Recently, pathogenic variants in the MLL2 gene were identified as the most common cause of Kabuki (Niikawa-Kuroki) syndrome (MIM#147920). To further elucidate the genotype-phenotype correlation, we studied a large cohort of 86 clinically defined patients with Kabuki syndrome (KS) for mutations in MLL2. All patients were assessed using a standardized phenotype list and all were scored using a newly developed clinical score list for KS (MLL2-Kabuki score 0-10). Sequencing of the full coding region and intron-exon boundaries of MLL2 identified a total of 45 likely pathogenic mutations (52%): 31 nonsense, 10 missense and four splice-site mutations, 34 of which were novel. In five additional patients, novel, i.e. non-dbSNP132 variants of clinically unknown relevance, were identified. Patients with likely pathogenic nonsense or missense MLL2 mutations were usually more severely affected (median 'MLL2-Kabuki score' of 6) as compared to the patients without MLL2 mutations (median 'MLL2-Kabuki score' of 5), a significant difference (p < 0.0014). Several typical facial features such as large dysplastic ears, arched eyebrows with sparse lateral third, blue sclerae, a flat nasal tip with a broad nasal root, and a thin upper and a full lower lip were observed more often in mutation positive patients.

Spinal muscular atrophy (SMA) is caused by mutations in the SMN1 gene. We have studied the molecular pathology of SMA in 745 unrelated Spanish patients using PCR-RFLP, SMN gene dosage analysis, linkage studies, long-range PCR and direct sequencing. Our systematic approach allowed us to complete genetic testing and risk assessment in 736 SMA patients (98.8%). Females were more frequently affected by the acute form of the disease (type I), whereas chronic forms (type II-III) predominated in males (p<0.008). Absence of the SMN1 gene was detected in 671 patients (90%), and hybrid SMN1-SMN2 genes were observed in 37 cases (5%). Furthermore, we detected 13 small mutations in 28 patients (3.8%), four of which were previously identified in other populations (c.91dupT; c.770_780dup11; p.Tyr272Cys and p.Thr274Ile), while five mutations were found to date only in Spanish patients (c.399_402delAGAG, p.Ile116Phe, p.Gln136Glu, c.740dupC and c.834+2T>G). The c.399_402delAGAG mutation accounted for 1.9% of all Spanish SMA patients. Finally, we discovered four novel mutations: c.312dupA, c.411delT, p.Trp190X and p.Met263Thr. Our results confirm that most SMA cases are due to large genetic rearrangements in the repetitive region of the SMA locus, resulting in absence-dysfunction of the SMN1 gene. By contrast, ancestrally inherited small mutations are responsible for only a small number of cases. Four prevalent changes in exons 3 and 6 (c.399_402delAGAG; c.770_780dup11; p.Tyr272Cys; p.Thr274Ile) accounted for almost 70% of our patients with these subtle mutations. An SMN-SMN dimer model featuring tight hydrophobic-aromatic interactions is proposed to explain the impact of mutations at the C-terminal end of the protein.

Phelan-McDermid syndrome (PMS, OMIM# 606232) results from either different rearrangements at the distal region of the long arm of chromosome 22 (22q13.3) or pathogenic sequence variants in the SHANK3 gene. SHANK3 codes for a structural protein that plays a central role in the formation of the postsynaptic terminals and the maintenance of synaptic structures. Clinically, patients with PMS often present with global developmental delay, absent or severely delayed speech, neonatal hypotonia, minor dysmorphic features, and autism spectrum disorders (ASD), among other findings. Here, we describe a cohort of 210 patients with genetically confirmed PMS. We observed multiple variant types, including a significant number of small deletions (&lt;0.5 Mb, 64/189) and SHANK3 sequence variants (21 cases). We also detected multiple types of rearrangements among microdeletion cases, including a significant number with post-zygotic mosaicism (9.0%, 17/189), ring chromosome 22 (10.6%, 20/189), unbalanced translocations (de novo or inherited, 6.4%), and additional rearrangements at 22q13 (6.3%, 12/189) as well as other copy number variations in other chromosomes, unrelated to 22q deletions (14.8%, 28/189). We compared the clinical and genetic characteristics among patients with different sizes of deletions and with SHANK3 variants. Our findings suggest that SHANK3 plays an important role in this syndrome but is probably not uniquely responsible for all the spectrum features in PMS. We emphasize that only an adequate combination of different molecular and cytogenetic approaches allows an accurate genetic diagnosis in PMS patients. Thus, a diagnostic algorithm is proposed.

Sotos syndrome is a well-described multiple anomaly syndrome characterized by overgrowth, distinctive craniofacial appearance, and variable learning disabilities. The diagnosis of Sotos syndrome relied solely on these clinical criteria until haploinsufficiency of the NSD1 gene was identified as causative. We describe a 63-year-old woman with classic features and a pathogenic NSD1 mutation, who we believe to be the oldest reported person with Sotos syndrome. She is notable for the diagnosis of Sotos syndrome late in life, mild cognitive limitation, and chronic kidney disease attributed to fibromuscular dysplasia for which she recently received a transplant. She has basal cell and squamous cell carcinoma for which her lifetime of sun exposure and fair cutaneous phototype are viewed as risk factors. We also reviewed previous literature reports (n = 11) for adults with Sotos syndrome, and studied patients ascertained in the Spanish Overgrowth Syndrome Registry (n = 15). Analysis was limited to 21/27 (78%) total patients who had molecular confirmation of Sotos syndrome (15 with a mutation, 6 with a microdeletion). With a mean age of 26 years, the most common features were learning disabilities (90%), scoliosis (52%), eye problems (43%), psychiatric issues (30%), and brain imaging anomalies (28%). Learning disabilities were more severe in patients with a microdeletion than in those with a point mutation. From this small study with heterogeneous ascertainment we suggest modest adjustments to the general healthcare monitoring of individuals with Sotos syndrome. Although this series includes neoplasia in four cases, this should not be interpreted as incidence. Age-appropriate cancer surveillance should be maintained.

Background: Broken chromosomes must acquire new telomeric ''caps'' to be structurally stable. Chromosome healing can be mediated either by telomerase through neo-telomere synthesis or by telomere capture. Aim: To unravel the mechanism(s) generating complex chromosomal mosaicisms and healing broken chromosomes. Methods: G banding, array comparative genomic hybridization (aCGH), fluorescence in-situ hybridisation (FISH) and short tandem repeat analysis (STR) was performed on a girl presenting with mental retardation, facial dysmorphism, urogenital malformations and limb anomalies carrying a complex chromosomal mosaicism. Results & discussion: The karyotype showed a de novo chromosome rearrangement with two cell lines: one cell line with a deletion 9pter and one cell line carrying an inverted duplication 9p and a non-reciprocal translocation 5pter fragment. aCGH, FISH and STR analysis enabled the deduction of the most likely sequence of events generating this complex mosaic. During embryogenesis, a double-strand break occurred on the paternal chromosome 9. Following mitotic separation of both broken sister chromatids, one acquired a telomere via neotelomere formation, while the other generated a dicentric chromosome which underwent breakage during anaphase, giving rise to the del inv dup(9) that was subsequently healed by chromosome 5 telomere capture. Conclusion: Broken chromosomes can coincidently be rescued by both telomere capture and neo-telomere synthesis.