Sajid Malik

Multiple familial trichoepithelioma caused by mutations in the cylindromatosis tumor suppressor gene

The recessive oncogene cylindromatosis (CYLD) mapping on 16q12-q13 is generally implicated in familial cylindromatosis, whereas a gene region for multiple familial trichoepithelioma has been assigned to 9p21. Markers from both chromosome intervals were subjected to linkage analysis in a large family with multiple hereditary trichoepithelioma (TE) from Algeria. Linkage to 9p21 was excluded, whereas CYLD remained as a candidate. Mutation analysis identified a single bp germ-line deletion expected to result in truncation or absence of the encoded protein, which segregated with the multiple TE phenotype. In individual tumors, loss of heterozygosity at 16q or a somatic point mutation in the CYLD gene was detected. Hence, mutations of the tumor suppressor gene CYLD at 16q12-q13 may give rise to familial TE indistinguishable from the phenotype assigned to 9p21.

Syndactyly: phenotypes, genetics and current classification

Syndactyly is one of the most common hereditary limb malformations depicting the fusion of certain fingers and/or toes. It may occur as an isolated entity or a component of more than 300 syndromic anomalies. Syndactylies exhibit great inter- and intra-familial clinical variability. Even within a subject, phenotype can be unilateral or bilateral and symmetrical or asymmetrical. At least nine non-syndromic syndactylies with additional sub-types have been characterized. Most of the syndactyly types are inherited as autosomal dominant but two autosomal recessive and an X-linked recessive entity have also been described. Whereas the underlying genes/mutations for types II-1, III, IV, V, and VII have been worked out, the etiology and molecular basis of the other syndactyly types remain unknown. In this communication, based on an overview of well-characterized isolated syndactylies, their cardinal phenotypes, inheritance patterns, and clinical and genetic heterogeneities, a ‘current classification scheme’ is presented. Despite considerable progress in the understanding of syndactyly at clinical and molecular levels, fundamental questions regarding the disturbed developmental mechanisms leading to fused digits, remain to be answered.

Human GLI3 Intragenic Conserved Non-Coding Sequences Are Tissue-Specific Enhancers

The zinc-finger transcription factor GLI3 is a key regulator of development, acting as a primary transducer of Sonic hedgehog (SHH) signaling in a combinatorial context dependent fashion controlling multiple patterning steps in different tissues/organs. A tight temporal and spatial control of gene expression is indispensable, however, cis-acting sequence elements regulating GLI3 expression have not yet been reported. We show that 11 ancient genomic DNA signatures, conserved from the pufferfish Takifugu (Fugu) rubripes to man, are distributed throughout the introns of human GLI3. They map within larger conserved non-coding elements (CNEs) that are found in the tetrapod lineage. Full length CNEs transiently transfected into human cell cultures acted as cell type specific enhancers of gene transcription. The regulatory potential of these elements is conserved and was exploited to direct tissue specific expression of a reporter gene in zebrafish embryos. Assays of deletion constructs revealed that the human-Fugu conserved sequences within the GLI3 intronic CNEs were essential but not sufficient for full-scale transcriptional activation. The enhancer activity of the CNEs is determined by a combinatorial effect of a core sequence conserved between human and teleosts (Fugu) and flanking tetrapod-specific sequences, suggesting that successive clustering of sequences with regulatory potential around an ancient, highly conserved nucleus might be a possible mechanism for the evolution of cis-acting regulatory elements.

Synpolydactyly: clinical and molecular advances

Synpolydactyly (SPD) is a rare limb deformity showing a distinctive combination of syndactyly and polydactyly. Of the nine non‐syndromic syndactylies, it is clinically and genetically one of the most heterogeneous malformation. SPD families may show clinical features consistent with the Temtamy and McKusick criteria as well as additional phenotypic variants, which vary from case to case. In certain instances, these variants predominate in a given family, while the typical SPD features remain less explicit. We have reviewed all the clinical variants occurring in well‐documented SPD families. We conclude that typical SPD features can be delineated from minor clinical variants. Then, we propose to lump all the phenotypic variants, manifesting themselves in SPD families into three categories: (i) typical SPD features, (ii) minor variants, and (iii) unusual phenotypes. Next, we discuss the likely reasons for the occurrence of minor variants and the obvious lack of penetrance in SPD families. Finally, we show that for the SPD phenotype associated with HOXD13 mutations, a straightforward genotype–phenotype correlation is weak. Our lumping and splitting scheme for SPD phenotypic variants could be useful for the understanding of this interesting malformation.

