Genes in Development

Short version. A small number of cell-cell signalling pathways pattern the embryo: WNT/β-catenin, Hedgehog (Sonic, Indian, Desert), Notch, BMP/TGF-β, FGF, and Hippo. The pathways were uncovered by genetic screens in Drosophila — Nüsslein-Volhard & Wieschaus 1980, Nature 287:795. The same components, often the same gene names, drive vertebrate development and a substantial fraction of human congenital malformation genetics. Examples: SHH haploinsufficiency causes holoprosencephaly; FGFR3 gain-of-function causes achondroplasia and its severity series; the RASopathies are germline activating mutations in RAS–MAPK; FBN1 mutations dysregulate TGF-β signalling and cause Marfan syndrome; NOTCH3 mutations cause CADASIL.

The Drosophila genetic screen and what it found

In the late 1970s, Christiane Nüsslein-Volhard and Eric Wieschaus, working at the European Molecular Biology Laboratory in Heidelberg, decided that the way to understand early embryonic development was to break it systematically with mutations and look at the cuticle phenotypes of the resulting embryos. The screen took years and was deliberately designed to be saturating — to find every gene whose loss disturbed segmentation. Their 1980 Nature paper (287:795) grouped the segmentation mutants into three classes:

Gap genes: deletion of contiguous body segments. Examples: Krüppel, knirps, hunchback, giant.
Pair-rule genes: deletion of every other segment. Examples: even-skipped, fushi tarazu, hairy, runt.
Segment-polarity genes: disturbed polarity within each segment. Examples: wingless, hedgehog, engrailed, patched.

Edward Lewis at Caltech, working in parallel on the bithorax complex, established that homeotic genes specify the identity of body segments. The 1995 Nobel Prize in Physiology or Medicine went jointly to all three. The reason this matters in clinical genetics is that the genes named in the fly screens turned out to be the master regulators of vertebrate development: wingless → the WNT family; hedgehog → SHH, IHH, DHH; decapentaplegic → BMP4 and the BMP / TGF-β family. The pathways that build a fly build a human.

The deep review of how these pathways translate into mammalian developmental biology is Pierpont, Bezuidenhout et al. 2018 (Cell 174:1127) on signalling and developmental disease.

The pathways, briefly

WNT/β-catenin. Secreted WNT ligands (19 in humans) bind Frizzled and LRP co-receptors, stabilising cytoplasmic β-catenin (CTNNB1), which translocates to the nucleus and partners with TCF/LEF to activate target transcription. WNT is implicated in axial patterning, limb development (WNT7A in dorsal–ventral limb polarity), and adult stem-cell homeostasis. Human disease examples: WNT7A mutations cause Fuhrmann syndrome and Schinzel phocomelia; WNT3 mutations cause tetra-amelia; LRP5 mutations cause osteoporosis-pseudoglioma syndrome.

Hedgehog. Three vertebrate ligands — Sonic (SHH), Indian (IHH), Desert (DHH). Ligand binding to Patched (PTCH1) relieves inhibition of Smoothened (SMO), activating GLI-family transcription factors. Loss-of-function in the pathway causes midline forebrain defects (holoprosencephaly); gain-of-function via PTCH1 loss causes basal cell carcinoma and the SHH-subgroup medulloblastomas (Foulkes et al. 2008, Nat Rev Cancer 8:851).

Notch. Single-pass transmembrane receptors (NOTCH1–4) activated by membrane-bound Delta-like and Jagged ligands on neighbouring cells. Activation triggers γ-secretase cleavage of the Notch intracellular domain, which translocates to the nucleus and partners with RBP-J to drive transcription. Notch is the canonical pathway of lateral inhibition. Disease examples: JAG1 haploinsufficiency causes Alagille syndrome; NOTCH3 stereotyped cysteine-altering missense variants cause CADASIL (cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy); NOTCH1 activating mutations are the commonest somatic driver in T-cell acute lymphoblastic leukaemia.

BMP / TGF-β. Bone morphogenetic proteins (BMP2, BMP4, BMP7 etc.) and TGF-β isoforms signal through type-I and type-II serine/threonine kinase receptors and SMAD transcription factors. BMP4 is critical for craniofacial and limb development. FBN1 mutations cause Marfan syndrome; the mechanism is now understood as dysregulated TGF-β signalling rather than purely structural connective-tissue defect. TGFBR1, TGFBR2, SMAD3, TGFB2, TGFB3: Loeys-Dietz syndrome.

