Phenotypic Mutation 'Punk' (pdf version)
Mutation Type missense
Coordinate72,262,076 bp (GRCm38)
Base Change A ⇒ G (forward strand)
Gene Slc13a5
Gene Name solute carrier family 13 (sodium-dependent citrate transporter), member 5
Synonym(s) Indy, Nact, mINDY, NaC2/NaCT
Chromosomal Location 72,241,989-72,267,222 bp (-)
MGI Phenotype FUNCTION: [Summary is not available for the mouse gene. This summary is for the human ortholog.] This gene encodes a protein belonging to the solute carrier family 13 group of proteins. This family member is a sodium-dependent citrate cotransporter that may regulate metabolic processes. Mutations in this gene cause early infantile epileptic encephalopathy 25. Alternative splicing results in multiple transcript variants. [provided by RefSeq, Aug 2014]
PHENOTYPE: Mice homozygous for a null allele display resistance to diet and age induced obesity, increased energy expenditure, improved glucose tolerance, and increased hepatic lipid oxidation. Mice homozygous for an ENU-induced allele exhibit reduced body weight. [provided by MGI curators]
Accession Number

NCBI RefSeq: NM_001004148; MGI:1919003

Mapped Yes 
Amino Acid Change Serine changed to Proline
Institutional SourceBeutler Lab
Gene Model predicted gene model for protein(s): [ENSMUSP00000021161] [ENSMUSP00000119417] [ENSMUSP00000119822] [ENSMUSP00000146922] [ENSMUSP00000146762]
SMART Domains Protein: ENSMUSP00000021161
Gene: ENSMUSG00000020805
AA Change: S60P

Pfam:Na_sulph_symp 8 558 1.3e-121 PFAM
Pfam:CitMHS 13 172 1.6e-14 PFAM
Pfam:CitMHS 202 498 6.4e-24 PFAM
Predicted Effect probably damaging

PolyPhen 2 Score 0.996 (Sensitivity: 0.55; Specificity: 0.98)
(Using ENSMUST00000021161)
SMART Domains Protein: ENSMUSP00000119417
Gene: ENSMUSG00000020805
AA Change: S60P

Pfam:Na_sulph_symp 7 115 1.3e-24 PFAM
Predicted Effect probably damaging

PolyPhen 2 Score 0.998 (Sensitivity: 0.27; Specificity: 0.99)
(Using ENSMUST00000137701)
SMART Domains Protein: ENSMUSP00000119822
Gene: ENSMUSG00000020805

Pfam:Na_sulph_symp 6 102 7.9e-20 PFAM
Predicted Effect probably benign
Predicted Effect probably damaging

PolyPhen 2 Score 0.997 (Sensitivity: 0.41; Specificity: 0.98)
(Using ENSMUST00000208056)
Predicted Effect probably benign
Meta Mutation Damage Score 0.6301 question?
Is this an essential gene? Non Essential (E-score: 0.000) question?
Phenotypic Category
Phenotypequestion? Literature verified References
Body Weight - decreased 21803289
Body Weight (DSS Female) - decreased 21803289
Body Weight (DSS) - decreased
Body Weight (DSS, z-score) - decreased
Body Weight (Female) - decreased 21803289
Body Weight (Male) - decreased 21803289
Body Weight (Z-score) - decreased
Candidate Explorer Status CE: excellent candidate; human score: 2; ML prob: 0.692
Single pedigree
Linkage Analysis Data
Alleles Listed at MGI

All Mutations and Alleles(8) : Chemically induced (other)(1) Targeted(7)

