaly with mental retardation, and 3) specific dysmorphic features. Recently, two disease loci have been mapped to chromosomes 3q22.1-q24 and 18p11.31-q11.2, respectively, by homozygosity mapping in consanguineous families. Here, we report on the exclusion of these loci in five consanguineous and one multiplex nonconsanguineous Seckel syndrome families and in two consanguineous families presenting type II osteodysplastic primordial dwarfism. These results support the view that Seckel syndrome is a clinically and genetically heterogeneous condition.     

Genetic and clinical heterogeneity of Stickler syndrome

We have studied 6 multigeneration Stickler syndrome families. Manifestations of the syndrome in the families included myopia, deafness, arthritis, characteristic facial changes with “flat” midface and cleft palate, although not all these were present in all families. COL2A1 has been implicated as a gene which can give rise to Stickler syndrome based on evidence from 2 large families which each showed significant linkage between the disease locus and restriction fragment length polymorphisms for the gene (Francomano CA, Lieberfarb RM, Hirose T, Maumenee IH, Streeten EA, Meyers DA, Pyeritz RE (1987): Genomics 1:293–296; Knowlton RG, Weaver EJ, Struyk AF, Knobloch WH, King RA, Norris K, Shamban A, Uitoo J, Jimenez SA, Prockop DJ (1989): Am J Hum Genet 45:681–688). We have found crossovers between the disease locus and COL2A1 in 2 families with Stickler syndrome. This could be explained by either genetic heterogeneity or the actual mutation being in a closely linked, currently unrecognized gene. We found a weakly positive overall lod score (z = 0.96 at θ = 0.10) suggesting that genetic heterogeneity is a more likely explanation. In one family, with typical findings, a translocation t5;17 (q15:q23) was found to segregate with the disease in 4 affected relatives. In view of the possible heterogeneity, although no crossovers with COL2A1 were seen in this family, either of these breakpoints could be the position of a further disease causing gene.     

Juvenile myoclonic epilepsy in chromosome 6p12-p11: Locus heterogeneity and recombinations

We recently analyzed under homogeneity a large pedigree from Belize with classic juvenile myoclonic epilepsy (JME). After a genome wide search with 146 microsatellites, we obtained significant linkage between chromosome 6p markers, D6S257 and D6S272, and both convulsive and EEG traits of JME. Recombinations in two affected members defined a 40 cM JME region flanked by D6S313 and D6S258. In the present communication, we explored if the same chromosome 6p11 microsatellites also have a role in JME mixed with pyknoleptic absences. We allowed for heterogeneity during linkage analyses. We tested for heterogeneity by the admixture test and looked for more recombinations. D6S272, D6S466, D6S294, and D6S257 were significantly linked (Z_max > 3.5) to the clinical and EEG traits of 22 families, assuming autosomal dominant inheritance with 70% penetrance. Pairwise Z_max were 4.230 for D6S294 (θ_{m = f} at 0.133) and 4.442 for D6S466 (θ_{m = f} at 0.111). Admixture test (H₂ vs. H₁) was significant (P = 0.0234 for D6S294 and 0.0128 for D6S272) supporting the hypotheses of linkage with heterogeneity. Estimated proportion of linked families, α, was 0.50 (95% confidence interval 0.05–0.99) for D6S294 and D6S272. Multipoint analyses and recombinations in three new families narrowed the JME locus to a 7 cM interval flanked by D6S272 and D6S257. © 1996 Wiley-Liss, Inc.     

Genetic heterogeneity of Meckel syndrome.

Meckel syndrome (MKS) is a lethal, autosomal recessive condition characterised by an occipital meningoencephalocele, enlarged kidneys with multicystic dysplasia, fibrotic changes of the liver in the portal area with ductal proliferation, and postaxial polydactyly. Recently, a MKS gene has been mapped to chromosome 17q21-q24 in Finnish families, with no evidence of locus heterogeneity in this population. Here, we report the exclusion of chromosome 17q21-q24 in eight typical MKS families of North African and Middle Eastern ancestry and provide evidence for genetic heterogeneity of this condition.