William Reardon

Recurrent rearrangements of chromosome 1q21.1 and variable pediatric phenotypes

BACKGROUND Duplications and deletions in the human genome can cause disease or predispose persons to disease. Advances in technologies to detect these changes allow for the routine identification of submicroscopic imbalances in large numbers of patients. METHODS We tested for the presence of microdeletions and microduplications at a specific region of chromosome 1q21.1 in two groups of patients with unexplained mental retardation, autism, or congenital anomalies and in unaffected persons. RESULTS We identified 25 persons with a recurrent 1.35-Mb deletion within 1q21.1 from screening 5218 patients. The microdeletions had arisen de novo in eight patients, were inherited from a mildly affected parent in three patients, were inherited from an apparently unaffected parent in six patients, and were of unknown inheritance in eight patients. The deletion was absent in a series of 4737 control persons (P=1.1x10(-7)). We found considerable variability in the level of phenotypic expression of the microdeletion; phenotypes included mild-to-moderate mental retardation, microcephaly, cardiac abnormalities, and cataracts. The reciprocal duplication was enriched in nine children with mental retardation or autism spectrum disorder and other variable features (P=0.02). We identified three deletions and three duplications of the 1q21.1 region in an independent sample of 788 patients with mental retardation and congenital anomalies. CONCLUSIONS We have identified recurrent molecular lesions that elude syndromic classification and whose disease manifestations must be considered in a broader context of development as opposed to being assigned to a specific disease. Clinical diagnosis in patients with these lesions may be most readily achieved on the basis of genotype rather than phenotype.

Heterozygous mutations in the gene encoding noggin affect human joint morphogenesis

The secreted polypeptide noggin (encoded by the Nog gene) binds and inactivates members of the transforming growth factor β superfamily of signalling proteins (TGFβ-FMs), such as BMP4 (ref. 1). By diffusing through extracellular matrices more efficiently than TGFβ-FMs, noggin may have a principal role in creating morphogenic gradients. During mouse embryogenesis, Nog is expressed at multiple sites, including developing bones. Nog-/- mice die at birth from multiple defects that include bony fusion of the appendicular skeleton. We have identified five dominant human NOG mutations in unrelated families segregating proximal symphalangism (SYM1; OMIM 185800) and a de novo mutation in a patient with unaffected parents. We also found a dominant NOG mutation in a family segregating multiple synostoses syndrome (SYNS1; OMIM 186500); both SYM1 and SYNS1 have multiple joint fusion as their principal feature. All seven NOG mutations alter evolutionarily conserved amino acid residues. The findings reported here confirm that NOG is essential for joint formation and suggest that NOG requirements during skeletogenesis differ between species and between specific skeletal elements within species.

A homeobox gene, HLXB9, is the major locus for dominantly inherited sacral agenesis

Partial absence of the sacrum is a rare congenital defect which also occurs as an autosomal dominant trait; association with anterior meningocoele, presacral teratoma and anorectal abnormalities constitutes the Currarino triad (MIM 176450). Malformation at the caudal end of the developing notochord at approximately Carnegie stage 7 (16 post-ovulatory days), which results in aberrant secondary neurulation, can explain the observed pattern of anomalies. We previously reported linkage to 7q36 markers in two dominantly inherited sacral agenesis families. We now present data refining the initial subchromosomal localization in several additional hereditary sacral agenesis (HSA) families. We excluded several candidate genes before identifying patient-specific mutations in a homeobox gene, HLXB9, which was previously reported to map to 1q41-q42.1 and to be expressed in lymphoid and pancreatic tissues.

Haploinsufficiency of TCF4 causes syndromal mental retardation with intermittent hyperventilation (Pitt-Hopkins syndrome).

Pitt-Hopkins syndrome is a rarely reported syndrome of so-far-unknown etiology characterized by mental retardation, wide mouth, and intermittent hyperventilation. By molecular karyotyping with GeneChip Human Mapping 100K SNP arrays, we detected a 1.2-Mb deletion on 18q21.2 in one patient. Sequencing of the TCF4 transcription factor gene, which is contained in the deletion region, in 30 patients with significant phenotypic overlap revealed heterozygous stop, splice, and missense mutations in five further patients with severe mental retardation and remarkable facial resemblance. Thus, we establish the Pitt-Hopkins syndrome as a distinct but probably heterogeneous entity caused by autosomal dominant de novo mutations in TCF4. Because of its phenotypic overlap, Pitt-Hopkins syndrome evolves as an important differential diagnosis to Angelman and Rett syndromes. Both null and missense mutations impaired the interaction of TCF4 with ASCL1 from the PHOX-RET pathway in transactivating an E box-containing reporter construct; therefore, hyperventilation and Hirschsprung disease in patients with Pitt-Hopkins syndrome might be explained by altered development of noradrenergic derivatives.

Prevalence, age of onset, and natural history of thyroid disease in Pendred syndrome

BACKGROUND We have sought to establish the prevalence of goitre within a Pendred syndrome (PS) cohort and to document the course of thyroid disease in this patient group. As part of a genetic study of PS we have assessed 57 subjects by perchlorate discharge test and in 52 (M 21, F 31, age range 9-54 years) a discharge of radioiodide of >10% was observed. RESULTS Goitre was present in 43 (83%) of the cohort (28 F, 15 M), generally developing after the age of 10 years, 56% remained euthyroid (age range 9-37 years), and 19 patients (44%) had objective evidence of hypothyroidism, all of whom had goitre. CONCLUSIONS In summary, thyroid dysfunction in PS is variable and inclusion of goitre as a diagnostic requirement will maintain significant underascertainment. The recent identification of the genetic defect underlying PS is likely to provide an important diagnostic aid in the identification of this disorder and this communication should assist clinicians in identifying deaf patients who ought to be considered for this investigation.

