Jeffrey Swensen

Using VAAST to identify an X-linked disorder resulting in lethality in male infants due to N-terminal acetyltransferase deficiency.

We have identified two families with a previously undescribed lethal X-linked disorder of infancy; the disorder comprises a distinct combination of an aged appearance, craniofacial anomalies, hypotonia, global developmental delays, cryptorchidism, and cardiac arrhythmias. Using X chromosome exon sequencing and a recently developed probabilistic algorithm aimed at discovering disease-causing variants, we identified in one family a c.109T>C (p.Ser37Pro) variant in NAA10, a gene encoding the catalytic subunit of the major human N-terminal acetyltransferase (NAT). A parallel effort on a second unrelated family converged on the same variant. The absence of this variant in controls, the amino acid conservation of this region of the protein, the predicted disruptive change, and the co-occurrence in two unrelated families with the same rare disorder suggest that this is the pathogenic mutation. We confirmed this by demonstrating a significantly impaired biochemical activity of the mutant hNaa10p, and from this we conclude that a reduction in acetylation by hNaa10p causes this disease. Here we provide evidence of a human genetic disorder resulting from direct impairment of N-terminal acetylation, one of the most common protein modifications in humans.

Familial occurrence of schwannomas and malignant rhabdoid tumour associated with a duplication in SMARCB1

Background: The role of germline and somatic SMARCB1 gene mutations in malignant rhabdoid tumour (MRT) predisposition is well known. Germline SMARCB1 mutations have also recently been identified in a subset of individuals with schwannomatosis. Surprisingly, MRT predisposition and schwannomatosis have never been reported to co-occur in a family. The correlation between genotype and phenotype for mutations in SMARCB1 has not been determined. Results: We have identified a germline 2631 bp duplication that includes exon 6 of SMARCB1 in a unique family with a four generation history of MRT predisposition and schwannomatosis. This duplication segregates with disease in individuals affected with both conditions, linking MRT predisposition and schwannomatosis as components of the same syndrome in this family. Conclusion: The unique combination of tumours that result from the duplication described in this report may provide important clues about the mechanisms that influence the phenotype associated with a given SMARCB1 mutation.

Clinical analysis of PMS2: mutation detection and avoidance of pseudogenes†

Germline mutation detection in PMS2, one of four mismatch repair genes associated with Lynch syndrome, is greatly complicated by the presence of numerous pseudogenes. We used a modification of a long‐range PCR method to evaluate PMS2 in 145 clinical samples. This modification avoids potential interference from the pseudogene PMS2CL by utilizing a long‐range product spanning exons 11–15, with the forward primer anchored in exon 10, an exon not shared by PMS2CL. Large deletions were identified by MLPA. Pathogenic PMS2 mutations were identified in 22 of 59 patients whose tumors showed isolated loss of PMS2 by immunohistochemistry (IHC), the IHC profile most commonly associated with a germline PMS2 mutation. Three additional patients with pathogenic mutations were identified from 53 samples without IHC data. Thirty‐seven percent of the identified mutations were large deletions encompassing one or more exons. In 27 patients whose tumors showed absence of either another protein or combination of proteins, no pathogenic mutations were identified. We conclude that modified long‐range PCR can be used to preferentially amplify the PMS2 gene and avoid pseudogene interference, thus providing a clinically useful germline analysis of PMS2. Our data also support the use of IHC screening to direct germline testing of PMS2. Hum Mutat 31:588–593, 2010.

Biochemical and cellular analysis of Ogden syndrome reveals downstream Nt-acetylation defects

The X-linked lethal Ogden syndrome was the first reported human genetic disorder associated with a mutation in an N-terminal acetyltransferase (NAT) gene. The affected males harbor an Ser37Pro (S37P) mutation in the gene encoding Naa10, the catalytic subunit of NatA, the major human NAT involved in the co-translational acetylation of proteins. Structural models and molecular dynamics simulations of the human NatA and its S37P mutant highlight differences in regions involved in catalysis and at the interface between Naa10 and the auxiliary subunit hNaa15. Biochemical data further demonstrate a reduced catalytic capacity and an impaired interaction between hNaa10 S37P and Naa15 as well as Naa50 (NatE), another interactor of the NatA complex. N-Terminal acetylome analyses revealed a decreased acetylation of a subset of NatA and NatE substrates in Ogden syndrome cells, supporting the genetic findings and our hypothesis regarding reduced Nt-acetylation of a subset of NatA/NatE-type substrates as one etiology for Ogden syndrome. Furthermore, Ogden syndrome fibroblasts display abnormal cell migration and proliferation capacity, possibly linked to a perturbed retinoblastoma pathway. N-Terminal acetylation clearly plays a role in Ogden syndrome, thus revealing the in vivo importance of N-terminal acetylation in human physiology and disease.

