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Headache Associated with High HCl (Gastric Acid Hypersecretion) - Physiological Explanation

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1. Histamine - The Central Link

• H2 receptors on parietal cells - stimulating HCl secretion (gastric effect)
• H1 receptors on cranial blood vessels - causing vasodilation of cerebral vessels

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2. The Gut-Brain Axis via the Vagus Nerve

• Afferent vagal fibers detect the chemical disturbance
• They transmit signals to the brainstem (nucleus tractus solitarius)
• This triggers autonomic responses including changes in vascular tone, nausea, and pain referral - which can manifest as a headache

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3. Alkaline Tide Effect

• Slight cerebral vasoconstriction/vasodilation shifts
• Hypocapnia-like effects
• Contributes to the sense of fullness, light-headedness, and headache

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4. Referred Pain / Visceral Hypersensitivity

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5. Serotonin Pathway

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Summary Table

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Types of Genes

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1. Based on What the Gene Encodes

A. Protein-Coding Genes

• Enzymes
• Structural proteins
• Receptor proteins
• Regulatory proteins (transcription factors, signaling molecules)

B. Non-Coding RNA (ncRNA) Genes

• Thompson & Thompson Genetics and Genomics in Medicine, 9th Ed

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2. Based on Function / Expression Pattern

A. Structural Genes

B. Regulatory Genes

C. Housekeeping Genes

D. Tissue-Specific (Luxury) Genes

E. Inducible Genes

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3. Based on Role in Cancer (Clinically Important)

A. Proto-Oncogenes

• Normal genes that regulate cell growth and proliferation
• When mutated or overexpressed, they become oncogenes
• Their protein products include: growth factors, growth factor receptors, signal transduction proteins (RAS), transcription factors (MYC)
• Mutations cause gain-of-function - only one allele needs to be mutated (dominant)
• Example: RAS proto-oncogene → mutant ras oncogene (found in lung, colon, bladder cancers)

B. Oncogenes

• Mutated forms of proto-oncogenes that drive uncontrolled cell proliferation
• Activation mechanisms: point mutation, gene amplification, chromosomal translocation
• "A transforming mutation in a proto-oncogene increases the activity or amount of the gene product - a gain-of-function mutation." - Basic Medical Biochemistry, 6th Ed

C. Tumor Suppressor Genes

• Normal genes that suppress cell proliferation and promote apoptosis
• Both alleles must be inactivated for cancer to result ("two-hit hypothesis")
• Loss-of-function mutations
• Examples: TP53 (p53), RB (retinoblastoma), APC, BRCA1/2
• "Tumor-suppressor genes contribute to the development of cancer when both copies of the gene are inactivated." - Basic Medical Biochemistry, 6th Ed

D. DNA Repair Genes (Caretaker Genes)

• Maintain genomic integrity by repairing damaged DNA
• Loss leads to increased mutation rate throughout the genome
• Examples: MLH1, MSH2 (mismatch repair genes - mutated in Lynch syndrome), BRCA1/2

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4. Based on Inheritance Pattern

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5. Based on Gene Family / Evolutionary Relationship

• Multigene families: Groups of genes with similar sequences and related functions (e.g., globin gene family - α-globin and β-globin clusters)
• Pseudogenes: Non-functional gene copies that have lost their ability to be expressed (due to accumulated mutations)
• Paralogues: Genes related by duplication within the same species
• Orthologues: Genes in different species that evolved from a common ancestor

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Summary Diagram (Conceptual)

GENES
├── By product
│   ├── Protein-coding (~20,000 genes)
│   └── Non-coding RNA (rRNA, tRNA, miRNA, lncRNA, snoRNA)
├── By expression
│   ├── Housekeeping (constitutive)
│   ├── Tissue-specific (luxury)
│   └── Inducible
├── By cancer role
│   ├── Proto-oncogenes → Oncogenes (gain-of-function)
│   ├── Tumor suppressor genes (loss-of-function)
│   └── DNA repair / caretaker genes
└── By inheritance
    ├── Autosomal dominant / recessive
    ├── X-linked
    ├── Mitochondrial
    └── Imprinted