Evidence for digenic inheritance in some cases of Antley-Bixler syndrome?

The Antley-Bixler syndrome has been thought to be caused by an autosomal recessive gene. However, patients with this phenotype have been reported with a new dominant mutation at theFGFR2 locus as well as in the offspring of mothers taking the antifungal agent fluconazole during early pregnancy. In addition to the craniosynostosis and joint ankylosis which are the clinical hallmarks of the condition, many patients, especially females, have genital abnormalities. We now report abnormalities of steroid biogenesis in seven of 16 patients with an Antley-Bixler phenotype. Additionally, we identify FGFR2 mutations in seven of these 16 patients, including one patient with abnormal steroidogenesis. These findings, suggesting that some cases of Antley-Bixler syndrome are the outcome of two distinct genetic events, allow a hypothesis to be formulated under which we may explain all the differing and seemingly contradictory circumstances in which the Antley-Bixler phenotype has been recognised.

A novel germline mutation of the PTEN gene in a patient with macrocephaly, ventricular dilatation, and features of VATER association

Mutations of the PTEN gene are associated with hamartoma-neoplasia syndromes. While germline mutations at this chromosome 10q22-23 locus have been observed in patients with Cowden syndrome (CS) and Bannayan-Riley-Ruvalcaba syndrome (BRR), both of which phenotypes are associated with hamartomata and neoplasia, somatic mutation of PTEN has been established in a wide variety of sporadically occurring neoplasia. CS and BRR share some clinical features, specifically hamartomata and lipomatosis. Investigation of other clinically distinct syndromes associated with lipomatosis and overgrowth has established germline and germline mosaic PTEN mutations in several patients with Proteus syndrome. To this expanding array of clinically distinct phenotypes associated with PTENmutations, we now report a novel heterozygous germline mutation, H61D, in a patient with features of VATER association with macrocephaly and ventriculomegaly.

Deletions in HOXD13 Segregate with an Identical, Novel Foot Malformation in Two Unrelated Families

Synpolydactyly (SPD) is a dominantly inherited congenital limb malformation consisting of 3/4 syndactyly in the hands and 4/5 syndactyly in the feet, with digit duplication in the syndactylous web. The condition recently has been found to result from different-sized expansions of an amino-terminal polyalanine tract in HOXD13. We report a novel type of mutation in HOXD13, associated in some cases with features of classic SPD and in all cases with a novel foot phenotype. In two unrelated families, each with a different intragenic deletion in HOXD13, all mutation carriers have a rudimentary extra digit between the first and second metatarsals and often between the fourth and fifth metatarsals as well. This phenotype has not been reported in any mice with genetic modifications of the HoxD gene cluster. The two different deletions affect the first exon and the homeobox, respectively, in each case producing frameshifts followed by a long stretch of novel sequence and a premature stop codon. Although the affected genes may encode proteins that exert a dominant negative or novel effect, they are most likely to act as null alleles. Either possibility has interesting implications for the role of HOXD13 in human autopod development.

How genetically heterogeneous is Kabuki syndrome?: MLL2 testing in 116 patients, review and analyses of mutation and phenotypic spectrum

MLL2 mutations are detected in 55 to 80% of patients with Kabuki syndrome (KS). In 20 to 45% patients with KS, the genetic basis remains unknown, suggesting possible genetic heterogeneity. Here, we present the largest yet reported cohort of 116 patients with KS. We identified MLL2 variants in 74 patients, of which 47 are novel and a majority are truncating. We show that pathogenic missense mutations were commonly located in exon 48. We undertook a systematic facial KS morphology study of patients with KS at our regional dysmorphology meeting. Our data suggest that nearly all patients with typical KS facial features have pathogenic MLL2 mutations, although KS can be phenotypically variable. Furthermore, we show that MLL2 mutation-positive KS patients are more likely to have feeding problems, kidney anomalies, early breast bud development, joint dislocations and palatal malformations in comparison with MLL2 mutation-negative patients. Our work expands the mutation spectrum of MLL2 that may help in better understanding of this molecule, which is important in gene expression, epigenetic control of active chromatin states, embryonic development and cancer. Our analyses of the phenotype indicates that MLL2 mutation-positive and -negative patients differ systematically, and genetic heterogeneity of KS is not as extensive as previously suggested. Moreover, phenotypic variability of KS suggests that MLL2 testing should be considered even in atypical patients.

Genetic analysis of the connexin-26 M34T variant: identification of genotype M34T/M34T segregating with mild-moderate non-syndromic sensorineural hearing loss

Mutations in the human gap junction β-2 gene (GJB2) that encodes connexin-26 have been shown to cause non-syndromic sensorineural hearing loss (NSSNHL) at theDFNB1 locus on 13q11. Functional and genetic data regarding the disease causing potential of one particularGJB2 sequence variant, 101 T→C (M34T), have proven contradictory. In this study, we found the prevalence of the M34T allele in a cohort of white sib pairs and sporadic cases with NSSNHL from the United Kingdom and Ireland to be 3.179% of chromosomes screened. Significantly, we identified the first M34T/M34T genotype cosegregating in a single family with mid to high frequency NSSNHL. Screening a control population of 630 subjects we identified 25 M34T heterozygotes; however, no M34T homozygotes were detected. Surprisingly, the majority of M34T alleles (88%) were incis with a 10 bp deletion in the 5′ non-coding sequence. This non-coding deletion was also homozygous in the homozygous M34T subjects. Microsatellite analysis of flanking loci in M34T heterozygotes and controls does not define an extensive ancestral haplotype but preliminary data suggest two common alleles in subjects with the M34T allele. In summary, we provide data that support M34T acting as a recessive GJB2 allele associated with mild-moderate prelingual hearing impairment.