A spectrum of LMX1B mutations in Nail-Patella syndrome: New point mutations,deletion, and evidence of mosaicism in unaffected parents

PurposeNail-Patella syndrome (MIM 161200) is a rare autosomal dominant disorder characterized by hypoplastic or absent patellae, dystrophic nails, dysplasia of the elbows, and iliac horn. In 40% of cases, a glomerular defect is present and, less frequently, ocular damage is observed. Inter- and intrafamilial variable expressivity of the clinical phenotype is a common finding. Mutations in the human LMX1B gene have been demonstrated to be responsible for Nail-Patella syndrome in around 80% of cases.MethodsStandard polymerase chain reaction and sequencing methods were used for mutation and single nucleotide polymorphism identification and control of cloned sequences. Array-CGH (Agilent, 244A Kit) was used for detection of deletions. Standard cloning techniques and the Snapshot method were used for analysis of mosaicism.ResultsIn this study, we present the results of LMX1B screening of 20 Nail-Patella syndrome patients. The molecular defect was found in 17 patients. We report five novel mutations and a ∼2 Mb deletion in chromosome 9q encompassing the entire LMX1B gene in a patient with a complex phenotype. We present evidence of somatic mosaicism in unaffected parents in two cases, which, to our knowledge, are the first reported cases of inheritance of a mutated LMX1B allele in Nail-Patella syndrome patients from a mosaic parent.ConclusionThe study of the described case series provides some original observations in an “old” genetic disorder.     

In vivo expression of putative LMX1B targets in nail-patella syndrome kidneys

The nail-patella syndrome (NPS) is characterized by nail and bone abnormalities, associated with glomerular involvement in approximately 40% of patients. Typical glomerular changes consist of fibrillar material in the irregularly thickened glomerular basement membrane. NPS is inherited as an autosomal dominant trait and caused by heterozygous loss of function mutations in LMX1B, a member of the LIM homeodomain protein family. Mice with homozygous inactivation of the gene exhibit nail and skeletal defects, similar to those observed in patients, associated with glomerular abnormalities. Strong reduction in the glomerular expression of the alpha3 and alpha4 chains of type IV collagen, and of podocin and CD2AP, two podocyte proteins critical for glomerular function, has been observed in Lmx1b null mice. The expression of these proteins appeared to be regulated by Lmx1b. To determine whether these changes in podocyte gene expression are involved in the development of NPS nephropathy, using immunohistological techniques, we analyzed the podocyte phenotype and the renal distribution of type IV collagen chains in the kidneys of seven NPS patients with severe glomerular disease. We also examined the nature of the fibrillar material present within the glomerular extracellular matrix. The glomerular basement membrane fibrillar material was specifically labeled with anti-type III collagen antibodies, suggesting a possible regulation of type III collagen expression by LMX1B. The expression of the alpha3 and alpha4 chains of type IV collagen, and of podocin and CD2AP, was found to be normal in the seven patients. These findings indicate that heterozygous mutations of LMX1B do not appear to dramatically affect the expression of type IV collagen chains, podocin, or CD2AP in NPS patients.     

Genitopatellar syndrome: expanding the phenotype and excluding mutations in LMX1B and TBX4

Genitopatellar syndrome is a newly described disorder characterized by absent/hypoplastic patellae, lower extremity contractures, urogenital anomalies, dysmorphic features, skeletal anomalies, and agenesis of the corpus callosum. More recently, cardiac anomalies and ectodermal dysplasia have been suggested as additional features of this syndrome. We report on two additional patients with genitopatellar syndrome and expand the spectrum of anomalies to include radio-ulnar synostosis. Since there exists significant overlap in the skeletal phenotype between genitopatellar syndrome and both the nail-patella and short patella syndromes, mutation screening of their causative genes, LMX1B and TBX4, was performed. Although there still does not appear to be an identifiable molecular etiology in genitopatellar syndrome, mutations in these two candidate genes have been excluded in our patients. Since both LMX1B and TBX4 are involved in a common molecular pathway, it is likely that the causative gene of genitopatellar syndrome functions within the same developmental process.     