Lab Alleles
AlleleSourceChrCoordTypePredicted EffectPPH Score
IGL02347:Slc13a5 APN 11 72258954 splice site probably null
IGL03392:Slc13a5 APN 11 72245178 missense probably damaging 1.00
punk2 UTSW 11 72253391 missense possibly damaging 0.65
R0018:Slc13a5 UTSW 11 72266475 missense probably benign
R0018:Slc13a5 UTSW 11 72266475 missense probably benign
R0042:Slc13a5 UTSW 11 72259114 missense probably benign 0.31
R0194:Slc13a5 UTSW 11 72245233 missense probably benign 0.22
R0194:Slc13a5 UTSW 11 72262130 missense possibly damaging 0.95
R0234:Slc13a5 UTSW 11 72250800 missense probably damaging 0.98
R1499:Slc13a5 UTSW 11 72250731 missense probably damaging 0.97
R1655:Slc13a5 UTSW 11 72257378 missense probably benign 0.00
R1728:Slc13a5 UTSW 11 72266459 splice site probably null
R1818:Slc13a5 UTSW 11 72253343 missense probably benign 0.02
R2304:Slc13a5 UTSW 11 72259039 missense probably damaging 1.00
R2352:Slc13a5 UTSW 11 72252321 missense probably benign 0.06
R2408:Slc13a5 UTSW 11 72262076 missense probably damaging 1.00
R2919:Slc13a5 UTSW 11 72247791 missense possibly damaging 0.92
R2920:Slc13a5 UTSW 11 72247791 missense possibly damaging 0.92
R3103:Slc13a5 UTSW 11 72257388 missense probably damaging 1.00
R4772:Slc13a5 UTSW 11 72250846 critical splice acceptor site probably null
R4906:Slc13a5 UTSW 11 72257418 missense probably damaging 0.99
R5385:Slc13a5 UTSW 11 72259077 missense probably benign 0.01
R5562:Slc13a5 UTSW 11 72262039 missense probably damaging 0.99
R5878:Slc13a5 UTSW 11 72253391 missense possibly damaging 0.65
R6173:Slc13a5 UTSW 11 72253197 missense probably benign 0.05
R6665:Slc13a5 UTSW 11 72260360 missense probably damaging 0.99
R7317:Slc13a5 UTSW 11 72245127 missense probably damaging 1.00
R7338:Slc13a5 UTSW 11 72266484 missense probably benign
R7908:Slc13a5 UTSW 11 72259064 missense probably benign 0.00
R8038:Slc13a5 UTSW 11 72253370 missense probably benign 0.31
R8420:Slc13a5 UTSW 11 72257384 missense probably damaging 1.00
Mode of Inheritance Autosomal Semidominant
Local Stock
Last Updated 2019-09-04 9:44 PM by Diantha La Vine
Record Created 2015-08-04 3:24 PM by Bruce Beutler
Record Posted 2016-09-19
Phenotypic Description

Figure 1. Punk mice exhibited reduced body weight compared to wild-type littermates. Scaled body weights are shown. Abbreviations: WT, wild-type; REF, homozygous reference mice; HET, heterozygous variant mice; VAR, homozygous variant mice. Mean (μ) and standard deviation (σ) are indicated.

The Punk phenotype was identified among G3 mice of the pedigree R2408, some of which exhibited reduced body weights compared to wild-type littermates (Figure 1).

Nature of Mutation

Figure 2. Linkage mapping of the reduced body weights using an additive model of inheritance. Manhattan plot shows -log10 P values (Y-axis) plotted against the chromosome positions of 36 mutations (X-axis) identified in the G1 male of pedigree R2408. Scaled body weight phenotype data are shown for single locus linkage analysis without consideration of G2 dam identity. Horizontal pink and red lines represent thresholds of P = 0.05, and the threshold for P = 0.05 after applying Bonferroni correction, respectively.

Whole exome HiSeq sequencing of the G1 grandsire identified 36 mutations. The body weight phenotype was linked to a mutation in Slc13a5: a T to C transition at base pair 72,262,076 (v38) on chromosome 11, or base pair 4,957 in the GenBank genomic region NC_000077 encoding Slc13a5. Linkage was found with an additive model of inheritance (P = 7.115 x 10-6), wherein 10 variant homozygotes and 23 heterozygotes departed phenotypically from nine homozygous reference mice (Figure 2).


The mutation corresponds to residue 217 in the mRNA sequence NM_001004148 within exon 2 of 12 total exons.



