Fibroblast Growth Factor 23

NEW INSIGHTS INTO CHRONIC KIDNEY DISEASE

ESPN Lyon 2008

J Bacchetta, Lyon

Outline FGF23, the new hormone of the bone/kidney axis

FGF23, biochemical and structural properties

FGF23, physiological roles: regulation of phosphate and 1-25 OH vitamin D

FGF23 deficiency and related diseases

FGF23 excess and related diseases

FGF23 and chronic kidney disease

Conclusion & perspectives

FGF23, the new hormone of the bone/kidney axis

A phosphatonin described in the early 2000’s FGF23 mutation in ADHR (Nat Genet. 2000) Tumor-induced osteomalacia (Shimada, PNAS 2001)

FROM BEDSIDE TO BENCH

A protein synthesized by bone Osteocytes, osteoblasts and odontoblasts

A complex of 3 elements for biological activity FGF23 FGF-R: tyrosine kinase receptor Klotho: cofactor

Anti-aging protein Tissue-specific expression (kidney and parathyroid gland)

FGF23, biochemical and structural properties

Chromosome 12p13 Protein - 251 amino-acids, 30 kDa

FGF19 subfamily, the ‘endocrine’ FGFs Systemic action Stabilization of the β-trefoil structure by a disulfide bond

Two forms: active/inactive

1 25 180 251

FGF homology region

Specific region

25 180 251 251

CLEAVAGE

25

+

ACTIVE FGF23

INACTIVE FGF23RXX

R

FGF23, different assays Two different types of assays (ELISA)

Intact FGF23: active form Antibody against C-term fragment: total FGF23, active and inactive forms

Limited data in children 1 study (total FGF23), 26 children (Jonsson, NEJM 2003)

Renal function: not defined

25 180 251 25125

+

ACTIVE FGF23

INACTIVE FGF23У

λ λ

УУ

λ

FGF23, physiology and regulation

FGF23 regulation not yet clear Potential regulatory mechanisms

FGF 23

P, Ca, PTH

DMP1

MEPE

PHEX

1,25 OH D3

(+) (-)

(-)

(+)

No direct effect on FGF23 promoter

Direct effect on FGF23 promoter

(VDR in osteoblasts)

Bone mineralization

(-)

(+)BONE

KIDNEY

(-)

Klotho

FGF-R

FGF23

Erk Phosphorylation

(+)

(+)

Distal tubule

Bone cell

(-)

Decreased 1α OHase Increased 24

OHase

Decreased 1,25 OH D3

TRPV5

(+)Calcium

reabsorption

Npt2a/2c

(-)Phosphaturia

Proximal tubule

Parathyroid cell

Inhibition of osteoblastic differenciatio

n and mineralizatio

n

Inhibition of PTH secretion and

gene expression

Bone cell

Active FGF23

(+)

PTH,1-25 OH D3

Inactive

FGF23

FP convertase

(-)

(-)

Choroid plexus

?

Animal models of FGF23 deficiency

FGF 23 -/- Klotho -/-

Life span Shortened; accelerated aging

Reproductive function

Infertility

Bone Osteopenia

Calcifications Soft tissue and medial vascular calcification

Clinical features Skin atrophy, neuronal degeneration, pulmonary emphysema,

hypoglycemia

Biology Increased tubular phosphate reabsorption Hyperphosphatemia

Increased expression of Npt2a in kidneyIncreased 1,25 OH vitamin D3 serum level

Hypercalcemia

Clinical symptoms probably arise from excess of phosphate rather than vitamin D

Low phosphate diet: rescues phenotype and prevents vascular calcificationsLow vitamin D diet: improves survival but does not prevent vascular calcificationsTransfer of 1αOHase deficiency: rescues phenotype

Memon 2008

WT FGF23 -/-

FGF23 deficiency and related diseases

Familial tumoral calcinosis Peri-articular, visceral and vascular calcifications Hyperostosis, specific dental abnormalities Biology: hyperphosphatemia, hypoparathyroidism

Genetics: recessive and dominant autosomal inheritance FGF23 inactivating mutation GALNT3 mutation (impaired glycosylation of FGF23)

KLOTHO mutation : high FGF23 concentration

Topaz 2006 Ichikawa, JCI 2007

FGF23 excess and related diseases Tumor-induced osteomalacia McCune-Albright Sd and fibrous

dysplasia of bone Epidermal nevus syndrome Hypophosphatemic rickets

X-linked HR PHEX mutation (Xp22)