---

• Thompson & Thompson Genetics and Genomics in Medicine, 9th Ed - gene structure, ncRNA types
• Basic Medical Biochemistry (Lippincott), 6th Ed - oncogenes, tumor suppressors
• Harper's Illustrated Biochemistry, 32nd Ed - housekeeping/constitutive genes
• Robbins & Kumar Basic Pathology - protein-coding vs non-coding DNA

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Monogenic Disorders

Definition

"Single-gene or monogenic disorders result from mutations that have large functional effects and manifest in families with Mendelian inheritance patterns."

• Robbins, Cotran & Kumar Pathologic Basis of Disease

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Types of Monogenic (Single-Gene) Disorders by Inheritance Pattern

1. Autosomal Dominant (AD)

• Mutation in one allele is sufficient to cause disease (heterozygous state)
• Both males and females are equally affected and can transmit
• Each child of an affected parent has a 50% chance of inheriting the disease
• Typically shows vertical transmission across generations

• Incomplete penetrance: some people carry the gene but show no symptoms (e.g., 50% penetrance = only 50% of carriers are affected)
• Variable expressivity: all carriers are affected, but to different degrees (e.g., neurofibromatosis type 1 - ranges from skin spots to tumors)
• De novo mutations: new germline mutations can cause AD disease with no family history, especially with advanced paternal age

• Huntington disease (HTT gene - CAG repeat expansion)
• Marfan syndrome (FBN1 gene - fibrillin)
• Familial hypercholesterolemia (LDLR gene)
• Neurofibromatosis type 1 (NF1 gene)
• Achondroplasia (FGFR3 gene)

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2. Autosomal Recessive (AR)

• Both alleles must be mutated for disease to manifest
• Parents are typically unaffected carriers (heterozygotes)
• Risk in offspring of two carriers: 25% affected, 50% carrier, 25% normal
• Often appears horizontally in a pedigree (siblings affected, parents not)
• More common with consanguinity

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3. X-Linked Recessive (XLR)

• Gene is on the X chromosome; males (XY) are hemizygous - one mutant copy is enough to cause disease
• Females (XX) are usually carriers - protected by their second normal X allele
• No father-to-son transmission (fathers give Y to sons)
• Affected males transmit the carrier state to all daughters, but to no sons

• Robbins, Cotran & Kumar Pathologic Basis of Disease

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4. X-Linked Dominant (XLD)

• Mutation on X chromosome causes disease in both males and females in heterozygous state
• Affected father → all daughters affected, no sons affected
• Affected mother → 50% daughters and 50% sons affected
• Pedigree resembles AD but with no father-to-son transmission

• Vitamin D-resistant rickets (X-linked hypophosphatemia)
• Alport syndrome (some forms)
• Rett syndrome (MECP2 - lethal in males, affects girls)

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5. Mitochondrial Inheritance

• Genes encoded in mitochondrial DNA (mtDNA)
• Transmitted exclusively through the mother (maternal inheritance) - only egg contributes mitochondria
• Affects both sexes, but an affected father CANNOT pass it to children
• Affects tissues with high energy demand - muscle, brain, eye

• MELAS (mitochondrial encephalomyopathy, lactic acidosis)
• Leber's hereditary optic neuropathy (LHON)
• MERRF (myoclonic epilepsy with ragged-red fibers)

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Summary: Key Distinguishing Features

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Related Concepts

• Codominance: Both alleles contribute to phenotype (e.g., ABO blood groups, HbAS in sickle cell trait)
• Anticipation: Severity increases / onset decreases in successive generations due to expanding trinucleotide repeats (e.g., Huntington, Myotonic dystrophy)
• Imprinting: Only one parental allele is expressed; disruption causes disease depending on which parent's copy is affected (e.g., Prader-Willi vs. Angelman syndrome, both involve chromosome 15)

