Kerstin U. Ludwig

Mendelian diseases are caused by rare pathogenic variants with large effects. However, phenotypes are often different between individuals carrying the same variant (“variable expressivity”), or even absent in some carriers (“incomplete penetrance”), suggesting that the genetic architecture of Mendelian diseases is more complex than previously thought. Common genetic variants are a potential modifier of observed phenotypes. Individual common variants usually have small effects, but in aggregate their effects can be substantial and be quantified as a polygenic score (PGS). The present review summarises available data on how PGS modify phenotypes in Mendelian diseases. We show that modifying effects of PGS on penetrance and expressivity have been observed in carriers of rare pathogenic variants across a broad range of conditions. This suggests that in Mendelian diseases, a modifying effect of the common variant background on penetrance and expressivity might be the rule rather than the exception. We anticipate that increasing data availability and methodological advances will improve our understanding of the genetic architecture of Mendelian diseases, including the joint consideration of both common and rare variants.

Orofacial clefting (OFC) is among the most common birth defects and can occur either as part of a syndrome or in isolation (nonsyndromic, ns). Cleft palate only (CPO) is an OFC subtype. Here, we searched for novel nsCPO risk genes carrying homozygous and compound-heterozygous variants, by analyzing exome data from six sibling pairs with nsCPO born to unaffected parents. After stringent quality control and filtering, we identified 6 homozygous variants and 32 compound-heterozygous variants in 5 and 16 candidate genes, respectively. We prioritized DDR2, a collagen-activated receptor-tyrosine-kinase influencing extracellular matrix composition, as our top candidate for functional follow-up, since variants in this gene can cause Warburg-Cinotti syndrome, the phenotypic spectrum of which includes palatal abnormalities. Knock-down and knock-out of DDR2-orthologs in zebrafish caused craniofacial abnormalities resembling CPO in humans. Zebrafish immunostaining indicated that DDR2-orthologs were expressed in mature head muscle cells, while murine single-cell RNA-Sequencing data detected Ddr2 expression only in head muscle progenitor cells; the latter finding was confirmed in human embryo sections stained for DDR2. DDR2-expressing head muscle progenitor cells may influence extracellular matrix composition through DDR2-mediated signaling, thereby affecting outgrowth, elevation, and fusion of the palatal shelves, a previously postulated mechanism involved in palatogenesis. Most established OFC genes (e.g. CDH1, CTNND1, IRF6, and GRHL3) act via mechanisms related to epithelial integrity and periderm differentiation, whereas our data provide evidence supporting DDR2 as a risk gene for nsOFC that functions by influencing extracellular matrix composition.

Epstein–Barr virus (EBV) infects approximately 90–95% of the global population ^1,2 and persists in B cells as a lifelong infection ³ . Previous EBV infection is associated with autoimmune and neoplastic disease ⁴ . Still, the biological basis of host control during EBV persistence remains unclear. Here we report the identification of non-genetic and genetic factors that are associated with EBV control during persistent infection. Using blood-based genome sequence data from 486,315 UK Biobank and 336,123 All of Us participants, we identified short-read pairs mapping to the EBV genome in 16.2% and 21.8% of individuals, respectively. EBV read detection (EBVread ⁺ ) reflects increased viral load in blood cells, as shown by orthogonal measurements, and was associated with HIV infection, immunosuppressive drug intake and current smoking. Genome-wide analyses of EBVread ⁺ identified strong associations at the major histocompatibility complex (MHC), including 54 independent human leukocyte antigen (HLA) alleles of MHC classes I and II, and at 27 genomic regions outside MHC. Epistasis with distinct HLA alleles of MHC class I was observed at the ERAP2 locus. Analysis of individuals with EBV-associated diseases ⁴ revealed a higher polygenic burden of EBVread ⁺ for HLA alleles at MHC class I in multiple sclerosis (driven by HLA-A*02:01) and at MHC class II in rheumatoid arthritis. Phenome-wide analyses identified a polygenic overlap of EBVread ⁺ with inflammatory bowel disease, hypothyroidism and type 1 diabetes. Our study establishes by-products of human genome sequencing as a surrogate marker of EBV viral load. This will facilitate investigation and treatment for EBV and other persistent viral infections.