55  -P--V--A--V--T--S--L--L--P--V--L-


The mutated nucleotide is indicated in red.  The mutation results in a serine (S) to proline (P) substitution at position 60 (S60P) in the SLC13A5 (NaCT) protein, and is strongly predicted by PolyPhen-2 to be damaging (score = 0.996).

Protein Prediction
Figure 3. Domain organization and topology of NaCT. NaCT has 11 putative transmembrane domains (SMART). The location of the Punk mutation results in a serine to proline substitution at position 60. Other mutations found in NaCT are noted. Click on each mutation to view more information. The topology of mouse NaCT was modeled after that of the Vibrio cholerae homolog of mouse NaCT (termed VcINDY); see Figure 4, PDB:4F35, and (2) for more information.

Slc13a5 encodes NaCT (Na+/citrate transporter; alternatively, mINDY (mouse I’m not dead yet) or NaC2), a member of the solute carrier (SLC) family of anion transporters. The SLC superfamily encodes intrinsic membrane transporters comprising 55 gene families and 362 putatively functional protein-coding genes. The gene products include passive transporters, symporters and antiporters that transport a wide variety of substrates including amino acids and oligopeptides, glucose and other sugars, inorganic cations and anions, bile salts, carboxylate and other organic anions, acetyl coenzyme A, essential metals, biogenic amines, neurotransmitters, vitamins, fatty acids and lipids, nucleosides, ammonium, choline, thyroid hormone and urea (1).


The members of the SLC13 family are divalent anion sodium symporters, which mediate sodium-coupled anion cotransport at the plasma membrane of epithelial cells of the kidney, small intestine, placenta, and liver. The SLC13 proteins can be grouped into two subfamilies based on different anion specificities: Na+-carboxylate (NaC; alternatively, Na+-dicarboxylate [NaDC]) cotransporters and Na+-sulfate (NaS) cotransporters. The NaC/NaDC cotransporters include SLC13A2 (NaC1; NaDC-1/SDCT1), SCL13A3 (NaC3; NaDC-3/SDCT2), and NaCT, while the NaS cotransporters are SLC13A1 (NaS1; NaSi-1) and SLC13A4 (NaS2; SUT-1). The NaC proteins mediate the transportation of tricaboxylic acid (TCA) cycle (alternatively, Krebs cycle or citric acid cycle) intermediates succinate citrate, succinate, and α-ketoglutarate, while the NaS proteins mediate the transportation of sulfate, selenite, and thiosulfate.


Mouse NaCT has 11 putative transmembrane domains (Figure 3; SMART). Each of the SLC13 proteins has a sodium:sulfate symporter family signature (TSFAFLLPVANPPNAIV); the function of this motif is unknown. The SLC13 proteins have consensus sites for protein kinase C, cAMP/cGMP, casein kinase II, tyrosine kinase phosphorylation as well as N-myristoylation sites; the functional significance of phosphorylation and myristoylation of NaCT are unknown. There are two putative N-linked glycosylation sites on NaCT at Asn382 and Asn566.


Figure 4. Crystal structure of the bacterial homolog of NaCT from Vibrio cholerae, VcINDY. Figure is modeled by UCSF Chimera using PDB:4F35.

The structure of the Vibrio cholerae homolog of mouse NaCT (termed VcINDY) has been solved by X-ray crystallography [PDB:4F35; (2); Figure 4]; one citrate and one sodium ion were bound to the peptide. VcINDY has 11 transmembrane domains (TM) and two opposing hairpin structures designated HPin and HPout. TM4, 5, 9, and 10 of VcINDY are each broken into two segments within the membrane; each pair are named “a” and “b”. The loops that connect TM5a and TM5b as well as the loop that connects TM10a and TM10b are eight amino acids long. VcINDY crystallized as a homodimer with contacts between one protomer at TM3, 4a, and 9b interacting with the other protomer at TM4b, 8, and 9a (2). The N-terminus of VcINDY is cytosolic, while the C-terminus is extracellular (3). The HPin structure inserts into the membrane from the cytosolic side and connects to TM4; the HPout structure inserts into the protein from the periplasm connecting to TM9. The substrate and cation binding sites of VcINDY are comprised of amino acids from the tips of the two opposing hairpin loops and portions of TM5 and 10 (2). The N- and C-terminal halves of VcINDY (TM2-6 and 7-11, respectively) have 26.2% identity and consist of a two-fold repeat (2).