FGF. 22 ligands and four receptors (FGFR1–4) in humans. Tyrosine-kinase signalling that drives limb-bud outgrowth, craniofacial development, and skeletal patterning. FGFR3 activating mutations are the textbook example of a gain-of-function developmental pathway disorder — see Skeletal dysplasias below. FGFR2 mutations cause Apert, Crouzon, and Pfeiffer syndromes.

Hippo. The most recently characterised of the major pathways. The MST1/2 and LATS1/2 kinase cascade phosphorylates YAP/TAZ, restricting their nuclear activity and limiting growth. Hippo controls organ size and is dysregulated in mesothelioma and other cancers; germline disease is rarer but emerging.

Holoprosencephaly — SHH and friends

Holoprosencephaly (HPE) is the most common congenital forebrain malformation, with a population prevalence in live-born infants of approximately 1 in 16,000 but a much higher prevalence among first-trimester miscarriages. The clinical spectrum runs from alobar HPE (no separation of the cerebral hemispheres, severe midline facial malformation) through semilobar and lobar forms to the microform that may be limited to a single central incisor.

HPE is genetically heterogeneous. The first major locus identified was SHH on 7q36.3: Roessler et al. 1996 (Nat Genet 14:357) reported heterozygous loss-of-function mutations in SHH as a cause of autosomal dominant HPE with reduced penetrance and variable expressivity. Other HPE genes include ZIC2 (13q32.3), SIX3 (2p21), TGIF1 (18p11.31), FGFR1, FGF8, GLI2, DISP1, NODAL, and others. Most heterozygous carriers in HPE families have mild or absent features, and the pedigrees show classic autosomal dominant transmission with reduced penetrance — a teaching example of why pedigree-pattern recognition is suggestive rather than diagnostic.

The mechanism is haploinsufficiency. One functional SHH allele is not enough to specify ventral midline forebrain identity at the right time. The same gene that builds the forebrain also patterns the limb (zone of polarising activity, ZPA), the gut, and the spinal cord; the developmental restriction of the HPE phenotype to the forebrain reflects the compensatory contributions of IHH and DHH in other tissues.

Skeletal dysplasias — FGFR3 gain-of-function

The FGFR3 series is the textbook example of how the same gene produces a graded series of phenotypes through different gain-of-function mutations:

Hypochondroplasia: mild short-limbed dwarfism. Most often caused by FGFR3 p.Asn540Lys.
Achondroplasia: the commonest form of disproportionate short stature. ~98% of cases are caused by a single recurrent mutation, FGFR3 p.Gly380Arg (c.1138G>A or rarely c.1138G>C) in the transmembrane domain. ~80% of cases are de novo, with a strong paternal-age effect. See our dedicated achondroplasia pedigree page.
Thanatophoric dysplasia: lethal in utero or in the neonatal period. Two types, both caused by stronger gain-of-function alleles — type I by FGFR3 p.Arg248Cys and other extracellular cysteine substitutions, type II by FGFR3 p.Lys650Glu in the kinase domain.
Muenke syndrome: craniosynostosis. Caused by the recurrent FGFR3 p.Pro250Arg mutation.

The mechanism is constitutive receptor activation: the wild-type receptor signals only in the presence of FGF ligand, but the mutant receptor signals constitutively, driving inappropriate downstream MAPK and STAT activation in chondrocytes and inhibiting their proliferation. This is a textbook gain-of-function mechanism, and it is why the inheritance pattern is autosomal dominant rather than recessive: one mutant allele is sufficient to produce the phenotype, and homozygous achondroplasia is lethal.

RASopathies — germline activation of RAS–MAPK

The RASopathies are a group of clinically related developmental syndromes caused by germline mutations in components of the RAS–MAPK signalling cascade. The shared mechanism is mild constitutive activation of RAS–MAPK signalling during development — the same pathway that, when activated more strongly somatically, drives cancer. Roberts & Tartaglia 2010 (Curr Opin Pediatr 22:587) is the canonical clinical review. The principal entities and their genes:

Noonan syndrome: PTPN11 (~50%), SOS1 (~13%), RAF1 (~5–10%), RIT1, KRAS, NRAS, BRAF, MAP2K1, LZTR1, SOS2.
Noonan syndrome with multiple lentigines (LEOPARD syndrome): PTPN11 (different alleles from Noonan), RAF1, BRAF.
Cardiofaciocutaneous (CFC) syndrome: BRAF, MAP2K1, MAP2K2, KRAS.
Costello syndrome: HRAS.
Neurofibromatosis type 1: NF1, encoding a RAS-GAP.
Legius syndrome: SPRED1.
Noonan-like / loose anagen hair: SHOC2 p.Ser2Gly.
Noonan syndrome with multiple giant cell lesions and CBL syndrome: CBL.