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Types of Genes

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1. Based on What the Gene Encodes

A. Protein-Coding Genes

• Transcribed into mRNA → translated into protein
• ~20,000 protein-coding genes in the human genome
• Products: enzymes, structural proteins, receptors, regulatory proteins
• Through alternative splicing and post-translational modification, these ~20,000 genes produce hundreds of thousands of different proteins (the proteome)

B. Non-Coding RNA (ncRNA) Genes

• Thompson & Thompson Genetics and Genomics in Medicine, 9th Ed

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2. Based on Expression Pattern

• Harper's Illustrated Biochemistry, 32nd Ed

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3. Based on Function

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4. Based on Role in Cancer (Clinically Most Important)

"A transforming mutation in a proto-oncogene increases the activity or amount of the gene product - a gain-of-function mutation. Tumor-suppressor genes... a transforming mutation results in a loss of activity."

• Basic Medical Biochemistry (Lippincott), 6th Ed

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5. Based on Inheritance Pattern

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6. Based on Gene Family / Evolutionary Relationship

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Quick Visual Summary

GENES
├── By product
│   ├── Protein-coding (~20,000)
│   └── Non-coding RNA (rRNA, tRNA, miRNA, lncRNA, snoRNA, snRNA)
├── By expression
│   ├── Housekeeping (always ON)
│   ├── Tissue-specific (cell-type restricted)
│   └── Inducible (stimulus-dependent)
├── By cancer role
│   ├── Proto-oncogenes → Oncogenes (gain-of-function, 1 hit)
│   ├── Tumor suppressor genes (loss-of-function, 2 hits)
│   └── DNA repair / caretaker genes
└── By inheritance
    ├── Autosomal dominant / recessive
    ├── X-linked (recessive / dominant)
    ├── Mitochondrial
    └── Imprinted

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Autosomal Recessive (AR) Disease

Definition

"Both alleles of an autosomal gene must be mutated in order to produce a phenotypic manifestation."

• Robbins, Cotran & Kumar Pathologic Basis of Disease

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Key Characteristics

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Pedigree Pattern (Horizontal Transmission)

   Cc  ×  Cc       (C = normal allele, c = mutant allele)
 (carrier) (carrier)

Offspring:
  CC (25%) - normal
  Cc (50%) - carrier (unaffected)
  cc (25%) - AFFECTED

• Parents are clinically normal
• Affected individuals appear in one generation (horizontal), often separated by unaffected generations
• No vertical (generation-to-generation) transmission like in AD

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Molecular Basis

Why does one normal allele compensate?

"Many of the mutated genes encode enzymes. In heterozygote carriers, cells with half the usual complement of the enzyme function normally."

• Robbins Pathology

Genotype possibilities in affected individuals

1. Homozygous: Inherited the same mutant allele from both parents (common when consanguinity is present or allele frequency is high)
2. Compound heterozygous: Inherited two different loss-of-function mutations in the same gene, one from each parent (e.g., one ΔF508 + one other CFTR mutation in cystic fibrosis)

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Distinguishing Features vs. Autosomal Dominant

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Major Categories and Examples

A. Inborn Errors of Metabolism (most common category)

• Basic Medical Biochemistry (Lippincott), 6th Ed; Bradley & Daroff's Neurology

B. Lysosomal Storage Diseases (a subset of above)

• Robbins & Kumar Basic Pathology

C. Ion Channel / Transporter Defects

• Gene: CFTR (chromosome 7)
• Most common: ΔF508 deletion mutation
• Defect: CFTR protein is a cAMP-activated Cl⁻/HCO₃⁻ channel - loss causes viscous secretions in lung, pancreas, GI tract
• Features: Recurrent pulmonary infections (Pseudomonas, Burkholderia), bronchiectasis, pancreatic insufficiency, male infertility (absent vas deferens), high sweat chloride
• Death: Most commonly from cardiopulmonary complications
• Treatment: Molecular therapies (CFTR modulators - ivacaftor, elexacaftor-tezacaftor) for certain mutations