Several genome wide association studies (GWASs) of orofacial cleft have been conducted. However only a few such studies to date have combined all cleft cases, focused on subtypes other than non-syndromic cleft lip with/without cleft palate, or investigated subtype heterogeneity. We conducted a GWAS of orofacial clefts within 2268 cases from the Cleft Collective and 7913 population-based controls; we performed analyses of all orofacial clefts, plus 7 subgroups. We replicated our findings in a meta-analysis of independent samples and investigated patterns of correlation across subgroups. We identified 27 regions at genome-wide significance, 8 of which were novel. We also conducted the first GWAS of Pierre Robin Sequence, despite the small sample size (n cases = 237), we found one genome wide significant SNP (P < 5 × 10−8), and another 21 suggestive associations (P < 10−5). Novel loci include those mapping to LHX8 and TSBP1 (combined clefts), ARHGEF18 and ARHGEF19 (cleft lip with/without palate), FBN2 (cleft lip only), SLC35B3 (cleft palate only), CASC20 (Pierre Robin Sequence) and CHRM2 (non-syndromic cleft palate only). Several novel hits were in regions previously associated with facial morphology in GWAS or were in regions involved in key developmental processes, including neural crest cell migration and craniofacial development. We identified genetic loci with similar effects across all subgroups and some loci which were subtype specific, we also identified 3 loci with opposing effects on cleft lip and Pierre Robin sequence. Our findings highlight the merit of including all orofacial cleft subtypes in GWAS studies and investigating heterogeneity of effects across subtypes.

Epstein-Barr virus (EBV) is a herpes virus that infects around 90-95% of the global population, and is associated with numerous autoimmune and neoplastic diseases. EBV persists in B cells as a life-long latent infection. Despite its relevance to disease development, the biological basis of host control during EBV latent infection remains unknown. Here, we report the identification of genetic and non-genetic factors that contribute to latent EBV infection control. In the genome sequence (GS) data of blood from 486,315 UK Biobank and 336,123 All of Us participants, we identified short-reads mapping to the EBV genome in 16.2% and 21.8% of individuals, respectively. The detection of EBV-reads (EBVread+) reflected increased viral load, and was associated with HIV infection, intake of immunosuppressive drugs, and current, but not former, smoking. Genome-wide association analyses identified strong associations at the Major Histocompatibility Complex (MHC), including 54 independent HLA-alleles of MHC class I and II, and at 27 genomic regions outside MHC (e.g., loci encompassing ERAP2, CTLA4). Associated genes included genes underlying monogenic susceptibility to EBV infection (e.g., CD70), and novel candidates thereof (e.g., CD226). In an analysis of individuals with EBV-associated diseases, we observed a higher polygenic burden of EBVread+ for HLA-alleles at MHC class I in multiple sclerosis (driven by HLA-A*02:01), and at MHC class II in rheumatoid arthritis (RA). A phenome-wide association analysis identified a polygenic overlap of EBVread+ with inflammatory bowel disease, hypothyroidism, and type I diabetes (T1D). Mendelian randomization analyses suggested potential causal effects of EBVread+ on RA and T1D. Our results will inform mechanistic studies and therapeutic approaches in EBV-associated diseases. More broadly, our study illustrates how by-products of human GS data can be used to investigate persistent viral infections.

Infections can lead to persistent symptoms and diseases such as shingles after varicella zoster or rheumatic fever after streptococcal infections. Similarly, severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) infection can result in long coronavirus disease (COVID), typically manifesting as fatigue, pulmonary symptoms and cognitive dysfunction. The biological mechanisms behind long COVID remain unclear. We performed a genome-wide association study for long COVID including up to 6,450 long COVID cases and 1,093,995 population controls from 24 studies across 16 countries. We discovered an association of FOXP4 with long COVID, independent of its previously identified association with severe COVID-19. The signal was replicated in 9,500 long COVID cases and 798,835 population controls. Given the transcription factor FOXP4’s role in lung physiology and pathology, our findings highlight the importance of lung function in the pathophysiology of long COVID.