TAF1 Variants Are Associated with Dysmorphic Features, Intellectual Disability, and Neurological Manifestations.

We describe an X-linked genetic syndrome associated with mutations in TAF1 and manifesting with global developmental delay, intellectual disability (ID), characteristic facial dysmorphology, generalized hypotonia, and variable neurologic features, all in male individuals. Simultaneous studies using diverse strategies led to the identification of nine families with overlapping clinical presentations and affected by de novo or maternally inherited single-nucleotide changes. Two additional families harboring large duplications involving TAF1 were also found to share phenotypic overlap with the probands harboring single-nucleotide changes, but they also demonstrated a severe neurodegeneration phenotype. Functional analysis with RNA-seq for one of the families suggested that the phenotype is associated with downregulation of a set of genes notably enriched with genes regulated by E-box proteins. In addition, knockdown and mutant studies of this gene in zebrafish have shown a quantifiable, albeit small, effect on a neuronal phenotype. Our results suggest that mutations in TAF1 play a critical role in the development of this X-linked ID syndrome.

Avoidance of pseudogene interference in the detection of 3′ deletions in PMS2

Lynch syndrome is characterized by mutations in the mismatch repair genes MLH1, MSH2, MSH6, and PMS2. In PMS2, detection of mutations is confounded by numerous pseudogenes. Detection of 3′ deletions is particularly complicated by the pseudogene PMS2CL, which has strong similarity to PMS2 exons 9 and 11–15, due to extensive gene conversion. A newly designed multiplex ligation‐dependent probe amplification (MLPA) kit incorporates probes for variants found in both PMS2 and PMS2CL. This provides detection of deletions, but does not allow localization of deletions to the gene or pseudogene. To address this, we have developed a methodology incorporating reference samples with known copy numbers of variants, and paired MLPA results with sequencing of PMS2 and PMS2CL. We tested a subset of clinically indicated samples for which mutations were either unidentified or not fully characterized using existing methods. We identified eight unrelated patients with deletions encompassing exons 9–15, 11–15, 13–15, 14–15, and 15. By incorporating specific, characterized reference samples and sequencing the gene and pseudogene it is possible to identify deletions in this region of PMS2 and provide clinically relevant results. This methodology represents a significant advance in the diagnosis of patients with Lynch syndrome caused by PMS2 mutations. Hum Mutat 32:1063–1071, 2011.

The frequency of previously undetectable deletions involving 3′ Exons of the PMS2 gene

Lynch syndrome is characterized by mutations in one of four mismatch repair genes, MLH1, MSH2, MSH6, or PMS2. Clinical mutation analysis of these genes includes sequencing of exonic regions and deletion/duplication analysis. However, detection of deletions and duplications in PMS2 has previously been confined to Exons 1–11 due to gene conversion between PMS2 and the pseudogene PMS2CL in the remaining 3′ exons (Exons 12–15). We have recently described an MLPA‐based method that permits detection of deletions of PMS2 Exons 12–15; however, the frequency of such deletions has not yet been determined. To address this question, we tested for 3′ deletions in 58 samples that were reported to be negative for PMS2 mutations using previously available methods. All samples were from individuals whose tumors exhibited loss of PMS2 immunohistochemical staining without concomitant loss of MLH1 immunostaining. We identified seven samples in this cohort with deletions in the 3′ region of PMS2, including three previously reported samples with deletions of Exons 13–15 (two samples) and Exons 14–15. Also detected were deletions of Exons 12–15, Exon 13, and Exon 14 (two samples). Breakpoint analysis of the intragenic deletions suggests they occurred through Alu‐mediated recombination. Our results indicate that ∼12% of samples suspected of harboring a PMS2 mutation based on immunohistochemical staining, for which mutations have not yet been identified, would benefit from testing using the new methodology.