Expression of the PAX2 gene in human embryos and exclusion in the CHARGE syndrome

The CHARGE syndrome comprises ocular coloboma, heart malformation, choanal atresia, retarded growth and development, central nervous system malformations, genital hypoplasia, ear abnormalities, or deafness. The cause of the CHARGE syndrome remains unknown. In the present study, we analyzed the distribution pattern of the PAX2 gene in human embryos and found that PAX2 gene expression occurs in the primordia affected in the CHARGE syndrome. These data prompted us to consider the PAX2 gene a candidate gene in the CHARGE "association." We analyzed the PAX2 gene in 34 patients fulfilling the diagnostic criteria of the CHARGE syndrome for deletion and nucleotidic variations of the coding sequence and identified only polymorphisms. Our data suggest that mutation of the PAX2 gene is not a cause of the CHARGE association. However, the pattern of expression of PAX2 suggests that genes encoding downstream targets effectors could be candidate genes for the CHARGE syndrome.     

Clinical and molecular basis of classical lissencephaly: Mutations in the LIS1 gene (PAFAH1B1)

Classical lissencephaly (LIS) and subcortical band heterotopia (SBH) are related cortical malformations secondary to abnormal migration of neurons during early brain development. Approximately 60% of patients with classical LIS, and one patient with atypical SBH have been found to have deletions or mutations of the LIS1 gene, located on 17p13.3. This gene encodes the LIS1 or PAFAH1B1 protein with a coiled‐coil domain at the N‐terminus and seven WD40 repeats at the C‐terminus. It is highly conserved between species and has been shown to interact with multiple proteins involved with cytoskeletal dynamics, playing a role in both cellular division and motility, as well as the regulation of brain levels of platelet activating factor. Here we report 65 large deletions of the LIS1 gene detected by FISH and 41 intragenic mutations, including four not previously reported, the majority of which have been found as a consequence of the investigation of 220 children with LIS or SBH by our group. All intragenic mutations are de novo, and there have been no familial recurrences. Eight‐eight percent (36/41) of the mutations result in a truncated or internally deleted protein—with missense mutations found in only 12% (5/41) thus far. Mutations occurred throughout the gene except for exon 7, with clustering of three of the five missense mutations in exon 6. Only five intragenic mutations were recurrent. In general, the most severe LIS phenotype was seen in patients with large deletions of 17p13.3, with milder phenotypes seen with intragenic mutations. Of these, the mildest phenotypes were seen in patients with missense mutations. Hum Mutat 19:4–15, 2002. © 2001 Wiley‐Liss, Inc.     

Autosomal recessive otofaciocervical syndrome type 2 with novel homozygous small insertion in PAX1 gene

Otofaciocervical syndrome (OTFCS) is described as a single gene disorder of both autosomal dominant and autosomal recessive inheritance. The major clinical features of OTFCS include ear malformations (external/middle/inner ear), facial dysmorphism, shoulder girdle abnormalities, vertebral anomalies, and mild intellectual disability. The autosomal recessive form of OTFCS syndrome (OTFCS2) has been recently reported to be caused due to homozygous mutations in PAX1 gene. Here we report a third family of OTFCS2 phenotype wherein whole exome sequencing identified a novel homozygous small insertion in PAX1 as the underlying genetic cause.     

Interaction between hamartin and tuberin, the TSC1 and TSC2 gene products

Tuberous sclerosis (TSC) is an autosomal dominant disorder caused by a mutation in either the TSC1 or TSC2 tumour suppressor gene. The disease is characterized by a broad phenotypic spectrum that can include seizures, mental retardation, renal dysfunction and dermatological abnormalities. TSC2 encodes tuberin, a putative GTPase activating protein for rap1 and rab5. The TSC1 gene was recently identified and codes for hamartin, a novel protein with no significant homology to tuberin or any other known vertebrate protein. Here, we show that hamartin and tuberin associate physically in vivo and that the interaction is mediated by predicted coiled-coil domains. Our data suggest that hamartin and tuberin function in the same complex rather than in separate pathways.      