The Slc13a5 mutation in the Punk mice results in a serine (S) to proline (P) substitution at position 60 (S60P) in NaCT. Ser60 is in putative TM2.


The members of the SLC13 family are ubiquitously expressed, with predominant expression in the kidney, small intestine, liver, placenta, testis and brain (4-7). In the mouse, NaCT is also expressed in the adipose tissue, skeletal muscle, and pancreas (6). In the liver, NaCT is expressed in the sinusoidal membrane of hepatocytes (8). In the brain, Slc13a5 is expressed in the cerebral cortex, cerebellum, hippocampus, and olfactory bulb (5). NaCT is expressed in mouse cerebrocortical neurons, but not in astrocytes (7). NaCT (and the other members of the SLC13 family) are located in the plasma membrane of epithelial cells. Slc13a5 mRNA expression levels are reduced during starvation in rat hepatocytes and mice liver (6).

Figure 5. NaCT mediates the transport of TCA cycle intermediates. NaCT facilitates citrate (and other TCA intermediates) uptake from the circulation. Citrate is a carbon source for the TCA cycle, fatty acid synthesis, cholesterol synthesis, glycolysis, neurotransmitter production, and gluconeogenesis.

Citrate is the major carbon source for several processes including energy production via the TCA cycle, fatty acid synthesis, cholesterol synthesis, glycolysis, neurotransmitter production, and gluconeogenesis (4;5;9). During fatty acid biosynthesis, citrate generated in the mitochondria by the TCA cycle enters the cytoplasm whereby it is converted to acetyl-CoA and oxaloacetate by ATP-citrate lyase. Conversion of acetyl-CoA to malonyl-CoA by acetyl-CoA carboxylase is the first committed step in fatty acid synthesis.


The TCA cycle is a set of chemical reactions within the mitochondria that mediate aerobic respiration in cells. The steps of the TCA are as follows: (i) pyruvate is transported into the mitochondria whereby it loses carbon dioxide to form acetyl-CoA. (ii) Citrate synthase mediates the combining of acetyl-CoA with oxaloacetate, which produces citrate. (iii) The citrate is converted into isocitrate by aconitase. (iv) Isocitrate is oxidized into alpha-ketogluterate by isocitrate dehydrogenase; byproducts of this reaction are NADH and CO2. (v) Alpha-ketogluterate is oxidized into succinyl-CoA by alpha-ketogluterate dehydrogenase; NADH and CO2 are products of this reaction. (vi) Succinyl-CoA is converted into succinate by succinyl-CoA synthetase, which yields one ATP/GTP. (vii) The enzyme succinate dehydrogenase converts succinate into fumerate; this reaction produces FADH2. (viii) Water is added to fumerate via fumerase to form malate. (ix) Malate is oxidized by malate dehydrogenase to form oxaloacetate; the byproduct of the reaction is NADH. In total, two carbons enter the cycle, two molecules of CO2 are released, three molecules of NADH and one of FADH2 are generated, and one molecule of ATP or GTP is produced.


NaCT mediates the transport of TCA cycle intermediates citrate, succinate, malate, and formate from the circulation (Figure 5). NaCT exhibits highest affinity for citrate (8;10), and has lower affinity for succinate, malate, and formate (4;5;8). NaCT exhibits the pH-sensitive cotransport of citrate, but pH-independent cotransport of succinate, and Li+-sensitive cotransport [reviewed in (11)]. Maximum NaCT activity is at pH 7.0-7.5; NaCT function is inhibited in acidic and alkaline pH (8). Species differences in the function of NaCT have been observed. Substrate sensitivity and cation dependence are reported to be different between mouse and human NaCT (12). For example, mouse NaCT has a higher affinity to citric acid intermediates than that of human NaCT (12). Also, mouse NaCT was fully active at physiologic levels of citrate, but human NaCT was not (12). Furthermore, human NaCT is less dependent on extracellular sodium than mouse NaCT (12). Human, chimpanzee, and monkey NaCT are stimulated by Li+ (9), while rat, mouse, dog, and zebrafish NaCT are inhibited by Li+ (13).