Autosomal dominant HR FGF23 mutation

Autosomal recessive HR DMP1 inactivating mutation

Other types HR + hypercalciuria

Npt2c: autosomal recessive ClCN5: X-linked

HR, hyperPTH and dysmorphy: Klotho activating translocation

FGF23 and CKD

Mineral and Bone Disorders in CKD children

GFR < 90 mL/min per 1.73 m2

Long term Bone damage

Drug toxicity Inflammation Anemia Metabolic acidosis Resistance to GH

Vascular calcifications

Morbidity and Morbidity and mortalitymortality

Ca-P HypocalcemiaHyperphosphatemia

Hormones HyperparathyroidismDecreased vitamin D

Bone Delayed bone maturation

Decreased bone massPain, deformationsIncreased fracture risk

Growth Resistance to GH

Muscles Proximal myopathy

Vessels Calcifications

Eye Corneal calcificationsBand keratopathy

Skin Soft tissue necrosisPruritus

Reduced nephron number

Decreased 1-25 OH vitamin D

Decreased number of CaR

and VDR

Decreased urine

phosphate

excretion

Hyperphosphatemia

Hypocalcemia

Vitamin D analog

Hyperparathyroidism

Increased FGF23 level

Urine phosphate excretion

(limitation due to reduced nephron

number)

Decreased FGF23

clearance?

FGF23 and CKD

Stimulatory and inhibitory effect

FGF23 and CKD Increased FGF23

Early stages of CKD, prior to hyperphosphatemia Both intact and C-term FGF23

FGF23: active in CKD? Accumulation in dialysis patients Relative Klotho deficiency?

Sites with co-expression of Klotho and FGF-R Cognitive function? Growth?

FGF23 and CKD in adult patients

FGF23: a novel independent risk factor of progression of CKD in 177 non diabetic adult patients

Prospective follow-up 53 months

Fliser et al., JASN 2007

GFR

Phosphate

PTH

C-t

erm

FG

F23

FGF23, CKD and therapeutic response

At short term (Imanishi, KI 2005) Intact FGF23 predictor of refractoriness to iv calcitriol

therapy In association with high serum PTH level (PPV 88 % and NPV 4 %) 24 weeks 62 HD patients

Nakanishi, KI 2005

At long term (Kazama, KI 2005) Baseline intact FGF23

predictor of refractory hyperparathyroidism

2 years 103 non diabetic HD

patients

FGF23 and CKD children

141 children, 10.8±4 years (4.4-19.9), GFRinulin 100±34 mL/min per 1.73 m² (27-234)

0

20

40

60

80

100

120

140

0 50 100 150 200 250

FGF23 Cterminal assay

Inul

in c

lear

ance

KDOQI I KDOQI II KDOQI IIIHyperfiltration

N 71 33 16 20

GFR * 110 ± 11 76 ± 9 46 ± 8 151 ± 27

FGF23 * 63 ± 89 96 ± 235 103 ± 62 66 ± 52

PTH * 32 ± 11 49 ± 19 69 ± 22 34 ± 17

25OH 23 ± 9 23 ± 8 21 ± 7 21 ± 11

120 children, KDOQI 1,2,3

r= -0.426; P < 0.001 J Bacchetta, et al.

Unpublished results

FGF23, CKD and vascular calcifications

Association between reduced BMD and vascular calcifications

FGF23: not a biomarker of coronary calcifications in subjects with normal kidney function Roos et al., Clin Endocrinol 2008 64 subjects with normal kidney function No correlation between intact FGF23 and coronary artery score

FGF23: biomarker of medial peripheral artery calcification in CKD patients? Inaba, Osteoporos Int. 2006 Inversely correlated to hand arteries, not to aortic calcifications

FGF23 and vascular calcifications: unclear pathophysiology Direct inhibition of calcification? Indirect inhibition of vascular calcification

By inhibiting hydroxylation of vitamin D? By lowering phosphate levels?

‘Take-home message’ FGF23/Klotho

New insight to the understanding of Ca-P metabolism Two hormones - a strong interplay Two key roles: phosphaturia and inhibition of

1OHase

FGF23 and CKD Toxic? Beneficial? Monitoring?

FGF23 and cardiovascular protection? Future: rh-FGF23? FGF23 Ab?

PTH

P

FGF23Vit D

Ca

Acknowledgements Centre de Référence des Maladies Rénales

Rares, Hôpital Femme Mère Enfant, Lyon P Cochat, B Ranchin, A Liutkus P Abou-Jaoude, A Pinçon, M Afanetti

Service d’Exploration Fonctionnelle Rénale et Métabolique, Hôpital Edouard Herriot, Lyon L Dubourg

Département de Biologie Ostéoarticulaire, Hôpital Edouard Herriot, Lyon M Richard S Arnaud

INSERM U831, Lyon PD Delmas I Rondy