Common features include short stature, cardiac defects (pulmonary valve stenosis classic in Noonan, hypertrophic cardiomyopathy in LEOPARD and Costello), craniofacial appearance (downslanting palpebral fissures, low-set ears, webbed neck), cryptorchidism, and a graded predisposition to specific malignancies that depends on the gene. Costello syndrome (HRAS) carries a meaningful childhood cancer risk; Noonan syndrome with PTPN11 mutations has a low but elevated risk of juvenile myelomonocytic leukaemia.

The RASopathies are useful teaching cases because they illustrate that activating mutations in the same pathway produce overlapping but distinguishable phenotypes — the precise gene matters, and the precise allele within the gene also matters (Noonan and LEOPARD PTPN11 alleles are different).

Marfan syndrome and Loeys-Dietz — TGF-β dysregulation

Marfan syndrome was historically considered a connective-tissue disorder — FBN1, encoding fibrillin-1, was identified as the gene in 1991. The understanding shifted in the 2000s: fibrillin-1 sequesters latent TGF-β complexes in the extracellular matrix, and FBN1 mutations release latent TGF-β, increasing TGF-β signalling. Loeys-Dietz syndrome — caused by mutations in TGF-β receptors and SMADs (TGFBR1, TGFBR2, SMAD3, TGFB2, TGFB3) — presents with overlapping aortopathy and skeletal features. The RASopathies, Marfan, and Loeys-Dietz are sometimes grouped together as TGF-β / RAS–MAPK signalling disorders to emphasise the shared mechanism.

CADASIL — NOTCH3

CADASIL is the most common monogenic cause of stroke and vascular dementia. The disease is caused by stereotyped missense variants in NOTCH3 that add or remove a cysteine residue in one of the EGF-like repeats of the extracellular domain, leading to abnormal NOTCH3 ectodomain accumulation in vascular smooth muscle cells. The unusual mechanism — pathogenic only when the substitution affects an odd cysteine count — makes NOTCH3 variant interpretation a teaching example of the limits of generic missense-deleteriousness predictors.

Hedgehog and medulloblastoma

The Hedgehog pathway sits at the intersection of developmental biology and cancer. PTCH1 germline loss-of-function causes Gorlin (nevoid basal cell carcinoma) syndrome, with predisposition to basal cell carcinoma and SHH-subgroup medulloblastoma. The same pathway is somatically activated in approximately 30% of medulloblastomas, defining the SHH subgroup. Foulkes et al. 2008 (Nat Rev Cancer 8:851) reviews the Hedgehog cancer-predisposition story. The pathway is druggable at SMO (vismodegib, sonidegib in BCC and cancer trials); we cite this neutrally as research literature.

Mechanism categories — haploinsufficiency, dominant negative, gain-of-function

Inheritance pattern alone does not tell you the molecular mechanism; the mechanism shapes the predicted recurrence and the variant interpretation. The four categories that matter most in developmental disease:

Haploinsufficiency — one functional allele is not enough. Examples: SHH in HPE; JAG1 in Alagille; FBN1 in Marfan (mostly); NF1 in NF1. Pedigree pattern: autosomal dominant. Loss-of-function variants are pathogenic.
Dominant negative — the mutant protein interferes with the wild-type. Examples: many COL1A1 / COL1A2 variants in osteogenesis imperfecta; FBN1 exon-skipping alleles. Loss-of-function variants may be milder than missense.
Gain-of-function — the mutant protein has a new or excess activity. Examples: FGFR3 in achondroplasia; the RASopathies; SOS1; HRAS in Costello. Loss-of-function variants produce a different phenotype or none.
Recessive loss-of-function — both alleles must be lost. Examples: most metabolic disease; many ciliopathies; some forms of skeletal dysplasia.

The mechanism is the diagnostic clue you take from the pedigree to the variant filter: HPE in a parent and child with reduced penetrance suggests autosomal dominant haploinsufficiency, which raises the prior on rare loss-of-function variants in SHH, ZIC2, SIX3, or TGIF1. Achondroplasia in a child of unaffected parents suggests a de novo gain-of-function variant, which raises the prior on the recurrent FGFR3 p.Gly380Arg.