Courses of SARS-CoV-2 infections are highly variable, ranging from asymptomatic to lethal COVID-19. Though research has shown that host genetic factors contribute to this variability, cohort-based joint analyses of variants from the entire allelic spectrum in individuals with confirmed SARS-CoV-2 infections are still lacking. Here, we present the results of whole genome sequencing in 1,220 mainly vaccine-naïve individuals with confirmed SARS-CoV-2 infection, including 827 hospitalized COVID-19 cases. We observed the presence of autosomal-recessive or likely compound heterozygous monogenic disorders in six individuals, all of which were hospitalized and significantly younger than the rest of the cohort. We did not observe any suggestive causal variants in or around the established risk gene TLR7. Burden testing in the largest population subgroup (i.e., Europeans) suggested nominal enrichments of rare variants in coding and non-coding regions of interferon immune response genes in the overall analysis and male subgroup. Case-control analyses of more common variants confirmed associations with previously reported risk loci, with the key locus at 3p21 reaching genome-wide significance. Polygenic scores accurately captured risk in an age-dependent manner. By enabling joint analyses of different types of variation across the entire frequency spectrum, this data will continue to contribute to the elucidation of COVID-19 etiology.

Orofacial clefting (OFC) is a frequent congenital anomaly and can occur either in the context of underlying syndromes or in isolation (nonsyndromic). The two common OFC phenotypes are cleft lip with/without cleft palate (CL/P) and cleft palate only (CPO). In this study, we searched for penetrant CL/P genes, by evaluating de novo copy number variants (CNV) from an exome sequencing dataset of 50 nonsyndromic patient-parent trios. We detected a heterozygous 86 kb de novo deletion affecting exons 4–11 of ZFHX4, a gene previously associated with OFC. Genetic and phenotypic data from our in-house and the AGORA cohort (710 and 229 individuals with nonsyndromic CL/P) together with literature and database reviews demonstrate that ZFHX4 variants can lead to both nonsyndromic and syndromic forms not only of CL/P but also CPO. Expression analysis in published single-cell RNA-sequencing data (mouse embryo, zebrafish larva) at relevant time-points support an important role of Zfhx4/zfhx4 in craniofacial development. To characterize the role of zfhx4 in zebrafish craniofacial development, we knocked out/down the zebrafish orthologue. Cartilage staining of the zfhx4 CRISPR F0 knockout and morpholino knockdown at 4 days post-fertilization showed an underdeveloped and abnormally shaped ethmoid plate and cartilaginous jaw (resembling micrognathia). While there is evidence for the dominant inheritance of ZFHX4 variants in OFC, we here present a patient with a possible recessive inheritance. In conclusion, ZFHX4 has a highly heterogeneous phenotypic spectrum and variable mode of inheritance. Our data highlight that ZFHX4 should be considered in genetic testing in patients with nonsyndromic clefting.

Article Bonn: Prof. Dr. rer. nat. Kerstin U. Ludwig W2-Professur Immungenetik was published on December 31, 2024 in the journal Medizinische Genetik (volume 36, issue 4).

Non-syndromic cleft lip with/without cleft palate (nsCL/P) is one of the most common birth defects and has a multifactorial etiology. To date, over 45 loci harboring common risk variants have been identified. However, the effector genes at these loci, and the cell types that are affected by risk alleles, remain largely unknown. To address this, we combined genetic data from an nsCL/P genome-wide association study (GWAS) with single-cell RNA sequencing data obtained from the heads of unaffected human embryos. Using the recently developed single-cell disease relevance score (scDRS) approach, we identified two major cell types involved in nsCL/P development, namely the epithelium and the HAND2+ pharyngeal arches (PA). Combining scDRS with co-expression networks and differential gene expression analysis, we prioritized nsCL/P candidate genes, some of which were additionally supported by GWAS data (e.g., CTNND1, PRTG, RPL35A, RAB11FIP1, KRT19). Our results suggest that specific epithelial and PA sub-cell types are involved in nsCL/P development, and harbor a substantial fraction of the genetic risk for nsCL/P.