Polydactyly: phenotypes, genetics and classification

Polydactyly is one of the most common hereditary limb malformations featuring additional digits in hands and/or feet. It constituted the highest proportion among the congenital limb defects in various epidemiological surveys. Polydactyly, primarily presenting as an additional pre‐axial or post‐axial digit of autopod, is a highly heterogeneous condition and depicts broad inter‐ and intra‐familial clinical variability. There is a plethora of polydactyly classification methods reported in the medical literature which approach the heterogeneity in polydactyly in various ways. In this communication, well‐characterized, non‐syndromic polydactylies in humans are reviewed. The cardinal features, phenotypic variability and molecular advances of each type have been presented. Polydactyly at cellular and developmental levels is mainly a failure in the control of digit number. Interestingly, GLI3 and SHH (ZRS/SHH enhancer), two antagonistic factors known to modulate digit number and identity during development, have also been implicated in polydactyly. Mutations in GLI3 and ZRS/SHH cause overlapping polydactyly phenotypes highlighting shared molecular cascades in the etiology of additional digits, and thus suggesting the lumping of at least six distinct polydactyly entities. However, owing to the extreme phenotypic and clinical heterogeneity witnessed in polydactyly a substantial genetic heterogeneity is expected across different populations and ethnic groups.

Ultraconserved non-coding sequence element controls a subset of spatiotemporal GLI3 expression

The zinc‐finger transcription factor GLI3 acts during vertebrate development in a combinatorial, context‐dependent fashion as a primary transducer of sonic hedgehog (SHH) signaling. In humans, mutations affecting this key regulator of development are associated with GLI3‐morphopathies, a group of congenital malformations in which forebrain and limb development are preferentially affected. We show that a non‐coding element from intron two of GLI3, ultraconserved in mammals and highly conserved in the pufferfish Fugu, is a transcriptional enhancer. In transient transfection assays, it activates reporter gene transcription in human cell cultures expressing endogenous GLI3 but not in GLI3 negative cells. The identified enhancer element is predicted to contain conserved binding sites for transcription factors crucial for developmental steps in which GLI3 is involved. The regulatory potential of this element is conserved and was used to direct tissue‐specific expression of a green fluorescent protein reporter gene in zebrafish embryos and of a beta‐galactosidase reporter in transgenic mouse embryos. Time, location, and quantity of reporter gene expression are congruent with part of the pattern previously reported for endogenous GLI3 transcription.

A locus for hereditary hypotrichosis localized to human chromosome 18q21.1.

Hereditary hypotrichosis is a rare autosomal recessive condition characterized clinically by alopecia. Three consanguineous kindreds with multiple affected individuals were ascertained from different regions of Pakistan. A novel hypotrichosis locus was mapped to a 5.5 cM region on chromosome 18q21.1. A maximum two-point LOD score of 5.25 was obtained at marker D18S36 (θ=0.0). Three genes each for desmoglein and desmocollin proteins are located in this region. The expression in epidermal desmosomes and their connection to the keratin intermediate filaments make these genes excellent candidates for recessive hypotrichosis.

