Trio exome sequencing of 100 patients with severe intellectual disability (IQ below 50) established de novo coding mutations as a major cause of the disorder but left 84% of patients without a molecular diagnosis (de Ligt et al., New England Journal of Medicine, 2012). The limitations of that exome study design — pathogenic variation outside captured exons, mosaic mutations, digenic/oligogenic inheritance, still-undescribed disease genes, and inheritance modes in populations beyond this outbred Dutch cohort — frame a single surviving research objective: to produce a validated accounting of the unsolved fraction of severe intellectual disability by determining and confirming the contribution of non-exonic (deep intronic, regulatory, and capture-missed structural) variants, mosaic (post-zygotic and parental) mutations, and non-monogenic architectures, together with discovery of the remaining recurrently mutated genes needed for evidence-based causal assignments. The inheritance-mode limitation has since been largely answered: exome sequencing of consanguineous families established autosomal recessive inheritance as a major contributor in those referral populations (Reuter et al., JAMA Psychiatry 2017; Harripaul et al., Molecular Psychiatry 2018), so that thread is retained here only as background and sampling context. A decade of whole-genome sequencing and cohort studies (the Deciphering Developmental Disorders study and the 100,000 Genomes Project) has partially quantified the remaining classes but, as of 2026, roughly half to two-thirds of patients remain undiagnosed, low-level mosaicism detection is still an analytical frontier, systematic oligogenic analyses have yet to confirm any definitive variant combination, and modelling suggests more than 1,000 developmental-disorder genes remain undescribed. The problem is open.

Background

Intellectual disability (ID), defined as an IQ below 70 with onset of impairment during development, affects roughly 1% of the population; the severe form (IQ below 50) affects about 0.5% of newborns and is thought to be largely genetic in origin, but its extreme clinical and genetic heterogeneity long kept most cases undiagnosed. Before 2012, known causes included X-linked gene variants traceable through family pedigrees, autosomal recessive variants exposed by homozygosity mapping (locating disease variants through chromosomal segments inherited identically from both parents) in consanguineous (cousin-marriage) populations — for example the discovery of 50 recessive cognitive-disorder genes in Iranian families (Najmabadi et al., Nature 2011) — and copy-number variants (CNVs, deletions or duplications of chromosomal segments) detected by microarrays. Exome sequencing, which reads only the protein-coding ~1-2% of the genome, made unbiased trio analysis possible: sequencing an affected child and both unaffected parents pinpoints de novo mutations (present in the child but not either parent), which microarray and candidate-gene approaches had systematically missed. De Ligt et al. (NEJM 2012) applied trio exome sequencing to 100 patients with severe ID of unknown cause, confirmed candidates by Sanger sequencing (the gold-standard orthogonal validation of the era), and reached a diagnostic yield of 16%, mostly from de novo mutations in known and novel genes, while finding no autosomal recessive diagnoses in this outbred Dutch cohort. These design limitations defined the next decade's agenda: the study interrogated only coding sequence, so variants outside captured exons (deep intronic variants affecting splicing, regulatory variants altering when and where genes are active, and structural variants invisible to exome capture) were missed; standard-depth trio data with Sanger confirmation could not detect mosaic mutations (post-zygotic mutations present in only a fraction of cells of the patient or of a parent, quantified by the variant allele fraction, VAF — the share of sequencing reads carrying the variant); and the analysis assumed monogenic causation, leaving digenic or oligogenic inheritance (pathogenic effects requiring damaging variants in two or more genes jointly) untested. Subsequent work transformed the technical substrate: whole-genome sequencing (WGS) removed capture bias and made structural, mitochondrial, and noncoding variants callable — the same Nijmegen group diagnosed 21 of 50 exome-negative severe-ID patients by WGS (Gilissen et al., Nature 2014), and the UK 100,000 Genomes Project pilot achieved a 25% diagnostic yield overall across rare diseases, with yields of 40-55% for intellectual disability, hearing disorders, and vision disorders (NEJM 2021). Large trio cohorts such as the Deciphering Developmental Disorders (DDD) study showed that about 42% of patients carry pathogenic de novo coding mutations and that diagnostic yield depends on sex, parental age, and parental relatedness (Nature 2017), and identified dozens of new disease genes by pooling research and clinical data (Kaplanis et al., Nature 2020). Yet the accounting is deliberately incomplete: de novo mutations in fetal-brain-active regulatory elements explain only about 1-3% of patients lacking a coding diagnosis (Short et al., Nature 2018), 6.5% of presumed germline de novo mutations are actually post-zygotic mosaic events (Acuna-Hidalgo et al., AJHG 2015), and a systematic exome-wide oligogenic analysis in a related neurodevelopmental phenotype could not definitively confirm a single variant combination (Duerinckx et al., Epilepsia 2026). The 2024 discovery that recurrent variants in the noncoding small RNA gene RNU4-2 cause one of the most frequent neurodevelopmental syndromes — found only through reanalysis of genome-sequenced cohorts — demonstrated both that the noncoding space still hides major causes and that roughly 60% of patients remain undiagnosed after comprehensive testing.

