Case Report: Homozygous Pathogenic Variant P209L in the TTC21B Gene: A Rare Cause of End Stage Renal Disease and Biliary Cirrhosis Requiring Combined Liver-Kidney Transplantation. A Case Report and Literature Review

Clinical report and genetic analysis of rare premature infant nephronophthisis caused by biallelic TTC21B variants

BACKGROUND

Nephronophthisis (NPHP) is a genetically heterogeneous disease that can lead to end-stage renal disease (ESRD) in children. The TTC21B variant is associated with NPHP12 and mainly characterized by cystic kidney disease, skeletal malformation, liver fibrosis, and retinopathy. Affected patients range from children to adults. Some patients experience ESRD in infancy or early childhood, but clinical reports on neonatal patients are rare. We report a case of NPHP12 in a premature infant and analyze its genetic etiology.

METHODS

Trio-whole exome sequencing analysis was performed on the patient and her parents; bioinformatics software was used to predict and analyze the hazards of the variants. Sanger sequencing was performed to verify variants. We calculated the free energy between mutant IFT139 and the IFT121-IFT122-IFT43 complex structure using molecular dynamics (MD). Finally, the clinical and genetic characteristics of patients with hotspot variant Cys518Arg were reviewed.

RESULTS

Genetic analysis revealed compound-heterozyous TTC21B variants in the patient, c.497delA (p.Lys166fs*36) and c.1552T>C (p.Cys518Arg). Her father and mother had heterozygous c.497delA (p.Lys166fs*36) and heterozygous c.1552T>C (p.Cys518Arg), respectively. Cys518Arg represents a hotspot variant, and the MD calculation results show that this can reduce the structural stability of the IFT121-IFT122-IFT139-IFT43 complex structure. A literature review showed that Cys518Arg might lead to the early occurrence of ESRD.

CONCLUSIONS

Compound-heterozygous TTC21B variants underlie the phenotype in this patient. Thus, Cys518Arg may be a hotspot variant in the Chinese population. Genetic testing should be recommended for NPHP in neonates and early infants.

Recent Advances in Clinical Diagnosis and Pharmacotherapy Options of Membranous Nephropathy

Membranous nephropathy (MN) is the most common cause of nephrotic syndrome among adults, which is the leading glomerular disease that recurs after kidney transplantation. Treatment for MN remained controversial and challenging, partly owing to absence of sensitive and specific biomarkers and effective therapy for prediction and diagnosis of disease activity. MN starts with the formation and deposition of circulating immune complexes on the outer area in the glomerular basement membrane, leading to complement activation. The identification of autoantibodies against the phospholipase A₂ receptor (PLA₂R) and thrombospondin type-1 domain-containing protein 7A (THSD7A) antigens illuminated a distinct pathophysiological rationale for MN treatments. Nowadays, detection of serum anti-PLA₂R antibodies and deposited glomerular PLA₂R antigen can be routinely applied to MN. Anti-PLA₂R antibodies exhibited much high specificity and sensitivity. Measurement of PLA₂R in immune complex deposition allows for the diagnosis of PLA₂R-associated MN in patients with renal biopsies. In the review, we critically summarized newer diagnosis biomarkers including PLA₂R and THSD7A tests and novel promising therapies by using traditional Chinese medicines such as Astragalus membranaceus, Tripterygium wilfordii, and Astragaloside IV for the treatment of MN patients. We also described unresolved questions and future challenges to reveal the diagnosis and treatments of MN. These unprecedented breakthroughs were quickly translated to clinical diagnosis and management. Considerable advances of detection methods played a critical role in diagnosis and monitoring of treatment.

Biallelic variants in TTC21B as a rare cause of early-onset arterial hypertension and tubuloglomerular kidney disease

Monogenic disorders of the kidney typically affect either the glomerular or tubulointerstitial compartment producing a distinct set of clinical phenotypes. Primary focal segmental glomerulosclerosis (FSGS), for instance, is characterized by glomerular scarring with proteinuria and hypertension while nephronophthisis (NPHP) is associated with interstitial fibrosis and tubular atrophy, salt wasting, and low- to normal blood pressure. For both diseases, an expanding number of non-overlapping genes with roles in glomerular filtration or primary cilium homeostasis, respectively, have been identified. TTC21B, encoding IFT139, however has been associated with disorders of both the glomerular and tubulointerstitial compartment, and linked with defective podocyte cytoskeleton and ciliary transport, respectively. Starting from a case report of extreme early-onset hypertension, proteinuria, and progressive kidney disease, as well as data from the Genomics England 100,000 Genomes Project, we illustrate here the difficulties in assigning this mixed phenotype to the correct genetic diagnosis. Careful literature review supports the notion that biallelic, often hypomorph, missense variants in TTC21B are commonly associated with early-onset hypertension and histological features of both FSGS and NPHP. Increased clinical recognition of this mixed glomerular and tubulointerstitial disease with often mild or absent features of a typical ciliopathy as well as inclusion of TTC21B on gene panels for early-onset arterial hypertension might shorten the diagnostic odyssey for patients affected by this rare tubuloglomerular kidney disease.