Human intronic enhancers control distinct sub-domains of Gli3 expression during mouse CNS and limb development

BackgroundThe zinc-finger transcription factor GLI3 is an important mediator of Sonic hedgehog signaling and crucial for patterning of many aspects of the vertebrate body plan. In vertebrates, the mechanism of SHH signal transduction and its action on target genes by means of activating or repressing forms of GLI3 have been studied most extensively during limb development and the specification of the central nervous system. From these studies it has emerged, that Gli3 expression must be subject to a tight spatiotemporal regulation. However, the genetic mechanisms and the cis-acting elements controlling the expression of Gli3 remained largely unknown.ResultsHere, we demonstrate in chicken and mouse transgenic embryos that human GLI3-intronic conserved non-coding sequence elements (CNEs) autonomously control individual aspects of Gli3 expression. Their combined action shows many aspects of a Gli3-specific pattern of transcriptional activity. In the mouse limb bud, different CNEs enhance Gli3-specific expression in evolutionary ancient stylopod and zeugopod versus modern skeletal structures of the autopod. Limb bud specificity is also found in chicken but had not been detected in zebrafish embryos. Three of these elements govern central nervous system specific gene expression during mouse embryogenesis, each targeting a subset of endogenous Gli3 transcription sites. Even though fish, birds, and mammals share an ancient repertoire of gene regulatory elements within Gli3, the functions of individual enhancers from this catalog have diverged significantly. During evolution, ancient broad-range regulatory elements within Gli3 attained higher specificity, critical for patterning of more specialized structures, by abolishing the potential for redundant expression control.ConclusionThese results not only demonstrate the high level of complexity in the genetic mechanisms controlling Gli3 expression, but also reveal the evolutionary significance of cis-acting regulatory networks of early developmental regulators in vertebrates.

A novel type of autosomal recessive syndactyly: clinical and molecular studies in a family of Pakistani origin.

Non‐syndromic syndactylies have been classified into five major types (I–V), all showing autosomal dominant mode of inheritance. Later, the classification was extended and three additional variants (VI–VIII) were defined. Type VII, the Cenani–Lenz syndactyly, is the only non‐syndromic, autosomal recessive type. It is characterized by fusion of all phalanges with metacarpal synostosis, dislocated and dysplastic carpals and infrequently, radio‐ulnar fusion. Here, we present a Pakistani family with a novel non‐syndromic autosomal recessive syndactyly manifesting a unique combination of clinical features. In both hands, reduction of certain phalanges is evident. Radiological examination shows synostosis of third and fourth metacarpals bearing single phalanges. The first three toes are webbed, with hypoplastic terminal phalanx in all the toes. Besides Cenani–Lenz syndactyly, the phenotype segregating in our family is the second well‐documented autosomal recessive, non‐syndromic syndactyly. A phenotype similar to our family was described in a Turkish kindred but was considered to be a homozygous expression of type I syndactyly. Since the clinical features in our family had minimal overlap with syndactyly types I, II, and III, we have performed microsatellite marker screening to look for the cosegregation of this phenotype with any of the known loci for these respective types. We show that the phenotype in our family is not linked to chromosomal regions 2q34‐q36, 2q31, and 6q22‐q23 encompassing loci for syndactyly types I, II, and III.

Clinical and descriptive genetic study of polydactyly: a Pakistani experience of 313 cases.

Polydactyly, a common hereditary condition with additional digits in hands and/or feet, is a very attractive model to appreciate clinical and genetic heterogeneity. In order to get an insight into its phenotypic manifestations, we ascertained a cohort of 313 independent families with polydactyly from Pakistan; 35% cases turned out to be familial while 65% were sporadic. In majority of the index cases, polydactyly was presented as an isolated digit defect. Preaxial polydactyly types were 48.24% and postaxial were 51.8%. Familial polydactylies mainly had bilateral and symmetrical presentations, whereas sporadic cases were mostly unilateral and less often symmetrical. In the 313 index subjects a total of 508 limbs with additional digits were recorded. Variable expression was evident as the involvement of upper limbs was more common than the lower, right hand than the left, and left foot than the right. The present cohort establishes interesting epidemiological attributes of polydactyly in the Pakistani population and highlights its extraordinary clinical heterogeneity. Molecular analyses of this cohort are anticipated to elucidate novel genetic factors involved in the origin of additional digits in the growing limb and may provide clues to the role of stochastic factors in the etiology of phenotypic variability in polydactyly.