D. Hemoglobin Disorders

E. Neurological AR Disorders

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Special Concepts

Consanguinity and Hardy-Weinberg

• When a rare recessive allele exists at low frequency (q), the chance of two unrelated carriers meeting and having an affected child is very low (q²)
• Consanguinity dramatically increases this risk - related individuals are far more likely to share the same rare allele
• A pseudodominant pattern can occur in highly inbred communities where the recessive allele is so common that disease appears in successive generations

Compound Heterozygosity

• Many AR patients are not simple homozygotes; they inherit two different loss-of-function mutations in the same gene
• Example: One parent carries ΔF508 CFTR, the other carries a different CFTR mutation → child is compound heterozygous but still has CF

Severity and Allelic Heterogeneity

• The severity of AR disease often depends on the type of mutation:
  • Null/truncating mutations → complete loss of protein → severe phenotype
  • Missense mutations → partial function retained → milder phenotype
  • Example: Severe GBE1 mutations → glycogen storage disease type IV (fatal in infancy); mild GBE1 mutations → adult polyglucosan body disorder

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Summary

AR DISEASE
├── BOTH alleles must be mutated
├── Parents = unaffected carriers
├── 25% risk per child
├── Horizontal pedigree, skips generations
├── Mostly enzyme deficiencies → substrate accumulation
├── More uniform phenotype, complete penetrance, early onset
└── Common categories:
    ├── Inborn errors of metabolism (PKU, galactosemia)
    ├── Lysosomal storage diseases (Tay-Sachs, Gaucher's)
    ├── Ion channel defects (Cystic fibrosis)
    ├── Hemoglobin disorders (Sickle cell, β-thalassemia)
    └── Neurological (Friedreich's ataxia, SMA)

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Diseases of Autosomal Recessive Inheritance

1. Inborn Errors of Metabolism

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2. Lysosomal Storage Diseases

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3. Ion Channel / Transporter Defects

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4. Hemoglobin / Red Cell Disorders

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5. Neurological / Neuromuscular Diseases

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6. Renal / Urinary Diseases

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7. Skeletal / Connective Tissue

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8. Endocrine / Reproductive

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9. Immune / Blood

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10. GI / Hepatic

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Quick Memory Aid - Common AR Diseases

"CF TWIST GAP" mnemonic:
C - Cystic fibrosis
F - Friedreich's ataxia
T - Tay-Sachs
W - Wilson's disease
I - Inborn errors of metabolism (PKU, galactosemia)
S - Sickle cell / SMA
T - Thalassemia (β)
G - Gaucher's disease
A - Ataxia-telangiectasia
P - Pompe / PKU

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"Autosomal recessive disorders include almost all inborn errors of metabolism... mutations in genes that encode enzymes."

• Robbins, Cotran & Kumar Pathologic Basis of Disease

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X-Linked Dominant and X-Linked Recessive Inheritance

Foundation: The X Chromosome and Hemizygosity

• Males are XY - they have only one X chromosome, so any gene on that X has no partner allele. This is called hemizygosity
• Females are XX - they have two X chromosomes, one from each parent
• Since males are hemizygous for X-linked genes, dominance/recessiveness only applies to females - a male with any mutant X-linked allele will always express the disease

"As males only possess a single X chromosome, they are hemizygous for all its genes, and consequently any pathogenic mutation is expressed by default. Because of this, dominance of X-linked genes applies with respect to whether female carriers express disease."