Confirmation of CLIM2/LMX1B interaction by yeast two-hybrid screening and analysis of its involvement in nail-patella syndrome

Nail-patella syndrome (NPS), an autosomal dominant disorder characterized by nail dysplasia, absent or hypoplastic patellae, iliac horns, and often associated with nephropathy and, less frequently, with open angle glaucoma, is caused by mutations in the LMX1B gene. Inter-familial and intra-familial phenotypic variability raises the question whether modifier genes can be identified to explain differences in the expression and severity of clinical features of NPS. Genes encoding proteins that interact with the LMX1B protein are good candidates and, therefore, methods to search for interactions can be used to this purpose. By the yeast two-hybrid screening we detected the CLIM2 gene as a LMX1B interactor, confirming previous reports which described the same interaction by biochemical methods. Sequencing of the CLIM2 coding region in seven NPS cases in which no LMX1B mutation had been found, did not detect any molecular variant in these patients. Moreover, by genotyping a polymorphic dinucleotide repeat close to the CLIM2 gene in affected members of a large Dutch NPS family with high incidence of nephropathy, we were unable to find a correlation between the presence of a specific allele and the expression of nephropathy. In conclusion, although the results of this study could not provide any proof of CLIM2 involvement in the pathogenesis of NPS or in determination of the clinical phenotype, we suggest that the CLIM2 gene can be considered as a good candidate for further studies on normal and disturbed kidney development associated with NPS or other hereditary glomerulopathies.     

Typical renal-coloboma syndrome phenotype in a patient with a submicroscopic deletion of the PAX2 gene

We present a patient with optic nerve hypoplasia, secondary strabismus, mild deafness, abnormal external ear helices, and renal hypoplasia. The clinical phenotype was consistent with renal-coloboma syndrome, but no point mutation in the PAX2 gene could be identified. High-resolution array comparative genomic hybridization (aCGH) analysis showed that this patient has a submicroscopic deletion on chromosome 10, affecting the entire coding region of the PAX2 gene. This finding provided the molecular confirmation of the patient's clinical diagnosis and showed that, in addition to point mutations, deletions of the PAX2 gene contribute to the etiology of the renal-coloboma syndrome.     

One gene,two phenotypes: ROR2 mutations in autosomal recessive Robinow syndrome and autosomal dominant brachydactyly type B

Autosomal recessive Robinow syndrome (RRS) is a severe skeletal dysplasia with short stature, generalized limb shortening, segmental defects of the spine, brachydactyly, and a dysmorphic facial appearance. The gene encoding receptor orphan receptor tyrosine kinase 2 (ROR2) is located on chromosome 9q22 and homozygous loss-of-function mutations in this gene are responsible for RRS. Moreover, knocking out the mouse Ror2 gene causes mesomelic dwarfism in the homozygous state, with almost identical features to recessive Robinow syndrome. The protein product of this gene is a cell membrane receptor, containing distinct motifs including an immunoglobulin-like (Ig) domain, a Frizzled-like cysteine-rich domain (FRZ or CRD), and a kringle domain (KD) in the extracellular region; and an intracellular region with tyrosine kinase (TK), serine/threonine-rich, and proline-rich structures. The extracellular motifs of the ROR2 protein are known to be involved in protein-protein interactions. The tyrosine kinase domain is involved in an as yet uncharacterized signaling pathway. Interestingly, heterozygous mutations in ROR2 have recently been shown to give rise to autosomal dominant brachydactyly type B1 (BDB1). This condition is characterized by terminal deficiency of fingers and toes. A variety of mutations have been reported in ROR2. Here, these genetic defects are compiled and possible genotype-phenotype correlations are discussed.