In Drosophila and C. elegans, the reduced expression of the NaCT homolog Indy (cdNAC-2 in C. elegans) promotes a prolonged life span with a concomitant reduction in whole body fat stores (10;14). Indy-deficient flies also exhibited reduced expression of insulin-like proteins compared to levels in calorically-restricted wild-type files (14).


Slc13a5-deficient (Slc13a5-/-) mice exhibit metabolic defects and changes in energy balance-regulating pathways. The Slc13a5-/- mice were smaller in size, have lower plasma glucose levels than wild-type mice, and are resistant to the effects (i.e., weight gain and insulin resistance) of high fat feeding (6). Citrate and malate levels in the plasma of Slc13a5-/- mice are slightly increased compared to that in wild-type mice; the levels of succinate or fumarate were not significantly changed upon loss of Slc13a5. The Slc13a5-/- mice exhibited reduced uptake of citrate from the circulation into the liver, but not the kidney or adipose tissue (6). The Slc13a5-/- mice also exhibited increased oxygen consumption, carbon dioxide generation, and resting energy expenditure. After a glucose tolerance test, plasma glucose and insulin concentrations were reduced in the Slc13a5-/- mice. The Slc13a5-/- mice had improved insulin sensitivity after hyperinsulinemic euglycemic clamp with reduced basal hepatic glucose production. The resistance to diet-induced obesity and insulin resistance is mediated by the function of NaCT on mitochondrial metabolism as the hepatocellular ATP/ADP ratio was reduced and induction of PGC-1α, inhibition of ACC-2, and reduction of SREBP-1c levels were observed (6).


In humans, mutations in SLC13A5 are linked to early-onset epileptic encephalopathy-25 (EOEE25; OMIM: #615905) (15;16). EOEE is a genetically heterogenous group of disorders. EOEE25 is an autosomal recessive condition marked by frequent tonic seizures or spasms starting in infancy. EOEE25 patients exhibit abnormal interictal (between seizures) electro-encephalogram, psychomotor delay, and/or cognitive deterioration. Approximately 75% of EOEE patients progress to West syndrome, which is a condition characterized by tonic spasms with clustering, arrest of psychomotor development, and hypsarrhythmia. The disease-causing SLC13A5 mutations in patients with EOEE25 are proposed to affect the ability of NaCT to transport citrate across the plasma membrane to the cytosol by preventing its ability to bind sodium (15).

Putative Mechanism

Similar to the Slc13a5-/- mice, the Punk mice are smaller in size compared to their wild-type littermates, indicating that NaCTPunk has lost citrate uptake function.

Primers PCR Primer

Sequencing Primer

PCR program

1) 94°C 2:00
2) 94°C 0:30
3) 55°C 0:30
4) 72°C 1:00
5) repeat steps (2-4) 40x
6) 72°C 10:00
7) 4°C hold

The following sequence of 404 nucleotides is amplified (chromosome 11, - strand):

1   acaaccaggc tcttcttggtgtcctgagc cttggaaagc caccctgccc aggagcagga
61  catagccttc ctcatcacag aggcagctgc cctgctatga ggttggtttg cagtgaagcc
121 tatggaacct cagaactgtt tcttcagttt gccaggtgtg cctatgttat agtcattatg
181 gctgtctact ggtgcacaga tgtcatccca gtggctgtta cctccctcct gcctgtctta
241 ctcttcccac ttttgaaggt tctggactcc aagcaggtga gtaagctgag cagggggctg
301 gtgacttccg ggtcctccca tgtacttcct tactcctctg agtccctctg agctgccgag
361 gcctagggat agctacagaa atccagaagt ggatccctca agga

Primer binding sites are underlined and the sequencing primers are highlighted; the mutated nucleotide is shown in red.

Science Writers Anne Murray
Illustrators Peter Jurek, Katherine Timer
AuthorsZhe Chen and Bruce Beutler