• Bradley & Daroff's Neurology in Clinical Practice

Critical Rule: No Father-to-Son (No Male-to-Male) Transmission

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X Inactivation (Lyonization) - Key Concept for Both Types

• All females are naturally mosaic for X-linked genes - some cells express the maternal X, others express the paternal X
• This is why carrier females of XLR conditions are usually unaffected - most of their cells happen to use the normal X as the active one
• However, if X inactivation is skewed (disproportionate activation of the mutant X), even XLR carrier females may show symptoms ("manifesting carriers")
• For XLD: the disease is expressed in heterozygous females, but severity varies depending on the pattern of lyonization

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PEDIGREE DIAGRAM

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Part 1: X-Linked Recessive (XLR)

Definition

Characteristics

Genetic Risk Calculation

Carrier mother (X^A X^a) × Normal father (X^A Y)

Sons:   X^A Y (50% normal)   |   X^a Y (50% AFFECTED)
Daughters: X^A X^A (25% normal) | X^A X^a (25% carrier, unaffected)

Affected father (X^a Y) × Normal mother (X^A X^A)

Sons:   All normal (X^A Y) ← get Y from dad
Daughters: All carriers (X^A X^a) ← all get X^a from dad

When Females Can Be Affected in XLR

1. Turner syndrome (45, XO): Only one X chromosome - if it carries the mutant allele, disease is expressed (e.g., females with Turner syndrome + hemophilia or DMD have been reported)
2. X-autosome translocation: If the translocation breakpoint disrupts an X-linked gene, the normal X is preferentially inactivated (to preserve autosomal gene dosage), so the mutant allele is always active
3. Skewed X inactivation: Random chance results in the mutant X being active in most cells ("manifesting carrier") - causes mild symptoms
4. Homozygous female: If both X chromosomes carry the mutant allele (rare, but possible in consanguineous families or high-frequency alleles like G6PD)

Diseases - X-Linked Recessive

Neuromuscular

Hematological

Immune / Metabolic

Neurological / Developmental

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Part 2: X-Linked Dominant (XLD)

Definition

Characteristics

Genetic Risk Calculation

Affected mother (X^A X^a) × Normal father (X^+ Y)

Sons:   50% X^+ Y (normal)  |  50% X^a Y (affected/lethal)
Daughters: 50% X^+ X^A (normal) | 50% X^A X^a (affected)

Affected father (X^a Y) × Normal mother (X^+ X^+)

Sons:   All X^+ Y = all NORMAL (sons get Y from dad)
Daughters: All X^+ X^a = ALL AFFECTED (all get X^a from dad)

Diseases - X-Linked Dominant

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Side-by-Side Comparison

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Important Special Note: Dominant vs. Recessive in X-Linked is a Spectrum

"Nearly a third of X-linked disorders are penetrant in some (but not all) female heterozygotes and cannot be classified as either dominant or recessive... they show incomplete penetrance that varies as a function of X inactivation patterns."

• Thompson & Thompson Genetics and Genomics in Medicine, 9th Ed

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Vitamin D-Resistant Rickets

Overview

"The most common cause of rickets in the United States is hypophosphatemic osteomalacia (also known as hypophosphatemic vitamin D-resistant rickets). This disorder is an X-linked dominant inherited trait characterized by renal phosphate wasting."

• Tietz Textbook of Laboratory Medicine, 7th Ed

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Classification of Vitamin D-Resistant Rickets

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PART 1: X-Linked Hypophosphatemia (XLH) - The Classic "Vitamin D-Resistant Rickets"

Genetics

• Inheritance: X-linked dominant (XLD)
• Gene: PHEX (Phosphate-Regulating Endopeptidase on the X chromosome)
• Prevalence: 1 in 20,000; high penetrance; both males and females affected
• Males tend to be more severely affected than females

Pathophysiology

PHEX mutation (loss of function)
        ↓
PHEX normally suppresses FGF23 production in osteocytes
(via binding to DMP1 - dentin matrix protein 1)
        ↓
Without PHEX: FGF23 levels rise markedly
        ↓
FGF23 acts on kidney proximal tubules:
  1. Blocks NaPi-IIa/IIc cotransporters → phosphate NOT reabsorbed → phosphaturia
  2. Inhibits CYP27B1 (1α-hydroxylase) → cannot convert 25(OH)D → 1,25(OH)2D
        ↓
Result: LOW serum phosphate + LOW/Normal 1,25(OH)2D
        ↓
Defective bone mineralization → RICKETS in children / OSTEOMALACIA in adults

"Binding of PHEX to DMP1 appears to be critical for the suppression of osteocyte production of FGF-23 in bone. Inactivating mutations in PHEX or DMP1 result in higher circulating levels of FGF-23 and resultant phosphate wasting. Abnormal synthesis of 1,25(OH)2D in XLH can be explained by increased levels of FGF-23, which suppress kidney 1α-hydroxylase activity."