Problem Statement

Despite trio exome and now whole-genome sequencing, a large fraction of individuals with severe intellectual disability (IQ below 50) ascertained in diagnostic referral populations — roughly half to two-thirds, spanning outbred and consanguineous ancestries — still receive no molecular diagnosis. For this undiagnosed fraction, determine and validate the contribution of the variant classes that the original exome study could not assess: (i) pathogenic variants outside captured exons, including deep intronic and regulatory variants and structural variants missed by exome capture; (ii) mosaic (post-zygotic and parental) mutations, including low allele-fraction events; and (iii) digenic/oligogenic inheritance and other non-monogenic architectures. These classes are defined by different variant properties — genomic location, mosaicism (post-zygotic or parental), and the number of loci jointly required — which can co-occur: a post-zygotic deep intronic variant belongs to both (i) and (ii), and a digenic architecture (iii) may consist entirely of coding variants. The classes are therefore not mutually exclusive, and the per-class contribution estimates are not required to form a disjoint partition of the unsolved fraction: each estimate is the share of unsolved patients for which an evidence-based causal assignment to that class is made, and a patient whose assignment exhibits properties of more than one class is counted under each class concerned. The objective is a validated accounting of the unsolved fraction of severe intellectual disability — per-class contribution estimates with evidence-based causal assignments to individual patients, alongside discovery of the remaining recurrently mutated genes needed to make such assignments. Resolution requires the three per-class contribution estimates with their evidence-based causal assignments and stated uncertainty; discovery of recurrently mutated genes is reported only to the extent needed to justify each claimed assignment, and no exhaustive inventory of the remaining undescribed genes is required.

References

  1. de Ligt J, Willemsen MH, van Bon BWM, Kleefstra T, Yntema HG, Kroes T, Vulto-van Silfhout AT, Koolen DA, de Vries P, Gilissen C, del Rosario M, Hoischen A, Scheffer H, de Vries BBA, Brunner HG, Veltman JA, Vissers LELM. Diagnostic exome sequencing in persons with severe intellectual disability. New England Journal of Medicine 2012;367(20):1921-1929. doi:10.1056/NEJMoa1206524. https://doi.org/10.1056/NEJMoa1206524
  2. Najmabadi H, Hu H, Garshasbi M, Zemojtel T, Abedini SS, Chen W, Hosseini M, Behjati F, Haas S, Jamali P, Zecha A, Mohseni M, Püttmann L, Nouri Vahid L, Jensen C, Abbasi Moheb L, Bienek M, Larti F, Mueller I, Weissmann R, Darvish H, Wrogemann K, Hadavi V, Lipkowitz B, Esmaeeli-Nieh S, Wieczorek D, Kariminejad R, Ghasemi Firouzabadi S, Cohen M, Fattahi Z, Rost I, Mojahedi F, Hertzberg C, Dehghan A, Rajab A, Soltani Banavandi MJ, Hoffer J, Falah M, Musante L, Kalscheuer V, Ullmann R, Kuss AW, Tzschach A, Kahrizi K, Ropers HH. Deep sequencing reveals 50 novel genes for recessive cognitive disorders. Nature 2011;478(7367):57-63. doi:10.1038/nature10423. https://doi.org/10.1038/nature10423
  3. Gilissen C, Hehir-Kwa JY, Thung DT, van de Vorst M, van Bon BWM, Willemsen MH, Kwint M, Janssen IM, Hoischen A, Schenck A, Leach R, Klein R, Tearle R, Bo T, Pfundt R, Yntema HG, de Vries BBA, Kleefstra T, Brunner HG, Vissers LELM, Veltman JA. Genome sequencing identifies major causes of severe intellectual disability. Nature 2014;511(7509):344-347. doi:10.1038/nature13394. https://doi.org/10.1038/nature13394
