Types
The older classification of NCL divided the condition into four types (CLN1, CLN2, CLN3, and CLN4) based upon age of onset, while newer classifications divide it by the associated gene.CLN4 (unlike CLN1, CLN2, and CLN3) has not been mapped to a specific gene. Mutations
Infantile form
Nonsense and frameshift mutations in the CLN1 gene (located at
1p32) always induce classical INCL, while some missense mutations have been associated with ANCL in addition to the infantile and juvenile forms. The mutation typically results in a deficient form of a lysosomal enzyme called palmitoyl protein thioesterase 1 (PPT1).The wild-type PPT1 is a 306-amino acid polypeptide that is typically targeted for transport into lysosomes by the mannose 6-phosphate (M6P) receptor-mediated pathway. Here, the protein appears to function in removing palmitate residues by cleaving thioester linkages in s-acylated (or palmitoylated) proteins, encouraging their breakdown. Defective polypeptides, however, are unable to exit the endoplasmic reticulum (ER), most likely due to misfolding; further analyses of this pathway could serve to categorize INCL among lysosomal enzyme deficiencies. The human PPT gene shows 91% similarity to bovine PPT and 85% similarity to rat PPT; these data indicate that the PPT gene is highly conserved and likely plays a vital role in cell metabolism. In addition, buildup of defective PPT1 in the ER has been shown to cause the increased release of Ca2+. This homeostasis-altering event leads to increased mitochondrial membrane permeability and subsequent activation of caspase-9, eventually leading to an accumulation of cleft and uncleft poly(ADP-ribose) polymerase  and eventual apoptosis.
Diagnosis
Affected infants have short arms and legs, a small chest with short ribs, and underdeveloped lungs. The spinal bones (vertebrae) in the neck and part of the pelvis (the sacrum) do not harden, or ossify, properly. The face appears flat and oval-shaped, with widely spaced eyes, a small chin, and, in some cases, an opening in the roof of the mouth called a cleft palate. The abdomen is enlarged, and excess fluid may build up in the body before birth (a condition called hydrops fetalis). As a result of these serious health problems, infants are usually premature and stillborn or die shortly after birth from respiratory failure. Some infants have lived for a time, however, with intensive medical support. Babies who live past the newborn period are usually reclassified as having spondyloepiphyseal dysplasia congenita, a related disorder on the spectrum of abnormal bone growth. Treatment
There is no cure or treatment for hypochondrogenesis. References
External links

This article incorporates public domain text from The U.S. National Library of Medicine