A new genomic mechanism leading to cri-du-chat syndrome†

Using standard banding techniques, a within‐arm intrachromosomal insertion can be mistakenly interpreted as a paracentric inversion. The need to correctly distinguish between these two types of chromosome rearrangements is emphasized by their different reproductive risks. For carriers of an intrachromosomal insertion, the empiric risk of having a liveborn child with a recombinant chromosome leading to a genetic imbalance is at least 15%, whereas the risk for a carrier of a paracentric inversion having a liveborn child with a recombinant chromosome leading to a genetic imbalance is thought to be practically negligible. We report a unique observation in which a paracentric inversion in the short arm of chromosome 5, 46,XX,inv(5)(p13.3p15.3), was identified in a women who had a daughter with an apparently terminal deletion in the distal short arm of chromosome 5, 46,XX,del(5)(p14.3), and the clinical diagnosis of cri‐du‐chat syndrome. We further characterized the rearrangement, and fluorescence in situ hybridization (FISH) and microsatellite analyses confirmed the paracentric inversion in the mother and showed the deletion in the daughter was maternal in origin. Therefore, this represents a case in which a confirmed paracentric inversion likely resulted in a viable terminal deletion. We propose a mechanism involving dicentric chromosome formation with subsequent breakage and telomere healing during meiosis. This illustrates a new genomic mechanism of chromosome rearrangement leading to cri‐du‐chat syndrome and should provide significant information for the medical management of patients with other terminal deletion syndromes.

Comprehensive profiling of metaplastic breast carcinomas reveals frequent overexpression of programmed death-ligand 1

Aims Metaplastic breast carcinoma (MBC) is a rare subtype of breast carcinoma less responsive to conventional chemotherapy than ductal carcinoma. In molecular terms, MBCs usually cluster with triple-negative breast cancers (TNBCs), but have a worse prognosis than TNBCs. Studies investigating MBCs for specific biomarkers of therapy response are rare and limited by the methodological approaches. The aim of the present study was to characterise MBCs on a molecular level and test programmed death-ligand 1 (PD-L1) biomarker expression in MBCs for future therapeutic interventions. Methods We profiled 297 samples (MBC (n=75), TNBC (n=106), human epidermal growth factor receptor 2 (HER2)-positive breast cancers (n=32) and hormone-positive breast cancers (n=84)) by next-generation sequencing. Immunohistochemistry for PD-L1 and programmed cell death 1 (PD-1) expression was performed using automated procedures. Results The most commonly mutated genes in MBCs included TP53 (56%) and PIK3CA (23%). Pathogenic mutations in other genes, including HRAS, FBXW7, PTEN, AKT1 and SMAD4, were rare. PD-L1 expression was detected in a significantly higher proportion of MBCs (46%) than in other subtypes (6% each in hormone-positive and HER2-positive breast cancers, and 9% in TNBC, not otherwise specified, p<0.001). PD-1-positive tumour infiltrating lymphocytes (TILs) varied greatly in MBCs. Conclusions Comprehensive profiling of a large cohort of this rare subtype of breast carcinoma highlighted the predominance of TP53 mutation and increased PD-L1 expression in carcinoma cells. These results can be exploited in clinical trials using immune checkpoint inhibitors.

Mutations in the Kinase Domain of the HER2/ERBB2 Gene Identified in a Wide Variety of Human Cancers

The HER2 (official name ERBB2) gene encodes a membrane receptor in the epidermal growth factor receptor family amplified and overexpressed in adenocarcinoma. Activating mutations also occur in several cancers. We report mutation analyses of the HER2 kinase domain in 7497 histologically diverse cancers. Forty-five genes, including the kinase domain of HER2 with HER2 IHC and dual in situ hybridization, were analyzed in tumors from 7497 patients with cancer, including 850 breast, 770 colorectal, 910 non-small cell lung, 823 uterine or cervical, 1372 ovarian, and 297 pancreatic cancers, as well as 323 melanomas and 2152 other solid tumors. Sixty-nine HER2 kinase domain mutations were identified in tumors from 68 patients (approximately 1% of all cases, ranging from absent in sarcomas to 4% in urothelial cancers), which included previously published activating mutations and 13 novel mutations. Fourteen cases with coexisting HER2 mutation and amplification and/or overexpression were identified. Fifty-two of 68 patients had additional mutations in other analyzed genes, whereas 16 patients (23%) had HER2 mutations identified as the sole driver mutation. HER2 mutations coexisted with HER2 gene amplification and overexpression and with mutations in other functionally important genes. HER2 mutations were identified as the only driver mutation in a significant proportion of solid cancers. Evaluation of anti-HER2 therapies in nonamplified, HER2-mutated cancers is warranted.