  4. Acuna-Hidalgo R, Bo T, Kwint MP, van de Vorst M, Pinelli M, Veltman JA, Hoischen A, Vissers LELM, Gilissen C. Post-zygotic point mutations are an underrecognized source of de novo genomic variation. American Journal of Human Genetics 2015;97(1):67-74. doi:10.1016/j.ajhg.2015.05.008. https://doi.org/10.1016/j.ajhg.2015.05.008
  5. Deciphering Developmental Disorders Study. Prevalence and architecture of de novo mutations in developmental disorders. Nature 2017;542(7642):433-438. doi:10.1038/nature21062. https://doi.org/10.1038/nature21062
  6. Reuter MS, Tawamie H, Buchert R, Hosny Gebril O, Froukh T, Thiel C, Uebe S, Ekici AB, Krumbiegel M, Zweier C, Hoyer J, Eberlein K, Bauer J, Scheller U, Strom TM, Hoffjan S, Abdelraouf ER, Meguid NA, Abboud A, Al Khateeb MA, Fakher M, Hamdan S, Ismael A, Muhammad S, Abdallah E, Sticht H, Wieczorek D, Reis A, Abou Jamra R. Diagnostic yield and novel candidate genes by exome sequencing in 152 consanguineous families with neurodevelopmental disorders. JAMA Psychiatry 2017;74(3):293-299. doi:10.1001/jamapsychiatry.2016.3798. https://doi.org/10.1001/jamapsychiatry.2016.3798
  7. Harripaul R, Vasli N, Mikhailov A, Rafiq MA, Mittal K, Windpassinger C, Sheikh TI, Noor A, Mahmood H, Downey S, Johnson M, Vleuten K, Bell L, Ilyas M, Khan FS, Khan V, Moradi M, Ayaz M, Naeem F, Heidari A, Ahmed I, Ghadami S, Agha Z, Zeinali S, Qamar R, Mozhdehipanah H, John P, Mir A, Ansar M, French L, Ayub M, Vincent JB. Mapping autosomal recessive intellectual disability: combined microarray and exome sequencing identifies 26 novel candidate genes in 192 consanguineous families. Molecular Psychiatry 2018;23(4):973-984. doi:10.1038/mp.2017.60. https://doi.org/10.1038/mp.2017.60
  8. Short PJ, McRae JF, Gallone G, Sifrim A, Won H, Geschwind DH, Wright CF, Firth HV, FitzPatrick DR, Barrett JC, Hurles ME. De novo mutations in regulatory elements in neurodevelopmental disorders. Nature 2018;555(7698):611-616. doi:10.1038/nature25983. https://doi.org/10.1038/nature25983
  9. Turner TN, Eichler EE. The role of de novo noncoding regulatory mutations in neurodevelopmental disorders. Trends in Neurosciences 2019;42(2):115-127. doi:10.1016/j.tins.2018.11.002. https://doi.org/10.1016/j.tins.2018.11.002
  10. Kaplanis J, Samocha KE, Wiel L, Zhang Z, Arvai KJ, Eberhardt RY, Gallone G, Lelieveld SH, Martin HC, McRae JF, Short PJ, Torene RI, de Boer E, Danecek P, Gardner EJ, Huang N, Lord J, Martincorena I, Pfundt R, Reijnders MRF, Yeung A, Yntema HG, Deciphering Developmental Disorders Study, Vissers LELM, Juusola J, Wright CF, Brunner HG, Firth HV, FitzPatrick DR, Barrett JC, Hurles ME, Gilissen C, Retterer K. Evidence for 28 genetic disorders discovered by combining healthcare and research data. Nature 2020;586(7831):757-762. doi:10.1038/s41586-020-2832-5. https://doi.org/10.1038/s41586-020-2832-5
  11. 100,000 Genomes Project Pilot Investigators; Smedley D, Smith KR, Martin A, Thomas EA, McDonagh EM, Cipriani V, Ellingford JM, Arno G, Tucci A, Vandrovcova J, Chan G, Williams HJ, Ratnaike T, Wei W, Stirrups K, Ibanez K, Moutsianas L, Wielscher M, Need A, Barnes MR, Vestito L, Buchanan J, Wordsworth S, Ashford S, Rehmström K, Li E, Fuller G, Twiss P, Spasic-Boskovic O, Halsall S, Floto RA, Poole K, Wagner A, Mehta SG, Gurnell M, Burrows N, James R, Penkott C, Dewhurst E, Gräf S, Mapeta R, Kasanicki M, Haworth A, Savage H, Babcock M, Reese MG, Bale M, Baple E, Boustred C, Brittain H, de Burca A, Bleda M, Devereau A, Halai D, Haraldsdottir E, Hyder Z, Kasperaviciute D, Patch C, Polychronopoulos D, Matchan A, Sultana R, Ryten M, Tavares ALT, Tregidgo C, Turnbull C, Welland M, Wood S, Snow C, Williams E, Leigh S, Foulger RE, Daugherty