Pedigree-level reading

Developmental signalling pathway disorders give characteristic pedigree patterns. Drawing fictional examples in Evagene's pedigree drawing tool is standard teaching practice:

Achondroplasia: typically a single affected child of unaffected parents (de novo dominant); when transmitted, classic autosomal dominant with one affected parent and ~50% of offspring affected. See achondroplasia pedigree.
Holoprosencephaly: variable expressivity within a family — one severely affected child and a parent with only a single central incisor, microcephaly, or no apparent features.
Noonan syndrome: classic autosomal dominant pedigrees; some SOS1-positive families have multiple mildly affected adults across generations. LZTR1 can act either dominantly or recessively, a useful teaching subtlety.
Marfan syndrome: vertical transmission across generations with variable severity; ~25% of cases are de novo.
CADASIL: late-onset autosomal dominant stroke / dementia; classically presents in middle age, so pedigrees often show several affected adults across two or three generations with no childhood phenotype.
Gorlin syndrome (PTCH1): autosomal dominant with cancer predisposition; multiple affected family members across generations.

The Mendelian inheritance calculator is useful for walking students through offspring probabilities under reduced penetrance; the germline mosaicism calculator covers the special case of apparent de novo dominance with recurrence in siblings (relevant in a small but real fraction of achondroplasia and other dominant skeletal-dysplasia families). Outputs are illustrative for teaching, not clinical.

Key references

Nüsslein-Volhard C, Wieschaus E. Mutations affecting segment number and polarity in Drosophila. Nature 1980; 287:795–801. PMID 6776413.
Roessler E, Belloni E, Gaudenz K, et al. Mutations in the human Sonic Hedgehog gene cause holoprosencephaly. Nat Genet 1996; 14:357–360. PMID 8896572.
Roberts AE, Allanson JE, Tartaglia M, Gelb BD. Noonan syndrome (and the RASopathies). Curr Opin Pediatr 2010; 22:587–592. PMID 20736950.
Foulkes WD. Inherited susceptibility to common cancers. Nat Rev Cancer 2008; 8:851–860. PMID 18948956.
Pierpont ME et al. Genetic causes of congenital malformation and developmental signalling. Cell 2018; 174:1127. PMID 30142344.
Joutel A, Corpechot C, Ducros A, et al. Notch3 mutations in CADASIL. Nature 1996; 383:707–710. PMID 8878478.
The Nobel Assembly. Nobel Prize in Physiology or Medicine 1995 (Nüsslein-Volhard, Wieschaus, Lewis). nobelprize.org.
OMIM: HPE3 / SHH 142945; achondroplasia 100800; Noonan syndrome 163950; Marfan syndrome 154700; CADASIL 125310.

Frequently asked questions

Why do flies and humans share developmental genes?

Because the genes were already in the common ancestor. Bilaterian metazoa share a small core of cell-cell signalling pathways — WNT, Hedgehog, Notch, BMP/TGF-β, FGF, Hippo — that have been retained because they work. The Nüsslein-Volhard–Wieschaus screen was reverse-engineered into clinical genetics once the homology of fly and vertebrate genes was recognised.

Why does the same pathway cause both birth defects and cancer?

Developmental signalling pathways tell cells when to divide and what fate to adopt. Germline perturbations during development cause a structural birth defect; somatic perturbations in adult tissues cause inappropriate proliferation. Hedgehog (HPE versus medulloblastoma / BCC) and RAS–MAPK (RASopathies versus the same pathway driving cancers somatically) are the textbook examples.

What is the difference between haploinsufficiency and dominant negative?

Haploinsufficiency: one functional copy is not enough; loss-of-function variants are pathogenic. Dominant negative: the mutant protein actively interferes with the wild-type, often through dimerisation; complete loss of the mutant allele can be milder than a missense variant. The distinction is important when interpreting variants in genes like COL1A1 or FBN1.

Why does FGFR3 produce a graded series of phenotypes?

Because the strength of the gain-of-function mutation varies. Hypochondroplasia mutations cause mild constitutive receptor activation; achondroplasia mutations cause moderate activation; thanatophoric dysplasia mutations cause strong activation that is lethal in utero or neonatally. The same gene, the same mechanism category, but a graded molecular consequence.

Are the RASopathies cancer syndromes?

Some are, to a varying degree. Costello syndrome (HRAS) carries a meaningful childhood cancer risk; Noonan syndrome with PTPN11 mutations has an elevated risk of juvenile myelomonocytic leukaemia. NF1 is a defined cancer-predisposition syndrome. We cover the literature here for context; clinical surveillance pathways are designed by clinicians with their patients.

How does pedigree drawing help in developmental signalling disorders?

A pedigree records who is affected, who has subtle features, and who carries the trait through to the next generation. With reduced penetrance, variable expressivity, and the possibility of de novo dominance, the pedigree is the integrating diagram for the family. Drawing fictional examples in Evagene's pedigree drawing tool with NSGC notation is standard teaching practice.