• Brenner & Rector's The Kidney

Key Biochemical Profile

Clinical Features

• Children: Rickets - bowing of legs, short stature, widened growth plates
• Skeletal: Short stature, bowed long bones, frontal bossing, rachitic rosary, short bowed legs
• Dental: Dental abscesses due to defective dentin mineralization (spontaneous, without caries)
• Cranial: Premature craniosynostosis (premature fusion of skull sutures)
• Adults: Osteomalacia, enthesopathy (calcification of tendons/ligaments), bone pain, pseudofractures, osteoarthritis, hearing loss
• Key difference from nutritional rickets: Does NOT respond to standard vitamin D doses

Treatment

• Oral phosphate supplements (1-3 g/day in divided doses) to replace lost phosphate
• Calcitriol (1,25(OH)2D - active vitamin D, 0.25-2 mcg/day) to improve calcium/phosphate absorption
• Limitation: Prolonged use leads to secondary/tertiary hyperparathyroidism (phosphate supplements stimulate PTH)

• Burosumab (Crysvita) - a human anti-FGF23 monoclonal antibody
  • FDA approved for children ≥1 year and adults
  • Blocks excess FGF23 directly → restores renal phosphate reabsorption and normalizes 1,25(OH)2D production
  • Dosing: 0.8-2 mg/kg SC every 2 weeks (children); 1 mg/kg up to 90 mg SC every 4 weeks (adults)
  • Significantly more effective than conventional therapy with fewer complications

"Burosumab, a human anti-FGF23 monoclonal antibody, has been approved for the treatment of XLH. It corrects hypophosphatemia, improves bone pain, and heals fractures in both children and adults."

• Harrison's Principles of Internal Medicine, 22nd Ed

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PART 2: Vitamin D-Dependent Rickets Type 1 (VDDR-1) / Pseudovitamin D-Deficiency Rickets

Genetics

• Inheritance: Autosomal recessive
• Gene: CYP27B1 encoding renal 1α-hydroxylase

Mechanism

• The enzyme that converts 25(OH)D (calcidiol) → 1,25(OH)2D (calcitriol) is absent/deficient
• The body cannot make the active form of vitamin D despite having plenty of 25(OH)D
• Presents in infancy with hypocalcemia, hypophosphatemia, elevated PTH, elevated ALP

Biochemical Profile

Treatment

• Calcitriol (1,25(OH)2D) - replaces the missing active vitamin D
• Responds well to physiological doses (0.25-0.5 mcg/day)

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PART 3: Vitamin D-Dependent Rickets Type 2 (VDDR-2) / Hereditary Vitamin D-Resistant Rickets (HVDRR)

Genetics

• Inheritance: Autosomal recessive
• Gene: VDR (vitamin D receptor)

Mechanism

• Vitamin D is activated normally - 1,25(OH)2D levels are actually very high (the body tries to compensate)
• But the receptor itself is non-functional → complete end-organ resistance
• Cells cannot respond to vitamin D at all

Biochemical Profile

Clinical Features

• Similar to VDDR-1 plus a unique feature: Alopecia (hair loss) - the VDR is required in hair follicles
• Alopecia is actually a defining distinguishing feature of VDDR-2

Treatment

• Very difficult - partially functional VDR mutations may respond to very large doses of calcitriol
• Most severe cases: IV calcium and phosphate infusions can correct rickets by bypassing the VDR
• After puberty: requirements often diminish spontaneously

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Comparison of All Types - Summary Table

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Also: Other Causes of Phosphate-Wasting Rickets (same mechanism as XLH)

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