LC, Niblock O, Leong IUS, Wright CF, Davies J, Crichton C, Welch J, Woods K, Abulhoul L, Aurora P, Bockenhauer D, Broomfield A, Cleary MA, Lam T, Dattani M, Footitt E, Ganesan V, Grunewald S, Compeyrot-Lacassagne S, Muntoni F, Pilkington C, Quinlivan R, Thapar N, Wallis C, Wedderburn LR, Worth A, Bueser T, Compton C, Deshpande C, Fassihi H, Haque E, Izatt L, Josifova D, Mohammed S, Robert L, Rose S, Ruddy D, Sarkany R, Say G, Shaw AC, Wolejko A, Habib B, Burns G, Hunter S, Grocock RJ, Humphray SJ, Robinson PN, Haendel M, Simpson MA, Banka S, Clayton-Smith J, Douzgou S, Hall G, Thomas HB, O'Keefe RT, Michaelides M, Moore AT, Malka S, Pontikos N, Browning AC, Straub V, Gorman GS, Horvath R, Quinton R, Schaefer AM, Yu-Wai-Man P, Turnbull DM, McFarland R, Taylor RW, O'Connor E, Yip J, Newland K, Morris HR, Polke J, Wood NW, Campbell C, Camps C, Gibson K, Koelling N, Lester T, Németh AH, Palles C, Patel S, Roy NBA, Sen A, Taylor J, Cacheiro P, Jacobsen JO, Seaby EG, Davison V, Chitty L, Douglas A, Naresh K, McMullan D, Ellard S, Temple IK, Mumford AD, Wilson G, Beales P, Bitner-Glindzicz M, Black G, Bradley JR, Brennan P, Burn J, Chinnery PF, Elliott P, Flinter F, Houlden H, Irving M, Newman W, Rahman S, Sayer JA, Taylor JC, Webster AR, Wilkie AOM, Ouwehand WH, Raymond FL, Chisholm J, Hill S, Bentley D, Scott RH, Fowler T, Rendon A, Caulfield M. 100,000 Genomes Pilot on Rare-Disease Diagnosis in Health Care — Preliminary Report. New England Journal of Medicine 2021;385(20):1868-1880. doi:10.1056/NEJMoa2035790. https://doi.org/10.1056/NEJMoa2035790
  12. Chen Y, Dawes R, Kim HC, Ljungdahl A, Stenton SL, Walker S, Lord J, Lemire G, Martin-Geary AC, Ganesh VS, Ma J, Ellingford JM, Delage E, D'Souza EN, Dong S, Adams DR, Allan K, Bakshi M, Baldwin EE, Berger SI, Bernstein JA, Bhatnagar I, Blair E, Brown NJ, Burrage LC, Chapman K, Coman DJ, Compton AG, Cunningham CA, D'Souza P, Danecek P, Délot EC, Dias KR, Elias ER, Elmslie F, Evans CA, Ewans L, Ezell K, Fraser JL, Gallacher L, Genetti CA, Goriely A, Grant CL, Haack T, Higgs JE, Hinch AG, Hurles ME, Kuechler A, Lachlan KL, Lalani SR, Lecoquierre F, Leitão E, Le Fevre A, Leventer RJ, Liebelt JE, Lindsay S, Lockhart PJ, Ma AS, Macnamara EF, Mansour S, Maurer TM, Mendez HR, Metcalfe K, Montgomery SB, Moosajee M, Nassogne MC, Neumann S, O'Donoghue M, O'Leary M, Palmer EE, Pattani N, Phillips J, Pitsava G, Pysar R, Rehm HL, Reuter CM, Revencu N, Riess A, Rius R, Rodan L, Roscioli T, Rosenfeld JA, Sachdev R, Shaw-Smith CJ, Simons C, Sisodiya SM, Snell P, St Clair L, Stark Z, Stewart HS, Tan TY, Tan NB, Temple SEL, Thorburn DR, Tifft CJ, Uebergang E, VanNoy GE, Vasudevan P, Vilain E, Viskochil DH, Wedd L, Wheeler MT, White SM, Wojcik M, Wolfe LA, Wolfenson Z, Wright CF, Xiao C, Zocche D, Rubenstein JL, Markenscoff-Papadimitriou E, Fica SM, Baralle D, Depienne C, MacArthur DG, Howson JMM, Sanders SJ, O'Donnell-Luria A, Whiffin N. De novo variants in the RNU4-2 snRNA cause a frequent neurodevelopmental syndrome. Nature 2024;632(8026):832-840. doi:10.1038/s41586-024-07773-7. https://doi.org/10.1038/s41586-024-07773-7
  13. Kang S, Oh Y, Kim S. Strategies for mosaic variant calling in brain disorders. Current Opinion in Genetics & Development 2026;100:102518. doi:10.1016/j.gde.2026.102518. https://doi.org/10.1016/j.gde.2026.102518
  14. Duerinckx S, Gravel B, Soblet J, Legros B, Ferrao Santos S, Van Paesschen W, Rikir E, Epi25 Collaborative, Solve-RD DITF-EpiCARE, Aeby A, Smits G, Depondt C. Oligogenic inheritance in epilepsy: a systematic exome-wide analysis. Epilepsia 2026 (online ahead of print, August 5, 2026). doi:10.1002/epi.70397. https://doi.org/10.1002/epi.70397