Our findings on 110 ARPKD parents and a maternal grandmother suggest that carrier status for PKHD1 mutations creates a predisposition to multiple liver cysts and renal involvement associated with increased medullary echogenicity on USG.
The ultrasound finding of increased medullary echogenicity is commonly reported as “nephrocalcinosis”, a term that describes the deposition of radiologically-demonstrable calcium salts in renal parenchyma11,12
. The differential diagnosis of “nephrocalcinosis” diagnosed radiologically includes true histopathological nephrocalcinosis and medullary sponge kidney (MSK), which itself can be associated with calcium depositions ()11,13
. Most cases of MSK are sporadic; the presence of rare dominantly inherited forms of MSK, and its association with different malformation syndromes, suggest that MSK is an inherited developmental disorder14–16
. The molecular basis of MSK remains largely unknown, two sequence variants of the GDNF
gene were recently reported in a few MSK patients14
. While intravenous pyelography is the gold standard for the diagnosis of MSK, computerized tomography has become the preferred diagnostic modality. MSK is clinically symptomatic in only a subset of patients and, therefore, is probably under-diagnosed15
. The prevalence of MSK is estimated at 5 in 10 000 – 100 00014,15
. Based upon our findings, we speculate that carrier (heterozygous) status for PKHD1
mutations may be one underlying molecular cause of MSK. Supporting this hypothesis, the microscopic pathology of MSK resembles that of ARPKD; both are characterized by dilated collecting ducts, although MSK involves only the precalyceal collecting ducts14,15
. Furthermore, MSK has been reported in patients with autosomal dominant polycystic kidney disease17
and congenital hepatic fibrosis18
, most commonly caused by homozygous mutations in PKHD1
. Given that the estimated carrier frequency for PKHD1
mutations is 1 in 70 and 5 % of ARPKD parents have increased medullary echogenicity, our data suggest that the frequency of PKHD1
-related MSK would be approximately 7 in 10 000.
Conditions associated with the radiological diagnosis of nephrocalcinosis
Polycystic liver disease (PLD) occurs most commonly as a part of ADPKD. PLD without renal cysts, often referred to as isolated PLD, is genetically distinct from ADPKD19,20
. Clinical diagnostic criteria for PLD are available only for at-risk individuals; 21
in patients younger than 40 years, the presence of any liver cysts is considered to be diagnostic of PLD, while in those older than 40, 4 or more cysts are required to differentiate PLD from simple liver cysts20
. These criteria, however, need to be further vetted in a larger cohort of PCLD patients. Isolated PLD is genetically heterogeneous19
; approximately 20 to 30% of the isolated cases are due to mutations in either PRKCSH22
the molecular cause of the remainder of isolated PLD is unknown. PRKCSH
encodes the beta-subunit of glucosidase II, an ER glucosidase that is involved in quality control of newly synthesized glycoproteins. SEC63
encodes Sec63, an integral ER membrane protein that is a part of the multi-protein translocon, the translocation machinery for integral membrane and secreted proteins. It was recently shown that glucosidase IIβ and Sec63p are required in mice for adequate expression of a functional complex of the polycystic kidney disease gene products, polycystin-1, polycystin-2 and fibrocystin/polyductin encoded by Pkhd123, 24
. This is supported by the fact that the histopathological features of isolated PLD are very similar to those of the PLD in ADPKD; in both disorders cysts originate from biliary microhamartomas, also termed von Meyenburg’s complexes25,26
. However, the reason why defects in glucosidase IIβ and Sec63 do not cause renal cystic disease is unknown.
The majority of PLD patients are clinically asymptomatic, with normal liver enzymes and intact synthetic function21,27
. The incidence of isolated PLD in autopsy series26,28,29
(2–3 in 10 000) is higher than its estimated clinical incidence (< 1 in 10 000), suggesting that most cases remain undetected. Our data suggest that heterozygous carriers of PKHD1
mutations are predisposed to PLD. In all ARPKD parents with multiple liver cysts, these cysts were initially detected by standard probe ultrasound; in every case the number of cysts on initial evaluation exceeded 4. Based on the estimated carrier frequency for PKHD1
mutations of 1 in 70, and our finding that 9 % of ARPKD parents have PLD, the frequency of PKHD1
-related PLD would be approximately 1 in 1000. Given the small size of the liver cysts and normal liver-related blood chemistries in ARPKD parents, this condition is more likely to remain undiagnosed, explaining the discrepancy between disease frequencies.
In addition to CHF, ARPKD patients might have liver cysts that are in continuity with the biliary tree, differing form the isolated cysts in PLD. MRCP images of the ARPKD parents were not conclusive in terms of the connectivity of the cysts to biliary ducts. However, the small size and peripheral location of these cysts suggest that they are isolated from the biliary tree, similar to the PLD cysts. This difference in the characteristics of liver cysts between children with ARPKD and their parents is surprising. The increased liver echogenicity associated with coarse echotexture identified in some parents and the biopsy proven CHF in the maternal grandmother presented suggest some PKHD1 carriers are also prone to CHF.
To exclude the possibility that these parents had two PKHD1
mutations, we performed complete sequencing of the gene. However, we found no additional PKHD1
mutations in these individuals. Other genes, including those encoding ciliary proteins, may contribute to cyst formation through synergistic heterozygosity30,31
; this could explain why only a subset of PKHD1
mutation carriers develop these liver and or kidney findings.
In summary, our data suggest that carrier status for ARPKD is a predisposition to renal involvement associated with increased medullary echogenicity on USG (possibly MSK), and liver involvement in the form of asymptomatic PLD and CHF in some cases. All these PKHD1 mutation carriers with abnormal kidney and/or liver imaging were clinically asymptomatic. It remains to be determined if some of these individuals might become symptomatic as they age.